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* gccint: (gccint).            Internals of the GNU Compiler Collection.
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   This file documents the internals of the GNU compilers.

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File: gccint.info,  Node: RTL Classes,  Next: Accessors,  Prev: RTL Objects,  Up: RTL

RTL Classes and Formats
=======================

   The various expression codes are divided into several "classes",
which are represented by single characters.  You can determine the class
of an RTX code with the macro `GET_RTX_CLASS (CODE)'.  Currently,
`rtx.def' defines these classes:

`o'
     An RTX code that represents an actual object, such as a register
     (`REG') or a memory location (`MEM', `SYMBOL_REF').  Constants and
     basic transforms on objects (`ADDRESSOF', `HIGH', `LO_SUM') are
     also included.  Note that `SUBREG' and `STRICT_LOW_PART' are not
     in this class, but in class `x'.

`<'
     An RTX code for a comparison, such as `NE' or `LT'.

`1'
     An RTX code for a unary arithmetic operation, such as `NEG',
     `NOT', or `ABS'.  This category also includes value extension
     (sign or zero) and conversions between integer and floating point.

`c'
     An RTX code for a commutative binary operation, such as `PLUS' or
     `AND'.  `NE' and `EQ' are comparisons, so they have class `<'.

`2'
     An RTX code for a non-commutative binary operation, such as
     `MINUS', `DIV', or `ASHIFTRT'.

`b'
     An RTX code for a bit-field operation.  Currently only
     `ZERO_EXTRACT' and `SIGN_EXTRACT'.  These have three inputs and
     are lvalues (so they can be used for insertion as well).  *Note
     Bit-Fields::.

`3'
     An RTX code for other three input operations.  Currently only
     `IF_THEN_ELSE'.

`i'
     An RTX code for an entire instruction:  `INSN', `JUMP_INSN', and
     `CALL_INSN'.  *Note Insns::.

`m'
     An RTX code for something that matches in insns, such as
     `MATCH_DUP'.  These only occur in machine descriptions.

`a'
     An RTX code for an auto-increment addressing mode, such as
     `POST_INC'.

`x'
     All other RTX codes.  This category includes the remaining codes
     used only in machine descriptions (`DEFINE_*', etc.).  It also
     includes all the codes describing side effects (`SET', `USE',
     `CLOBBER', etc.) and the non-insns that may appear on an insn
     chain, such as `NOTE', `BARRIER', and `CODE_LABEL'.

   For each expression code, `rtl.def' specifies the number of
contained objects and their kinds using a sequence of characters called
the "format" of the expression code.  For example, the format of
`subreg' is `ei'.

   These are the most commonly used format characters:

`e'
     An expression (actually a pointer to an expression).

`i'
     An integer.

`w'
     A wide integer.

`s'
     A string.

`E'
     A vector of expressions.

   A few other format characters are used occasionally:

`u'
     `u' is equivalent to `e' except that it is printed differently in
     debugging dumps.  It is used for pointers to insns.

`n'
     `n' is equivalent to `i' except that it is printed differently in
     debugging dumps.  It is used for the line number or code number of
     a `note' insn.

`S'
     `S' indicates a string which is optional.  In the RTL objects in
     core, `S' is equivalent to `s', but when the object is read, from
     an `md' file, the string value of this operand may be omitted.  An
     omitted string is taken to be the null string.

`V'
     `V' indicates a vector which is optional.  In the RTL objects in
     core, `V' is equivalent to `E', but when the object is read from
     an `md' file, the vector value of this operand may be omitted.  An
     omitted vector is effectively the same as a vector of no elements.

`0'
     `0' means a slot whose contents do not fit any normal category.
     `0' slots are not printed at all in dumps, and are often used in
     special ways by small parts of the compiler.

   There are macros to get the number of operands and the format of an
expression code:

`GET_RTX_LENGTH (CODE)'
     Number of operands of an RTX of code CODE.

`GET_RTX_FORMAT (CODE)'
     The format of an RTX of code CODE, as a C string.

   Some classes of RTX codes always have the same format.  For example,
it is safe to assume that all comparison operations have format `ee'.

`1'
     All codes of this class have format `e'.

`<'
`c'
`2'
     All codes of these classes have format `ee'.

`b'
`3'
     All codes of these classes have format `eee'.

`i'
     All codes of this class have formats that begin with `iuueiee'.
     *Note Insns::.  Note that not all RTL objects linked onto an insn
     chain are of class `i'.

`o'
`m'
`x'
     You can make no assumptions about the format of these codes.


File: gccint.info,  Node: Accessors,  Next: Flags,  Prev: RTL Classes,  Up: RTL

Access to Operands
==================

   Operands of expressions are accessed using the macros `XEXP',
`XINT', `XWINT' and `XSTR'.  Each of these macros takes two arguments:
an expression-pointer (RTX) and an operand number (counting from zero).
Thus,

     XEXP (X, 2)

accesses operand 2 of expression X, as an expression.

     XINT (X, 2)

accesses the same operand as an integer.  `XSTR', used in the same
fashion, would access it as a string.

   Any operand can be accessed as an integer, as an expression or as a
string.  You must choose the correct method of access for the kind of
value actually stored in the operand.  You would do this based on the
expression code of the containing expression.  That is also how you
would know how many operands there are.

   For example, if X is a `subreg' expression, you know that it has two
operands which can be correctly accessed as `XEXP (X, 0)' and `XINT (X,
1)'.  If you did `XINT (X, 0)', you would get the address of the
expression operand but cast as an integer; that might occasionally be
useful, but it would be cleaner to write `(int) XEXP (X, 0)'.  `XEXP
(X, 1)' would also compile without error, and would return the second,
integer operand cast as an expression pointer, which would probably
result in a crash when accessed.  Nothing stops you from writing `XEXP
(X, 28)' either, but this will access memory past the end of the
expression with unpredictable results.

