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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: Arithmetic,  Next: Comparisons,  Prev: Regs and Memory,  Up: RTL

RTL Expressions for Arithmetic
==============================

   Unless otherwise specified, all the operands of arithmetic
expressions must be valid for mode M.  An operand is valid for mode M
if it has mode M, or if it is a `const_int' or `const_double' and M is
a mode of class `MODE_INT'.

   For commutative binary operations, constants should be placed in the
second operand.

`(plus:M X Y)'
     Represents the sum of the values represented by X and Y carried
     out in machine mode M.

`(lo_sum:M X Y)'
     Like `plus', except that it represents that sum of X and the
     low-order bits of Y.  The number of low order bits is
     machine-dependent but is normally the number of bits in a `Pmode'
     item minus the number of bits set by the `high' code (*note
     Constants::).

     M should be `Pmode'.

`(minus:M X Y)'
     Like `plus' but represents subtraction.

`(ss_plus:M X Y)'
     Like `plus', but using signed saturation in case of an overflow.

`(us_plus:M X Y)'
     Like `plus', but using unsigned saturation in case of an overflow.

`(ss_minus:M X Y)'
     Like `minus', but using signed saturation in case of an overflow.

`(us_minus:M X Y)'
     Like `minus', but using unsigned saturation in case of an overflow.

`(compare:M X Y)'
     Represents the result of subtracting Y from X for purposes of
     comparison.  The result is computed without overflow, as if with
     infinite precision.

     Of course, machines can't really subtract with infinite precision.
     However, they can pretend to do so when only the sign of the
     result will be used, which is the case when the result is stored
     in the condition code.  And that is the _only_ way this kind of
     expression may validly be used: as a value to be stored in the
     condition codes, either `(cc0)' or a register.  *Note
     Comparisons::.

     The mode M is not related to the modes of X and Y, but instead is
     the mode of the condition code value.  If `(cc0)' is used, it is
     `VOIDmode'.  Otherwise it is some mode in class `MODE_CC', often
     `CCmode'.  *Note Condition Code::.  If M is `VOIDmode' or
     `CCmode', the operation returns sufficient information (in an
     unspecified format) so that any comparison operator can be applied
     to the result of the `COMPARE' operation.  For other modes in
     class `MODE_CC', the operation only returns a subset of this
     information.

     Normally, X and Y must have the same mode.  Otherwise, `compare'
     is valid only if the mode of X is in class `MODE_INT' and Y is a
     `const_int' or `const_double' with mode `VOIDmode'.  The mode of X
     determines what mode the comparison is to be done in; thus it must
     not be `VOIDmode'.

     If one of the operands is a constant, it should be placed in the
     second operand and the comparison code adjusted as appropriate.

     A `compare' specifying two `VOIDmode' constants is not valid since
     there is no way to know in what mode the comparison is to be
     performed; the comparison must either be folded during the
     compilation or the first operand must be loaded into a register
     while its mode is still known.

`(neg:M X)'
     Represents the negation (subtraction from zero) of the value
     represented by X, carried out in mode M.

`(mult:M X Y)'
     Represents the signed product of the values represented by X and Y
     carried out in machine mode M.

     Some machines support a multiplication that generates a product
     wider than the operands.  Write the pattern for this as

          (mult:M (sign_extend:M X) (sign_extend:M Y))

     where M is wider than the modes of X and Y, which need not be the
     same.

     For unsigned widening multiplication, use the same idiom, but with
     `zero_extend' instead of `sign_extend'.

`(div:M X Y)'
     Represents the quotient in signed division of X by Y, carried out
     in machine mode M.  If M is a floating point mode, it represents
     the exact quotient; otherwise, the integerized quotient.

     Some machines have division instructions in which the operands and
     quotient widths are not all the same; you should represent such
     instructions using `truncate' and `sign_extend' as in,

          (truncate:M1 (div:M2 X (sign_extend:M2 Y)))

`(udiv:M X Y)'
     Like `div' but represents unsigned division.