   Access to operands which are vectors is more complicated.  You can
use the macro `XVEC' to get the vector-pointer itself, or the macros
`XVECEXP' and `XVECLEN' to access the elements and length of a vector.

`XVEC (EXP, IDX)'
     Access the vector-pointer which is operand number IDX in EXP.

`XVECLEN (EXP, IDX)'
     Access the length (number of elements) in the vector which is in
     operand number IDX in EXP.  This value is an `int'.

`XVECEXP (EXP, IDX, ELTNUM)'
     Access element number ELTNUM in the vector which is in operand
     number IDX in EXP.  This value is an RTX.

     It is up to you to make sure that ELTNUM is not negative and is
     less than `XVECLEN (EXP, IDX)'.

   All the macros defined in this section expand into lvalues and
therefore can be used to assign the operands, lengths and vector
elements as well as to access them.


File: gccint.info,  Node: Flags,  Next: Machine Modes,  Prev: Accessors,  Up: RTL

Flags in an RTL Expression
==========================

   RTL expressions contain several flags (one-bit bit-fields) that are
used in certain types of expression.  Most often they are accessed with
the following macros, which expand into lvalues:

`CONSTANT_POOL_ADDRESS_P (X)'
     Nonzero in a `symbol_ref' if it refers to part of the current
     function's constant pool.  For most targets these addresses are in
     a `.rodata' section entirely separate from the function, but for
     some targets the addresses are close to the beginning of the
     function.  In either case GCC assumes these addresses can be
     addressed directly, perhaps with the help of base registers.
     Stored in the `unchanging' field and printed as `/u'.

`CONST_OR_PURE_CALL_P (X)'
     In a `call_insn', `note', or an `expr_list' for notes, indicates
     that the insn represents a call to a const or pure function.
     Stored in the `unchanging' field and printed as `/u'.

`INSN_ANNULLED_BRANCH_P (X)'
     In an `insn' in the delay slot of a branch insn, indicates that an
     annulling branch should be used.  See the discussion under
     `sequence' below.  Stored in the `unchanging' field and printed as
     `/u'.

`INSN_DEAD_CODE_P (X)'
     In an `insn' during the dead-code elimination pass, nonzero if the
     insn is dead.  Stored in the `in_struct' field and printed as `/s'.

`INSN_DELETED_P (X)'
     In an `insn', nonzero if the insn has been deleted.  Stored in the
     `volatil' field and printed as `/v'.

`INSN_FROM_TARGET_P (X)'
     In an `insn' in a delay slot of a branch, indicates that the insn
     is from the target of the branch.  If the branch insn has
     `INSN_ANNULLED_BRANCH_P' set, this insn will only be executed if
     the branch is taken.  For annulled branches with
     `INSN_FROM_TARGET_P' clear, the insn will be executed only if the
     branch is not taken.  When `INSN_ANNULLED_BRANCH_P' is not set,
     this insn will always be executed.  Stored in the `in_struct'
     field and printed as `/s'.

`LABEL_OUTSIDE_LOOP_P (X)'
     In `label_ref' expressions, nonzero if this is a reference to a
     label that is outside the innermost loop containing the reference
     to the label.  Stored in the `in_struct' field and printed as `/s'.

`LABEL_PRESERVE_P (X)'
     In a `code_label', indicates that the label is referenced by code
     or data not visible to the RTL of a given function.  Labels
     referenced by a non-local goto will have this bit set.  Stored in
     the `in_struct' field and printed as `/s'.

`LABEL_REF_NONLOCAL_P (X)'
     In `label_ref' and `reg_label' expressions, nonzero if this is a
     reference to a non-local label.  Stored in the `volatil' field and
     printed as `/v'.

`LINK_COST_FREE (X)'
     In the `LOG_LINKS' `insn_list' during scheduling, nonzero when the
     cost of executing an instruction through the link is zero, i.e.,
     the link makes the cost free.  Stored in the `call' field and
     printed as `/c'.

`LINK_COST_ZERO (X)'
     In the `LOG_LINKS' `insn_list' during scheduling, nonzero when the
     cost of executing an instruction through the link varies and is
     unchanged, i.e., the link has zero additional cost.  Stored in the
     `jump' field and printed as `/j'.

`MEM_IN_STRUCT_P (X)'
     In `mem' expressions, nonzero for reference to an entire structure,
     union or array, or to a component of one.  Zero for references to a
     scalar variable or through a pointer to a scalar.  If both this
     flag and `MEM_SCALAR_P' are clear, then we don't know whether this
     `mem' is in a structure or not.  Both flags should never be
     simultaneously set.  Stored in the `in_struct' field and printed
     as `/s'.

`MEM_KEEP_ALIAS_SET_P (X)'
     In `mem' expressions, 1 if we should keep the alias set for this
     mem unchanged when we access a component.  Set to 1, for example,
     when we are already in a non-addressable component of an aggregate.
     Stored in the `jump' field and printed as `/j'.

`MEM_SCALAR_P (X)'
     In `mem' expressions, nonzero for reference to a scalar known not
     to be a member of a structure, union, or array.  Zero for such
     references and for indirections through pointers, even pointers
     pointing to scalar types.  If both this flag and `MEM_IN_STRUCT_P'
     are clear, then we don't know whether this `mem' is in a structure
     or not.  Both flags should never be simultaneously set.  Stored in
     the `frame_related' field and printed as `/f'.

`MEM_VOLATILE_P (X)'
     In `mem' and `asm_operands' expressions, nonzero for volatile
     memory references.  Stored in the `volatil' field and printed as
     `/v'.