`(mod:M X Y)'
`(umod:M X Y)'
     Like `div' and `udiv' but represent the remainder instead of the
     quotient.

`(smin:M X Y)'
`(smax:M X Y)'
     Represents the smaller (for `smin') or larger (for `smax') of X
     and Y, interpreted as signed integers in mode M.

`(umin:M X Y)'
`(umax:M X Y)'
     Like `smin' and `smax', but the values are interpreted as unsigned
     integers.

`(not:M X)'
     Represents the bitwise complement of the value represented by X,
     carried out in mode M, which must be a fixed-point machine mode.

`(and:M X Y)'
     Represents the bitwise logical-and of the values represented by X
     and Y, carried out in machine mode M, which must be a fixed-point
     machine mode.

`(ior:M X Y)'
     Represents the bitwise inclusive-or of the values represented by X
     and Y, carried out in machine mode M, which must be a fixed-point
     mode.

`(xor:M X Y)'
     Represents the bitwise exclusive-or of the values represented by X
     and Y, carried out in machine mode M, which must be a fixed-point
     mode.

`(ashift:M X C)'
     Represents the result of arithmetically shifting X left by C
     places.  X have mode M, a fixed-point machine mode.  C be a
     fixed-point mode or be a constant with mode `VOIDmode'; which mode
     is determined by the mode called for in the machine description
     entry for the left-shift instruction.  For example, on the VAX,
     the mode of C is `QImode' regardless of M.

`(lshiftrt:M X C)'
`(ashiftrt:M X C)'
     Like `ashift' but for right shift.  Unlike the case for left shift,
     these two operations are distinct.

`(rotate:M X C)'
`(rotatert:M X C)'
     Similar but represent left and right rotate.  If C is a constant,
     use `rotate'.

`(abs:M X)'
     Represents the absolute value of X, computed in mode M.

`(sqrt:M X)'
     Represents the square root of X, computed in mode M.  Most often M
     will be a floating point mode.

`(ffs:M X)'
     Represents one plus the index of the least significant 1-bit in X,
     represented as an integer of mode M.  (The value is zero if X is
     zero.)  The mode of X need not be M; depending on the target
     machine, various mode combinations may be valid.


File: gccint.info,  Node: Comparisons,  Next: Bit-Fields,  Prev: Arithmetic,  Up: RTL

Comparison Operations
=====================

   Comparison operators test a relation on two operands and are
considered to represent a machine-dependent nonzero value described by,
but not necessarily equal to, `STORE_FLAG_VALUE' (*note Misc::) if the
relation holds, or zero if it does not.  The mode of the comparison
operation is independent of the mode of the data being compared.  If
the comparison operation is being tested (e.g., the first operand of an
`if_then_else'), the mode must be `VOIDmode'.  If the comparison
operation is producing data to be stored in some variable, the mode
must be in class `MODE_INT'.  All comparison operations producing data
must use the same mode, which is machine-specific.

   There are two ways that comparison operations may be used.  The
comparison operators may be used to compare the condition codes `(cc0)'
against zero, as in `(eq (cc0) (const_int 0))'.  Such a construct
actually refers to the result of the preceding instruction in which the
condition codes were set.  The instruction setting the condition code
must be adjacent to the instruction using the condition code; only
`note' insns may separate them.

   Alternatively, a comparison operation may directly compare two data
objects.  The mode of the comparison is determined by the operands; they
must both be valid for a common machine mode.  A comparison with both
operands constant would be invalid as the machine mode could not be
deduced from it, but such a comparison should never exist in RTL due to
constant folding.

   In the example above, if `(cc0)' were last set to `(compare X Y)',
the comparison operation is identical to `(eq X Y)'.  Usually only one
style of comparisons is supported on a particular machine, but the
combine pass will try to merge the operations to produce the `eq' shown
in case it exists in the context of the particular insn involved.

   Inequality comparisons come in two flavors, signed and unsigned.
Thus, there are distinct expression codes `gt' and `gtu' for signed and
unsigned greater-than.  These can produce different results for the same
pair of integer values: for example, 1 is signed greater-than -1 but not
unsigned greater-than, because -1 when regarded as unsigned is actually
`0xffffffff' which is greater than 1.