`REG_FUNCTION_VALUE_P (X)'
     Nonzero in a `reg' if it is the place in which this function's
     value is going to be returned.  (This happens only in a hard
     register.)  Stored in the `integrated' field and printed as `/i'.

`REG_LOOP_TEST_P (X)'
     In `reg' expressions, nonzero if this register's entire life is
     contained in the exit test code for some loop.  Stored in the
     `in_struct' field and printed as `/s'.

`REG_POINTER (X)'
     Nonzero in a `reg' if the register holds a pointer.  Stored in the
     `frame_related' field and printed as `/f'.

`REG_USERVAR_P (X)'
     In a `reg', nonzero if it corresponds to a variable present in the
     user's source code.  Zero for temporaries generated internally by
     the compiler.  Stored in the `volatil' field and printed as `/v'.

     The same hard register may be used also for collecting the values
     of functions called by this one, but `REG_FUNCTION_VALUE_P' is zero
     in this kind of use.

`RTX_FRAME_RELATED_P (X)'
     Nonzero in an `insn' or `set' which is part of a function prologue
     and sets the stack pointer, sets the frame pointer, or saves a
     register.  This flag should also be set on an instruction that
     sets up a temporary register to use in place of the frame pointer.
     Stored in the `frame_related' field and printed as `/f'.

     In particular, on RISC targets where there are limits on the sizes
     of immediate constants, it is sometimes impossible to reach the
     register save area directly from the stack pointer.  In that case,
     a temporary register is used that is near enough to the register
     save area, and the Canonical Frame Address, i.e., DWARF2's logical
     frame pointer, register must (temporarily) be changed to be this
     temporary register.  So, the instruction that sets this temporary
     register must be marked as `RTX_FRAME_RELATED_P'.

     If the marked instruction is overly complex (defined in terms of
     what `dwarf2out_frame_debug_expr' can handle), you will also have
     to create a `REG_FRAME_RELATED_EXPR' note and attach it to the
     instruction.  This note should contain a simple expression of the
     computation performed by this instruction, i.e., one that
     `dwarf2out_frame_debug_expr' can handle.

     This flag is required for exception handling support on targets
     with RTL prologues.

`RTX_INTEGRATED_P (X)'
     Nonzero in an `insn', `insn_list', or `const' if it resulted from
     an in-line function call.  Stored in the `integrated' field and
     printed as `/i'.

`RTX_UNCHANGING_P (X)'
     Nonzero in a `reg' or `mem' if the memory is set at most once,
     anywhere.  This does not mean that it is function invariant.
     Stored in the `unchanging' field and printed as `/u'.

`SCHED_GROUP_P (X)'
     During instruction scheduling, in an `insn', indicates that the
     previous insn must be scheduled together with this insn.  This is
     used to ensure that certain groups of instructions will not be
     split up by the instruction scheduling pass, for example, `use'
     insns before a `call_insn' may not be separated from the
     `call_insn'.  Stored in the `in_struct' field and printed as `/s'.

`SET_IS_RETURN_P (X)'
     For a `set', nonzero if it is for a return.  Stored in the `jump'
     field and printed as `/j'.

`SIBLING_CALL_P (X)'
     For a `call_insn', nonzero if the insn is a sibling call.  Stored
     in the `jump' field and printed as `/j'.

`STRING_POOL_ADDRESS_P (X)'
     For a `symbol_ref' expression, nonzero if it addresses this
     function's string constant pool.  Stored in the `frame_related'
     field and printed as `/f'.

`SUBREG_PROMOTED_UNSIGNED_P (X)'
     Nonzero in a `subreg' that has `SUBREG_PROMOTED_VAR_P' nonzero if
     the object being referenced is kept zero-extended and zero if it
     is kept sign-extended.  Stored in the `unchanging' field and
     printed as `/u'.

`SUBREG_PROMOTED_VAR_P (X)'
     Nonzero in a `subreg' if it was made when accessing an object that
     was promoted to a wider mode in accord with the `PROMOTED_MODE'
     machine description macro (*note Storage Layout::).  In this case,
     the mode of the `subreg' is the declared mode of the object and
     the mode of `SUBREG_REG' is the mode of the register that holds
     the object.  Promoted variables are always either sign- or
     zero-extended to the wider mode on every assignment.  Stored in
     the `in_struct' field and printed as `/s'.

`SYMBOL_REF_FLAG (X)'
     In a `symbol_ref', this is used as a flag for machine-specific
     purposes.  Stored in the `volatil' field and printed as `/v'.

`SYMBOL_REF_USED (X)'
     In a `symbol_ref', indicates that X has been used.  This is
     normally only used to ensure that X is only declared external
     once.  Stored in the `used' field.

`SYMBOL_REF_WEAK (X)'
     In a `symbol_ref', indicates that X has been declared weak.
     Stored in the `integrated' field and printed as `/i'.

   These are the fields to which the above macros refer:

`call'
     In the `LOG_LINKS' of an `insn_list' during scheduling, 1 means
     that the cost of executing an instruction through the link is zero.

     In an RTL dump, this flag is represented as `/c'.

`frame_related'
     In an `insn' or `set' expression, 1 means that it is part of a
     function prologue and sets the stack pointer, sets the frame
     pointer, saves a register, or sets up a temporary register to use
     in place of the frame pointer.

     In `reg' expressions, 1 means that the register holds a pointer.

     In `symbol_ref' expressions, 1 means that the reference addresses
     this function's string constant pool.

     In `mem' expressions, 1 means that the reference is to a scalar.

     In an RTL dump, this flag is represented as `/f'.