   The signed comparisons are also used for floating point values.
Floating point comparisons are distinguished by the machine modes of
the operands.

`(eq:M X Y)'
     `STORE_FLAG_VALUE' if the values represented by X and Y are equal,
     otherwise 0.

`(ne:M X Y)'
     `STORE_FLAG_VALUE' if the values represented by X and Y are not
     equal, otherwise 0.

`(gt:M X Y)'
     `STORE_FLAG_VALUE' if the X is greater than Y.  If they are
     fixed-point, the comparison is done in a signed sense.

`(gtu:M X Y)'
     Like `gt' but does unsigned comparison, on fixed-point numbers
     only.

`(lt:M X Y)'
`(ltu:M X Y)'
     Like `gt' and `gtu' but test for "less than".

`(ge:M X Y)'
`(geu:M X Y)'
     Like `gt' and `gtu' but test for "greater than or equal".

`(le:M X Y)'
`(leu:M X Y)'
     Like `gt' and `gtu' but test for "less than or equal".

`(if_then_else COND THEN ELSE)'
     This is not a comparison operation but is listed here because it is
     always used in conjunction with a comparison operation.  To be
     precise, COND is a comparison expression.  This expression
     represents a choice, according to COND, between the value
     represented by THEN and the one represented by ELSE.

     On most machines, `if_then_else' expressions are valid only to
     express conditional jumps.

`(cond [TEST1 VALUE1 TEST2 VALUE2 ...] DEFAULT)'
     Similar to `if_then_else', but more general.  Each of TEST1,
     TEST2, ... is performed in turn.  The result of this expression is
     the VALUE corresponding to the first nonzero test, or DEFAULT if
     none of the tests are nonzero expressions.

     This is currently not valid for instruction patterns and is
     supported only for insn attributes.  *Note Insn Attributes::.


File: gccint.info,  Node: Bit-Fields,  Next: Vector Operations,  Prev: Comparisons,  Up: RTL

Bit-Fields
==========

   Special expression codes exist to represent bit-field instructions.
These types of expressions are lvalues in RTL; they may appear on the
left side of an assignment, indicating insertion of a value into the
specified bit-field.

`(sign_extract:M LOC SIZE POS)'
     This represents a reference to a sign-extended bit-field contained
     or starting in LOC (a memory or register reference).  The bit-field
     is SIZE bits wide and starts at bit POS.  The compilation option
     `BITS_BIG_ENDIAN' says which end of the memory unit POS counts
     from.

     If LOC is in memory, its mode must be a single-byte integer mode.
     If LOC is in a register, the mode to use is specified by the
     operand of the `insv' or `extv' pattern (*note Standard Names::)
     and is usually a full-word integer mode, which is the default if
     none is specified.

     The mode of POS is machine-specific and is also specified in the
     `insv' or `extv' pattern.

     The mode M is the same as the mode that would be used for LOC if
     it were a register.

`(zero_extract:M LOC SIZE POS)'
     Like `sign_extract' but refers to an unsigned or zero-extended
     bit-field.  The same sequence of bits are extracted, but they are
     filled to an entire word with zeros instead of by sign-extension.


File: gccint.info,  Node: Vector Operations,  Next: Conversions,  Prev: Bit-Fields,  Up: RTL

Vector Operations
=================

   All normal RTL expressions can be used with vector modes; they are
interpreted as operating on each part of the vector independently.
Additionally, there are a few new expressions to describe specific
vector operations.

`(vec_merge:M VEC1 VEC2 ITEMS)'
     This describes a merge operation between two vectors.  The result
     is a vector of mode M; its elements are selected from either VEC1
     or VEC2.  Which elements are selected is described by ITEMS, which
     is a bit mask represented by a `const_int'; a zero bit indicates
     the corresponding element in the result vector is taken from VEC2
     while a set bit indicates it is taken from VEC1.

`(vec_select:M VEC1 SELECTION)'
     This describes an operation that selects parts of a vector.  VEC1
     is the source vector, SELECTION is a `parallel' that contains a
     `const_int' for each of the subparts of the result vector, giving
     the number of the source subpart that should be stored into it.

`(vec_concat:M VEC1 VEC2)'
     Describes a vector concat operation.  The result is a
     concatenation of the vectors VEC1 and VEC2; its length is the sum
     of the lengths of the two inputs.

`(vec_const:M SUBPARTS)'
     This describes a constant vector.  SUBPARTS is a `parallel' that
     contains a constant for each of the subparts of the vector.

`(vec_duplicate:M VEC)'
     This operation converts a small vector into a larger one by
     duplicating the input values.  The output vector mode must have
     the same submodes as the input vector mode, and the number of
     output parts must be an integer multiple of the number of input
     parts.



File: gccint.info,  Node: Conversions,  Next: RTL Declarations,  Prev: Vector Operations,  Up: RTL

Conversions
===========

   All conversions between machine modes must be represented by
explicit conversion operations.  For example, an expression which is
the sum of a byte and a full word cannot be written as `(plus:SI
(reg:QI 34) (reg:SI 80))' because the `plus' operation requires two
operands of the same machine mode.  Therefore, the byte-sized operand
is enclosed in a conversion operation, as in

     (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))

   The conversion operation is not a mere placeholder, because there
may be more than one way of converting from a given starting mode to
the desired final mode.  The conversion operation code says how to do
it.

   For all conversion operations, X must not be `VOIDmode' because the
mode in which to do the conversion would not be known.  The conversion
must either be done at compile-time or X must be placed into a register.

`(sign_extend:M X)'
     Represents the result of sign-extending the value X to machine
     mode M.  M must be a fixed-point mode and X a fixed-point value of
     a mode narrower than M.

`(zero_extend:M X)'
     Represents the result of zero-extending the value X to machine
     mode M.  M must be a fixed-point mode and X a fixed-point value of
     a mode narrower than M.

`(float_extend:M X)'
     Represents the result of extending the value X to machine mode M.
     M must be a floating point mode and X a floating point value of a
     mode narrower than M.

`(truncate:M X)'
     Represents the result of truncating the value X to machine mode M.
     M must be a fixed-point mode and X a fixed-point value of a mode
     wider than M.

`(ss_truncate:M X)'
     Represents the result of truncating the value X to machine mode M,
     using signed saturation in the case of overflow.  Both M and the
     mode of X must be fixed-point modes.

`(us_truncate:M X)'
     Represents the result of truncating the value X to machine mode M,
     using unsigned saturation in the case of overflow.  Both M and the
     mode of X must be fixed-point modes.

`(float_truncate:M X)'
     Represents the result of truncating the value X to machine mode M.
     M must be a floating point mode and X a floating point value of a
     mode wider than M.

`(float:M X)'
     Represents the result of converting fixed point value X, regarded
     as signed, to floating point mode M.

`(unsigned_float:M X)'
     Represents the result of converting fixed point value X, regarded
     as unsigned, to floating point mode M.

`(fix:M X)'
     When M is a fixed point mode, represents the result of converting
     floating point value X to mode M, regarded as signed.  How
     rounding is done is not specified, so this operation may be used
     validly in compiling C code only for integer-valued operands.

`(unsigned_fix:M X)'
     Represents the result of converting floating point value X to
     fixed point mode M, regarded as unsigned.  How rounding is done is
     not specified.

`(fix:M X)'
     When M is a floating point mode, represents the result of
     converting floating point value X (valid for mode M) to an
     integer, still represented in floating point mode M, by rounding
     towards zero.


File: gccint.info,  Node: RTL Declarations,  Next: Side Effects,  Prev: Conversions,  Up: RTL

Declarations
============

   Declaration expression codes do not represent arithmetic operations
but rather state assertions about their operands.