`in_struct'
     In `mem' expressions, it is 1 if the memory datum referred to is
     all or part of a structure or array; 0 if it is (or might be) a
     scalar variable.  A reference through a C pointer has 0 because
     the pointer might point to a scalar variable.  This information
     allows the compiler to determine something about possible cases of
     aliasing.

     In `reg' expressions, it is 1 if the register has its entire life
     contained within the test expression of some loop.

     In `subreg' expressions, 1 means that the `subreg' is accessing an
     object that has had its mode promoted from a wider mode.

     In `label_ref' expressions, 1 means that the referenced label is
     outside the innermost loop containing the insn in which the
     `label_ref' was found.

     In `code_label' expressions, it is 1 if the label may never be
     deleted.  This is used for labels which are the target of
     non-local gotos.  Such a label that would have been deleted is
     replaced with a `note' of type `NOTE_INSN_DELETED_LABEL'.

     In an `insn' during dead-code elimination, 1 means that the insn is
     dead code.

     In an `insn' during reorg for an insn in the delay slot of a
     branch, 1 means that this insn is from the target of the branch.

     In an `insn' during instruction scheduling, 1 means that this insn
     must be scheduled as part of a group together with the previous
     insn.

     In an RTL dump, this flag is represented as `/s'.

`integrated'
     In an `insn', `insn_list', or `const', 1 means the RTL was
     produced by procedure integration.

     In `reg' expressions, 1 means the register contains the value to
     be returned by the current function.  On machines that pass
     parameters in registers, the same register number may be used for
     parameters as well, but this flag is not set on such uses.

     In `symbol_ref' expressions, 1 means the referenced symbol is weak.

     In an RTL dump, this flag is represented as `/i'.

`jump'
     In a `mem' expression, 1 means we should keep the alias set for
     this mem unchanged when we access a component.

     In a `set', 1 means it is for a return.

     In a `call_insn', 1 means it is a sibling call.

     In the `LOG_LINKS' of an `insn_list' during scheduling, 1 means the
     cost of executing an instruction through the link varies and is
     unchanging.

     In an RTL dump, this flag is represented as `/j'.

`unchanging'
     In `reg' and `mem' expressions, 1 means that the value of the
     expression never changes.

     In `subreg' expressions, it is 1 if the `subreg' references an
     unsigned object whose mode has been promoted to a wider mode.

     In an `insn', 1 means that this is an annulling branch.

     In a `symbol_ref' expression, 1 means that this symbol addresses
     something in the per-function constant pool.

     In a `call_insn', `note', or an `expr_list' of notes, 1 means that
     this instruction is a call to a const or pure function.

     In an RTL dump, this flag is represented as `/u'.

`used'
     This flag is used directly (without an access macro) at the end of
     RTL generation for a function, to count the number of times an
     expression appears in insns.  Expressions that appear more than
     once are copied, according to the rules for shared structure
     (*note Sharing::).

     For a `reg', it is used directly (without an access macro) by the
     leaf register renumbering code to ensure that each register is only
     renumbered once.

     In a `symbol_ref', it indicates that an external declaration for
     the symbol has already been written.

`volatil'
     In a `mem' or `asm_operands' expression, it is 1 if the memory
     reference is volatile.  Volatile memory references may not be
     deleted, reordered or combined.

     In a `symbol_ref' expression, it is used for machine-specific
     purposes.

     In a `reg' expression, it is 1 if the value is a user-level
     variable.  0 indicates an internal compiler temporary.

     In an `insn', 1 means the insn has been deleted.

     In `label_ref' and `reg_label' expressions, 1 means a reference to
     a non-local label.

     In an RTL dump, this flag is represented as `/v'.


File: gccint.info,  Node: Machine Modes,  Next: Constants,  Prev: Flags,  Up: RTL

Machine Modes
=============

   A machine mode describes a size of data object and the
representation used for it.  In the C code, machine modes are
represented by an enumeration type, `enum machine_mode', defined in
`machmode.def'.  Each RTL expression has room for a machine mode and so
do certain kinds of tree expressions (declarations and types, to be
precise).

   In debugging dumps and machine descriptions, the machine mode of an
RTL expression is written after the expression code with a colon to
separate them.  The letters `mode' which appear at the end of each
machine mode name are omitted.  For example, `(reg:SI 38)' is a `reg'
expression with machine mode `SImode'.  If the mode is `VOIDmode', it
is not written at all.

   Here is a table of machine modes.  The term "byte" below refers to an
object of `BITS_PER_UNIT' bits (*note Storage Layout::).

`BImode'
     "Bit" mode represents a single bit, for predicate registers.

`QImode'
     "Quarter-Integer" mode represents a single byte treated as an
     integer.

`HImode'
     "Half-Integer" mode represents a two-byte integer.

`PSImode'
     "Partial Single Integer" mode represents an integer which occupies
     four bytes but which doesn't really use all four.  On some
     machines, this is the right mode to use for pointers.

`SImode'
     "Single Integer" mode represents a four-byte integer.

`PDImode'
     "Partial Double Integer" mode represents an integer which occupies
     eight bytes but which doesn't really use all eight.  On some
     machines, this is the right mode to use for certain pointers.

`DImode'
     "Double Integer" mode represents an eight-byte integer.

`TImode'
     "Tetra Integer" (?) mode represents a sixteen-byte integer.

`OImode'
     "Octa Integer" (?) mode represents a thirty-two-byte integer.

`QFmode'
     "Quarter-Floating" mode represents a quarter-precision (single
     byte) floating point number.

`HFmode'
     "Half-Floating" mode represents a half-precision (two byte)
     floating point number.

`TQFmode'
     "Three-Quarter-Floating" (?) mode represents a
     three-quarter-precision (three byte) floating point number.

`SFmode'
     "Single Floating" mode represents a four byte floating point
     number.  In the common case, of a processor with IEEE arithmetic
     and 8-bit bytes, this is a single-precision IEEE floating point
     number; it can also be used for double-precision (on processors
     with 16-bit bytes) and single-precision VAX and IBM types.

`DFmode'
     "Double Floating" mode represents an eight byte floating point
     number.  In the common case, of a processor with IEEE arithmetic
     and 8-bit bytes, this is a double-precision IEEE floating point
     number.

`XFmode'
     "Extended Floating" mode represents a twelve byte floating point
     number.  This mode is used for IEEE extended floating point.  On
     some systems not all bits within these bytes will actually be used.

`TFmode'
     "Tetra Floating" mode represents a sixteen byte floating point
     number.  This gets used for both the 96-bit extended IEEE
     floating-point types padded to 128 bits, and true 128-bit extended
     IEEE floating-point types.

`CCmode'
     "Condition Code" mode represents the value of a condition code,
     which is a machine-specific set of bits used to represent the
     result of a comparison operation.  Other machine-specific modes
     may also be used for the condition code.  These modes are not used
     on machines that use `cc0' (see *note Condition Code::).

`BLKmode'
     "Block" mode represents values that are aggregates to which none of
     the other modes apply.  In RTL, only memory references can have
     this mode, and only if they appear in string-move or vector
     instructions.  On machines which have no such instructions,
     `BLKmode' will not appear in RTL.

`VOIDmode'
     Void mode means the absence of a mode or an unspecified mode.  For
     example, RTL expressions of code `const_int' have mode `VOIDmode'
     because they can be taken to have whatever mode the context
     requires.  In debugging dumps of RTL, `VOIDmode' is expressed by
     the absence of any mode.

`QCmode, HCmode, SCmode, DCmode, XCmode, TCmode'
     These modes stand for a complex number represented as a pair of
     floating point values.  The floating point values are in `QFmode',
     `HFmode', `SFmode', `DFmode', `XFmode', and `TFmode', respectively.

`CQImode, CHImode, CSImode, CDImode, CTImode, COImode'
     These modes stand for a complex number represented as a pair of
     integer values.  The integer values are in `QImode', `HImode',
     `SImode', `DImode', `TImode', and `OImode', respectively.

   The machine description defines `Pmode' as a C macro which expands
into the machine mode used for addresses.  Normally this is the mode
whose size is `BITS_PER_WORD', `SImode' on 32-bit machines.

   The only modes which a machine description must support are
`QImode', and the modes corresponding to `BITS_PER_WORD',
`FLOAT_TYPE_SIZE' and `DOUBLE_TYPE_SIZE'.  The compiler will attempt to
use `DImode' for 8-byte structures and unions, but this can be
prevented by overriding the definition of `MAX_FIXED_MODE_SIZE'.
Alternatively, you can have the compiler use `TImode' for 16-byte
structures and unions.  Likewise, you can arrange for the C type `short
int' to avoid using `HImode'.

   Very few explicit references to machine modes remain in the compiler
and these few references will soon be removed.  Instead, the machine
modes are divided into mode classes.  These are represented by the
enumeration type `enum mode_class' defined in `machmode.h'.  The
possible mode classes are:

`MODE_INT'
     Integer modes.  By default these are `BImode', `QImode', `HImode',
     `SImode', `DImode', `TImode', and `OImode'.

`MODE_PARTIAL_INT'
     The "partial integer" modes, `PQImode', `PHImode', `PSImode' and
     `PDImode'.

`MODE_FLOAT'
     Floating point modes.  By default these are `QFmode', `HFmode',
     `TQFmode', `SFmode', `DFmode', `XFmode' and `TFmode'.

`MODE_COMPLEX_INT'
     Complex integer modes.  (These are not currently implemented).

`MODE_COMPLEX_FLOAT'
     Complex floating point modes.  By default these are `QCmode',
     `HCmode', `SCmode', `DCmode', `XCmode', and `TCmode'.

`MODE_FUNCTION'
     Algol or Pascal function variables including a static chain.
     (These are not currently implemented).

`MODE_CC'
     Modes representing condition code values.  These are `CCmode' plus
     any modes listed in the `EXTRA_CC_MODES' macro.  *Note Jump
     Patterns::, also see *Note Condition Code::.

`MODE_RANDOM'
     This is a catchall mode class for modes which don't fit into the
     above classes.  Currently `VOIDmode' and `BLKmode' are in
     `MODE_RANDOM'.

   Here are some C macros that relate to machine modes:

`GET_MODE (X)'
     Returns the machine mode of the RTX X.

`PUT_MODE (X, NEWMODE)'
     Alters the machine mode of the RTX X to be NEWMODE.

`NUM_MACHINE_MODES'
     Stands for the number of machine modes available on the target
     machine.  This is one greater than the largest numeric value of any
     machine mode.

`GET_MODE_NAME (M)'
     Returns the name of mode M as a string.

`GET_MODE_CLASS (M)'
     Returns the mode class of mode M.

`GET_MODE_WIDER_MODE (M)'
     Returns the next wider natural mode.  For example, the expression
     `GET_MODE_WIDER_MODE (QImode)' returns `HImode'.

`GET_MODE_SIZE (M)'
     Returns the size in bytes of a datum of mode M.

`GET_MODE_BITSIZE (M)'
     Returns the size in bits of a datum of mode M.

`GET_MODE_MASK (M)'
     Returns a bitmask containing 1 for all bits in a word that fit
     within mode M.  This macro can only be used for modes whose
     bitsize is less than or equal to `HOST_BITS_PER_INT'.