`(strict_low_part (subreg:M (reg:N R) 0))'
     This expression code is used in only one context: as the
     destination operand of a `set' expression.  In addition, the
     operand of this expression must be a non-paradoxical `subreg'
     expression.

     The presence of `strict_low_part' says that the part of the
     register which is meaningful in mode N, but is not part of mode M,
     is not to be altered.  Normally, an assignment to such a subreg is
     allowed to have undefined effects on the rest of the register when
     M is less than a word.


File: gccint.info,  Node: Side Effects,  Next: Incdec,  Prev: RTL Declarations,  Up: RTL

Side Effect Expressions
=======================

   The expression codes described so far represent values, not actions.
But machine instructions never produce values; they are meaningful only
for their side effects on the state of the machine.  Special expression
codes are used to represent side effects.

   The body of an instruction is always one of these side effect codes;
the codes described above, which represent values, appear only as the
operands of these.

`(set LVAL X)'
     Represents the action of storing the value of X into the place
     represented by LVAL.  LVAL must be an expression representing a
     place that can be stored in: `reg' (or `subreg' or
     `strict_low_part'), `mem', `pc', `parallel', or `cc0'.

     If LVAL is a `reg', `subreg' or `mem', it has a machine mode; then
     X must be valid for that mode.

     If LVAL is a `reg' whose machine mode is less than the full width
     of the register, then it means that the part of the register
     specified by the machine mode is given the specified value and the
     rest of the register receives an undefined value.  Likewise, if
     LVAL is a `subreg' whose machine mode is narrower than the mode of
     the register, the rest of the register can be changed in an
     undefined way.

     If LVAL is a `strict_low_part' of a `subreg', then the part of the
     register specified by the machine mode of the `subreg' is given
     the value X and the rest of the register is not changed.

     If LVAL is `(cc0)', it has no machine mode, and X may be either a
     `compare' expression or a value that may have any mode.  The
     latter case represents a "test" instruction.  The expression `(set
     (cc0) (reg:M N))' is equivalent to `(set (cc0) (compare (reg:M N)
     (const_int 0)))'.  Use the former expression to save space during
     the compilation.

     If LVAL is a `parallel', it is used to represent the case of a
     function returning a structure in multiple registers.  Each element
     of the `parallel' is an `expr_list' whose first operand is a `reg'
     and whose second operand is a `const_int' representing the offset
     (in bytes) into the structure at which the data in that register
     corresponds.  The first element may be null to indicate that the
     structure is also passed partly in memory.

     If LVAL is `(pc)', we have a jump instruction, and the
     possibilities for X are very limited.  It may be a `label_ref'
     expression (unconditional jump).  It may be an `if_then_else'
     (conditional jump), in which case either the second or the third
     operand must be `(pc)' (for the case which does not jump) and the
     other of the two must be a `label_ref' (for the case which does
     jump).  X may also be a `mem' or `(plus:SI (pc) Y)', where Y may
     be a `reg' or a `mem'; these unusual patterns are used to
     represent jumps through branch tables.

     If LVAL is neither `(cc0)' nor `(pc)', the mode of LVAL must not
     be `VOIDmode' and the mode of X must be valid for the mode of LVAL.

     LVAL is customarily accessed with the `SET_DEST' macro and X with
     the `SET_SRC' macro.

`(return)'
     As the sole expression in a pattern, represents a return from the
     current function, on machines where this can be done with one
     instruction, such as VAXen.  On machines where a multi-instruction
     "epilogue" must be executed in order to return from the function,
     returning is done by jumping to a label which precedes the
     epilogue, and the `return' expression code is never used.

     Inside an `if_then_else' expression, represents the value to be
     placed in `pc' to return to the caller.

     Note that an insn pattern of `(return)' is logically equivalent to
     `(set (pc) (return))', but the latter form is never used.

`(call FUNCTION NARGS)'
     Represents a function call.  FUNCTION is a `mem' expression whose
     address is the address of the function to be called.  NARGS is an
     expression which can be used for two purposes: on some machines it
     represents the number of bytes of stack argument; on others, it
     represents the number of argument registers.