`GET_MODE_ALIGNMENT (M)'
     Return the required alignment, in bits, for an object of mode M.

`GET_MODE_UNIT_SIZE (M)'
     Returns the size in bytes of the subunits of a datum of mode M.
     This is the same as `GET_MODE_SIZE' except in the case of complex
     modes.  For them, the unit size is the size of the real or
     imaginary part.

`GET_MODE_NUNITS (M)'
     Returns the number of units contained in a mode, i.e.,
     `GET_MODE_SIZE' divided by `GET_MODE_UNIT_SIZE'.

`GET_CLASS_NARROWEST_MODE (C)'
     Returns the narrowest mode in mode class C.

   The global variables `byte_mode' and `word_mode' contain modes whose
classes are `MODE_INT' and whose bitsizes are either `BITS_PER_UNIT' or
`BITS_PER_WORD', respectively.  On 32-bit machines, these are `QImode'
and `SImode', respectively.


File: gccint.info,  Node: Constants,  Next: Regs and Memory,  Prev: Machine Modes,  Up: RTL

Constant Expression Types
=========================

   The simplest RTL expressions are those that represent constant
values.

`(const_int I)'
     This type of expression represents the integer value I.  I is
     customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)',
     which is equivalent to `XWINT (EXP, 0)'.

     There is only one expression object for the integer value zero; it
     is the value of the variable `const0_rtx'.  Likewise, the only
     expression for integer value one is found in `const1_rtx', the only
     expression for integer value two is found in `const2_rtx', and the
     only expression for integer value negative one is found in
     `constm1_rtx'.  Any attempt to create an expression of code
     `const_int' and value zero, one, two or negative one will return
     `const0_rtx', `const1_rtx', `const2_rtx' or `constm1_rtx' as
     appropriate.

     Similarly, there is only one object for the integer whose value is
     `STORE_FLAG_VALUE'.  It is found in `const_true_rtx'.  If
     `STORE_FLAG_VALUE' is one, `const_true_rtx' and `const1_rtx' will
     point to the same object.  If `STORE_FLAG_VALUE' is -1,
     `const_true_rtx' and `constm1_rtx' will point to the same object.

`(const_double:M ADDR I0 I1 ...)'
     Represents either a floating-point constant of mode M or an
     integer constant too large to fit into `HOST_BITS_PER_WIDE_INT'
     bits but small enough to fit within twice that number of bits (GCC
     does not provide a mechanism to represent even larger constants).
     In the latter case, M will be `VOIDmode'.

`(const_vector:M [X0 X1 ...])'
     Represents a vector constant.  The square brackets stand for the
     vector containing the constant elements.  X0, X1 and so on are the
     `const_int' or `const_double' elements.

     The number of units in a `const_vector' is obtained with the macro
     `CONST_VECTOR_NUNITS' as in `CONST_VECTOR_NUNITS (V)'.

     Individual elements in a vector constant are accessed with the
     macro `CONST_VECTOR_ELT' as in `CONST_VECTOR_ELT (V, N)' where V
     is the vector constant and N is the element desired.

     ADDR is used to contain the `mem' expression that corresponds to
     the location in memory that at which the constant can be found.  If
     it has not been allocated a memory location, but is on the chain
     of all `const_double' expressions in this compilation (maintained
     using an undisplayed field), ADDR contains `const0_rtx'.  If it is
     not on the chain, ADDR contains `cc0_rtx'.  ADDR is customarily
     accessed with the macro `CONST_DOUBLE_MEM' and the chain field via
     `CONST_DOUBLE_CHAIN'.

     If M is `VOIDmode', the bits of the value are stored in I0 and I1.
     I0 is customarily accessed with the macro `CONST_DOUBLE_LOW' and
     I1 with `CONST_DOUBLE_HIGH'.

     If the constant is floating point (regardless of its precision),
     then the number of integers used to store the value depends on the
     size of `REAL_VALUE_TYPE' (*note Cross-compilation::).  The
     integers represent a floating point number, but not precisely in
     the target machine's or host machine's floating point format.  To
     convert them to the precise bit pattern used by the target
     machine, use the macro `REAL_VALUE_TO_TARGET_DOUBLE' and friends
     (*note Data Output::).

     The macro `CONST0_RTX (MODE)' refers to an expression with value 0
     in mode MODE.  If mode MODE is of mode class `MODE_INT', it
     returns `const0_rtx'.  If mode MODE is of mode class `MODE_FLOAT',
     it returns a `CONST_DOUBLE' expression in mode MODE.  Otherwise,
     it returns a `CONST_VECTOR' expression in mode MODE.  Similarly,
     the macro `CONST1_RTX (MODE)' refers to an expression with value 1
     in mode MODE and similarly for `CONST2_RTX'.  The `CONST1_RTX' and
     `CONST2_RTX' macros are undefined for vector modes.

`(const_string STR)'
     Represents a constant string with value STR.  Currently this is
     used only for insn attributes (*note Insn Attributes::) since
     constant strings in C are placed in memory.

`(symbol_ref:MODE SYMBOL)'
     Represents the value of an assembler label for data.  SYMBOL is a
     string that describes the name of the assembler label.  If it
     starts with a `*', the label is the rest of SYMBOL not including
     the `*'.  Otherwise, the label is SYMBOL, usually prefixed with
     `_'.

     The `symbol_ref' contains a mode, which is usually `Pmode'.
     Usually that is the only mode for which a symbol is directly valid.

`(label_ref LABEL)'
     Represents the value of an assembler label for code.  It contains
     one operand, an expression, which must be a `code_label' or a
     `note' of type `NOTE_INSN_DELETED_LABEL' that appears in the
     instruction sequence to identify the place where the label should
     go.

     The reason for using a distinct expression type for code label
     references is so that jump optimization can distinguish them.

`(const:M EXP)'
     Represents a constant that is the result of an assembly-time
     arithmetic computation.  The operand, EXP, is an expression that
     contains only constants (`const_int', `symbol_ref' and `label_ref'
     expressions) combined with `plus' and `minus'.  However, not all
     combinations are valid, since the assembler cannot do arbitrary
     arithmetic on relocatable symbols.

     M should be `Pmode'.

`(high:M EXP)'
     Represents the high-order bits of EXP, usually a `symbol_ref'.
     The number of bits is machine-dependent and is normally the number
     of bits specified in an instruction that initializes the high
     order bits of a register.  It is used with `lo_sum' to represent
     the typical two-instruction sequence used in RISC machines to
     reference a global memory location.

     M should be `Pmode'.


File: gccint.info,  Node: Regs and Memory,  Next: Arithmetic,  Prev: Constants,  Up: RTL

Registers and Memory
====================

   Here are the RTL expression types for describing access to machine
registers and to main memory.

`(reg:M N)'
     For small values of the integer N (those that are less than
     `FIRST_PSEUDO_REGISTER'), this stands for a reference to machine
     register number N: a "hard register".  For larger values of N, it
     stands for a temporary value or "pseudo register".  The compiler's
     strategy is to generate code assuming an unlimited number of such
     pseudo registers, and later convert them into hard registers or
     into memory references.

     M is the machine mode of the reference.  It is necessary because
     machines can generally refer to each register in more than one
     mode.  For example, a register may contain a full word but there
     may be instructions to refer to it as a half word or as a single
     byte, as well as instructions to refer to it as a floating point
     number of various precisions.

     Even for a register that the machine can access in only one mode,
     the mode must always be specified.

     The symbol `FIRST_PSEUDO_REGISTER' is defined by the machine
     description, since the number of hard registers on the machine is
     an invariant characteristic of the machine.  Note, however, that
     not all of the machine registers must be general registers.  All
     the machine registers that can be used for storage of data are
     given hard register numbers, even those that can be used only in
     certain instructions or can hold only certain types of data.

     A hard register may be accessed in various modes throughout one
     function, but each pseudo register is given a natural mode and is
     accessed only in that mode.  When it is necessary to describe an
     access to a pseudo register using a nonnatural mode, a `subreg'
     expression is used.

     A `reg' expression with a machine mode that specifies more than
     one word of data may actually stand for several consecutive
     registers.  If in addition the register number specifies a
     hardware register, then it actually represents several consecutive
     hardware registers starting with the specified one.

     Each pseudo register number used in a function's RTL code is
     represented by a unique `reg' expression.

     Some pseudo register numbers, those within the range of
     `FIRST_VIRTUAL_REGISTER' to `LAST_VIRTUAL_REGISTER' only appear
     during the RTL generation phase and are eliminated before the
     optimization phases.  These represent locations in the stack frame
     that cannot be determined until RTL generation for the function
     has been completed.  The following virtual register numbers are
     defined:

    `VIRTUAL_INCOMING_ARGS_REGNUM'
          This points to the first word of the incoming arguments
          passed on the stack.  Normally these arguments are placed
          there by the caller, but the callee may have pushed some
          arguments that were previously passed in registers.

          When RTL generation is complete, this virtual register is
          replaced by the sum of the register given by
          `ARG_POINTER_REGNUM' and the value of `FIRST_PARM_OFFSET'.

    `VIRTUAL_STACK_VARS_REGNUM'
          If `FRAME_GROWS_DOWNWARD' is defined, this points to
          immediately above the first variable on the stack.
          Otherwise, it points to the first variable on the stack.

          `VIRTUAL_STACK_VARS_REGNUM' is replaced with the sum of the
          register given by `FRAME_POINTER_REGNUM' and the value
          `STARTING_FRAME_OFFSET'.

    `VIRTUAL_STACK_DYNAMIC_REGNUM'
          This points to the location of dynamically allocated memory
          on the stack immediately after the stack pointer has been
          adjusted by the amount of memory desired.

          This virtual register is replaced by the sum of the register
          given by `STACK_POINTER_REGNUM' and the value
          `STACK_DYNAMIC_OFFSET'.

    `VIRTUAL_OUTGOING_ARGS_REGNUM'
          This points to the location in the stack at which outgoing
          arguments should be written when the stack is pre-pushed
          (arguments pushed using push insns should always use
          `STACK_POINTER_REGNUM').

          This virtual register is replaced by the sum of the register
          given by `STACK_POINTER_REGNUM' and the value
          `STACK_POINTER_OFFSET'.

`(subreg:M REG BYTENUM)'
     `subreg' expressions are used to refer to a register in a machine
     mode other than its natural one, or to refer to one register of a
     multi-part `reg' that actually refers to several registers.

     Each pseudo-register has a natural mode.  If it is necessary to
     operate on it in a different mode--for example, to perform a
     fullword move instruction on a pseudo-register that contains a
     single byte--the pseudo-register must be enclosed in a `subreg'.
     In such a case, BYTENUM is zero.

     Usually M is at least as narrow as the mode of REG, in which case
     it is restricting consideration to only the bits of REG that are
     in M.

     Sometimes M is wider than the mode of REG.  These `subreg'
     expressions are often called "paradoxical".  They are used in
     cases where we want to refer to an object in a wider mode but do
     not care what value the additional bits have.  The reload pass
     ensures that paradoxical references are only made to hard
     registers.

     The other use of `subreg' is to extract the individual registers of
     a multi-register value.  Machine modes such as `DImode' and
     `TImode' can indicate values longer than a word, values which
     usually require two or more consecutive registers.  To access one
     of the registers, use a `subreg' with mode `SImode' and a BYTENUM
     offset that says which register.