     Each machine has a standard machine mode which FUNCTION must have.
     The machine description defines macro `FUNCTION_MODE' to expand
     into the requisite mode name.  The purpose of this mode is to
     specify what kind of addressing is allowed, on machines where the
     allowed kinds of addressing depend on the machine mode being
     addressed.

`(clobber X)'
     Represents the storing or possible storing of an unpredictable,
     undescribed value into X, which must be a `reg', `scratch',
     `parallel' or `mem' expression.

     One place this is used is in string instructions that store
     standard values into particular hard registers.  It may not be
     worth the trouble to describe the values that are stored, but it
     is essential to inform the compiler that the registers will be
     altered, lest it attempt to keep data in them across the string
     instruction.

     If X is `(mem:BLK (const_int 0))', it means that all memory
     locations must be presumed clobbered.  If X is a `parallel', it
     has the same meaning as a `parallel' in a `set' expression.

     Note that the machine description classifies certain hard
     registers as "call-clobbered".  All function call instructions are
     assumed by default to clobber these registers, so there is no need
     to use `clobber' expressions to indicate this fact.  Also, each
     function call is assumed to have the potential to alter any memory
     location, unless the function is declared `const'.

     If the last group of expressions in a `parallel' are each a
     `clobber' expression whose arguments are `reg' or `match_scratch'
     (*note RTL Template::) expressions, the combiner phase can add the
     appropriate `clobber' expressions to an insn it has constructed
     when doing so will cause a pattern to be matched.

     This feature can be used, for example, on a machine that whose
     multiply and add instructions don't use an MQ register but which
     has an add-accumulate instruction that does clobber the MQ
     register.  Similarly, a combined instruction might require a
     temporary register while the constituent instructions might not.

     When a `clobber' expression for a register appears inside a
     `parallel' with other side effects, the register allocator
     guarantees that the register is unoccupied both before and after
     that insn.  However, the reload phase may allocate a register used
     for one of the inputs unless the `&' constraint is specified for
     the selected alternative (*note Modifiers::).  You can clobber
     either a specific hard register, a pseudo register, or a `scratch'
     expression; in the latter two cases, GCC will allocate a hard
     register that is available there for use as a temporary.

     For instructions that require a temporary register, you should use
     `scratch' instead of a pseudo-register because this will allow the
     combiner phase to add the `clobber' when required.  You do this by
     coding (`clobber' (`match_scratch' ...)).  If you do clobber a
     pseudo register, use one which appears nowhere else--generate a
     new one each time.  Otherwise, you may confuse CSE.

     There is one other known use for clobbering a pseudo register in a
     `parallel': when one of the input operands of the insn is also
     clobbered by the insn.  In this case, using the same pseudo
     register in the clobber and elsewhere in the insn produces the
     expected results.

`(use X)'
     Represents the use of the value of X.  It indicates that the value
     in X at this point in the program is needed, even though it may
     not be apparent why this is so.  Therefore, the compiler will not
     attempt to delete previous instructions whose only effect is to
     store a value in X.  X must be a `reg' expression.

     In some situations, it may be tempting to add a `use' of a
     register in a `parallel' to describe a situation where the value
     of a special register will modify the behavior of the instruction.
     An hypothetical example might be a pattern for an addition that can
     either wrap around or use saturating addition depending on the
     value of a special control register:

          (parallel [(set (reg:SI 2) (unspec:SI [(reg:SI 3)
                                                 (reg:SI 4)] 0))
                     (use (reg:SI 1))])

     This will not work, several of the optimizers only look at
     expressions locally; it is very likely that if you have multiple
     insns with identical inputs to the `unspec', they will be
     optimized away even if register 1 changes in between.

     This means that `use' can _only_ be used to describe that the
     register is live.  You should think twice before adding `use'
     statements, more often you will want to use `unspec' instead.  The
     `use' RTX is most commonly useful to describe that a fixed
     register is implicitly used in an insn.  It is also safe to use in
     patterns where the compiler knows for other reasons that the result
     of the whole pattern is variable, such as `movstrM' or `call'
     patterns.

     During the reload phase, an insn that has a `use' as pattern can
     carry a reg_equal note.  These `use' insns will be deleted before
     the reload phase exits.

     During the delayed branch scheduling phase, X may be an insn.
     This indicates that X previously was located at this place in the
     code and its data dependencies need to be taken into account.
     These `use' insns will be deleted before the delayed branch
     scheduling phase exits.

`(parallel [X0 X1 ...])'
     Represents several side effects performed in parallel.  The square
     brackets stand for a vector; the operand of `parallel' is a vector
     of expressions.  X0, X1 and so on are individual side effect
     expressions--expressions of code `set', `call', `return',
     `clobber' or `use'.

     "In parallel" means that first all the values used in the
     individual side-effects are computed, and second all the actual
     side-effects are performed.  For example,

          (parallel [(set (reg:SI 1) (mem:SI (reg:SI 1)))
                     (set (mem:SI (reg:SI 1)) (reg:SI 1))])

     says unambiguously that the values of hard register 1 and the
     memory location addressed by it are interchanged.  In both places
     where `(reg:SI 1)' appears as a memory address it refers to the
     value in register 1 _before_ the execution of the insn.

     It follows that it is _incorrect_ to use `parallel' and expect the
     result of one `set' to be available for the next one.  For
     example, people sometimes attempt to represent a jump-if-zero
     instruction this way:

          (parallel [(set (cc0) (reg:SI 34))
                     (set (pc) (if_then_else
                                  (eq (cc0) (const_int 0))
                                  (label_ref ...)
                                  (pc)))])

     But this is incorrect, because it says that the jump condition
     depends on the condition code value _before_ this instruction, not
     on the new value that is set by this instruction.

     Peephole optimization, which takes place together with final
     assembly code output, can produce insns whose patterns consist of
     a `parallel' whose elements are the operands needed to output the
     resulting assembler code--often `reg', `mem' or constant
     expressions.  This would not be well-formed RTL at any other stage
     in compilation, but it is ok then because no further optimization
     remains to be done.  However, the definition of the macro
     `NOTICE_UPDATE_CC', if any, must deal with such insns if you
     define any peephole optimizations.

`(cond_exec [COND EXPR])'
     Represents a conditionally executed expression.  The EXPR is
     executed only if the COND is nonzero.  The COND expression must
     not have side-effects, but the EXPR may very well have
     side-effects.

`(sequence [INSNS ...])'
     Represents a sequence of insns.  Each of the INSNS that appears in
     the vector is suitable for appearing in the chain of insns, so it
     must be an `insn', `jump_insn', `call_insn', `code_label',
     `barrier' or `note'.

     A `sequence' RTX is never placed in an actual insn during RTL
     generation.  It represents the sequence of insns that result from a
     `define_expand' _before_ those insns are passed to `emit_insn' to
     insert them in the chain of insns.  When actually inserted, the
     individual sub-insns are separated out and the `sequence' is
     forgotten.

     After delay-slot scheduling is completed, an insn and all the
     insns that reside in its delay slots are grouped together into a
     `sequence'.  The insn requiring the delay slot is the first insn
     in the vector; subsequent insns are to be placed in the delay slot.

     `INSN_ANNULLED_BRANCH_P' is set on an insn in a delay slot to
     indicate that a branch insn should be used that will conditionally
     annul the effect of the insns in the delay slots.  In such a case,
     `INSN_FROM_TARGET_P' indicates that the insn is from the target of
     the branch and should be executed only if the branch is taken;
     otherwise the insn should be executed only if the branch is not
     taken.  *Note Delay Slots::.

   These expression codes appear in place of a side effect, as the body
of an insn, though strictly speaking they do not always describe side
effects as such:

`(asm_input S)'
     Represents literal assembler code as described by the string S.