     Storing in a non-paradoxical `subreg' has undefined results for
     bits belonging to the same word as the `subreg'.  This laxity makes
     it easier to generate efficient code for such instructions.  To
     represent an instruction that preserves all the bits outside of
     those in the `subreg', use `strict_low_part' around the `subreg'.

     The compilation parameter `WORDS_BIG_ENDIAN', if set to 1, says
     that byte number zero is part of the most significant word;
     otherwise, it is part of the least significant word.

     The compilation parameter `BYTES_BIG_ENDIAN', if set to 1, says
     that byte number zero is the most significant byte within a word;
     otherwise, it is the least significant byte within a word.

     On a few targets, `FLOAT_WORDS_BIG_ENDIAN' disagrees with
     `WORDS_BIG_ENDIAN'.  However, most parts of the compiler treat
     floating point values as if they had the same endianness as
     integer values.  This works because they handle them solely as a
     collection of integer values, with no particular numerical value.
     Only real.c and the runtime libraries care about
     `FLOAT_WORDS_BIG_ENDIAN'.

     Between the combiner pass and the reload pass, it is possible to
     have a paradoxical `subreg' which contains a `mem' instead of a
     `reg' as its first operand.  After the reload pass, it is also
     possible to have a non-paradoxical `subreg' which contains a
     `mem'; this usually occurs when the `mem' is a stack slot which
     replaced a pseudo register.

     Note that it is not valid to access a `DFmode' value in `SFmode'
     using a `subreg'.  On some machines the most significant part of a
     `DFmode' value does not have the same format as a single-precision
     floating value.

     It is also not valid to access a single word of a multi-word value
     in a hard register when less registers can hold the value than
     would be expected from its size.  For example, some 32-bit
     machines have floating-point registers that can hold an entire
     `DFmode' value.  If register 10 were such a register `(subreg:SI
     (reg:DF 10) 1)' would be invalid because there is no way to
     convert that reference to a single machine register.  The reload
     pass prevents `subreg' expressions such as these from being formed.

     The first operand of a `subreg' expression is customarily accessed
     with the `SUBREG_REG' macro and the second operand is customarily
     accessed with the `SUBREG_BYTE' macro.

`(scratch:M)'
     This represents a scratch register that will be required for the
     execution of a single instruction and not used subsequently.  It is
     converted into a `reg' by either the local register allocator or
     the reload pass.

     `scratch' is usually present inside a `clobber' operation (*note
     Side Effects::).

`(cc0)'
     This refers to the machine's condition code register.  It has no
     operands and may not have a machine mode.  There are two ways to
     use it:

        * To stand for a complete set of condition code flags.  This is
          best on most machines, where each comparison sets the entire
          series of flags.

          With this technique, `(cc0)' may be validly used in only two
          contexts: as the destination of an assignment (in test and
          compare instructions) and in comparison operators comparing
          against zero (`const_int' with value zero; that is to say,
          `const0_rtx').

        * To stand for a single flag that is the result of a single
          condition.  This is useful on machines that have only a
          single flag bit, and in which comparison instructions must
          specify the condition to test.

          With this technique, `(cc0)' may be validly used in only two
          contexts: as the destination of an assignment (in test and
          compare instructions) where the source is a comparison
          operator, and as the first operand of `if_then_else' (in a
          conditional branch).

     There is only one expression object of code `cc0'; it is the value
     of the variable `cc0_rtx'.  Any attempt to create an expression of
     code `cc0' will return `cc0_rtx'.

     Instructions can set the condition code implicitly.  On many
     machines, nearly all instructions set the condition code based on
     the value that they compute or store.  It is not necessary to
     record these actions explicitly in the RTL because the machine
     description includes a prescription for recognizing the
     instructions that do so (by means of the macro
     `NOTICE_UPDATE_CC').  *Note Condition Code::.  Only instructions
     whose sole purpose is to set the condition code, and instructions
     that use the condition code, need mention `(cc0)'.

     On some machines, the condition code register is given a register
     number and a `reg' is used instead of `(cc0)'.  This is usually the
     preferable approach if only a small subset of instructions modify
     the condition code.  Other machines store condition codes in
     general registers; in such cases a pseudo register should be used.

     Some machines, such as the Sparc and RS/6000, have two sets of
     arithmetic instructions, one that sets and one that does not set
     the condition code.  This is best handled by normally generating
     the instruction that does not set the condition code, and making a
     pattern that both performs the arithmetic and sets the condition
     code register (which would not be `(cc0)' in this case).  For
     examples, search for `addcc' and `andcc' in `sparc.md'.

`(pc)'
     This represents the machine's program counter.  It has no operands
     and may not have a machine mode.  `(pc)' may be validly used only
     in certain specific contexts in jump instructions.

     There is only one expression object of code `pc'; it is the value
     of the variable `pc_rtx'.  Any attempt to create an expression of
     code `pc' will return `pc_rtx'.

     All instructions that do not jump alter the program counter
     implicitly by incrementing it, but there is no need to mention
     this in the RTL.

`(mem:M ADDR ALIAS)'
     This RTX represents a reference to main memory at an address
     represented by the expression ADDR.  M specifies how large a unit
     of memory is accessed.  ALIAS specifies an alias set for the
     reference.  In general two items are in different alias sets if
     they cannot reference the same memory address.

`(addressof:M REG)'
     This RTX represents a request for the address of register REG.
     Its mode is always `Pmode'.  If there are any `addressof'
     expressions left in the function after CSE, REG is forced into the
     stack and the `addressof' expression is replaced with a `plus'
     expression for the address of its stack slot.