`(unspec [OPERANDS ...] INDEX)'
`(unspec_volatile [OPERANDS ...] INDEX)'
     Represents a machine-specific operation on OPERANDS.  INDEX
     selects between multiple machine-specific operations.
     `unspec_volatile' is used for volatile operations and operations
     that may trap; `unspec' is used for other operations.

     These codes may appear inside a `pattern' of an insn, inside a
     `parallel', or inside an expression.

`(addr_vec:M [LR0 LR1 ...])'
     Represents a table of jump addresses.  The vector elements LR0,
     etc., are `label_ref' expressions.  The mode M specifies how much
     space is given to each address; normally M would be `Pmode'.

`(addr_diff_vec:M BASE [LR0 LR1 ...] MIN MAX FLAGS)'
     Represents a table of jump addresses expressed as offsets from
     BASE.  The vector elements LR0, etc., are `label_ref' expressions
     and so is BASE.  The mode M specifies how much space is given to
     each address-difference.  MIN and MAX are set up by branch
     shortening and hold a label with a minimum and a maximum address,
     respectively.  FLAGS indicates the relative position of BASE, MIN
     and MAX to the containing insn and of MIN and MAX to BASE.  See
     rtl.def for details.

`(prefetch:M ADDR RW LOCALITY)'
     Represents prefetch of memory at address ADDR.  Operand RW is 1 if
     the prefetch is for data to be written, 0 otherwise; targets that
     do not support write prefetches should treat this as a normal
     prefetch.  Operand LOCALITY specifies the amount of temporal
     locality; 0 if there is none or 1, 2, or 3 for increasing levels
     of temporal locality; targets that do not support locality hints
     should ignore this.

     This insn is used to minimize cache-miss latency by moving data
     into a cache before it is accessed.  It should use only
     non-faulting data prefetch instructions.


File: gccint.info,  Node: Incdec,  Next: Assembler,  Prev: Side Effects,  Up: RTL

Embedded Side-Effects on Addresses
==================================

   Six special side-effect expression codes appear as memory addresses.

`(pre_dec:M X)'
     Represents the side effect of decrementing X by a standard amount
     and represents also the value that X has after being decremented.
     X must be a `reg' or `mem', but most machines allow only a `reg'.
     M must be the machine mode for pointers on the machine in use.
     The amount X is decremented by is the length in bytes of the
     machine mode of the containing memory reference of which this
     expression serves as the address.  Here is an example of its use:

          (mem:DF (pre_dec:SI (reg:SI 39)))

     This says to decrement pseudo register 39 by the length of a
     `DFmode' value and use the result to address a `DFmode' value.

`(pre_inc:M X)'
     Similar, but specifies incrementing X instead of decrementing it.

`(post_dec:M X)'
     Represents the same side effect as `pre_dec' but a different
     value.  The value represented here is the value X has before being
     decremented.

`(post_inc:M X)'
     Similar, but specifies incrementing X instead of decrementing it.

`(post_modify:M X Y)'
     Represents the side effect of setting X to Y and represents X
     before X is modified.  X must be a `reg' or `mem', but most
     machines allow only a `reg'.  M must be the machine mode for
     pointers on the machine in use.  The amount X is decremented by is
     the length in bytes of the machine mode of the containing memory
     reference of which this expression serves as the address.  Note
     that this is not currently implemented.

     The expression Y must be one of three forms:
          `(plus:M X Z)', `(minus:M X Z)', or `(plus:M X I)',
     where Z is an index register and I is a constant.

     Here is an example of its use:

          (mem:SF (post_modify:SI (reg:SI 42) (plus (reg:SI 42)
                                                    (reg:SI 48))))

     This says to modify pseudo register 42 by adding the contents of
     pseudo register 48 to it, after the use of what ever 42 points to.

`(pre_modify:M X EXPR)'
     Similar except side effects happen before the use.

   These embedded side effect expressions must be used with care.
Instruction patterns may not use them.  Until the `flow' pass of the
compiler, they may occur only to represent pushes onto the stack.  The
`flow' pass finds cases where registers are incremented or decremented
in one instruction and used as an address shortly before or after;
these cases are then transformed to use pre- or post-increment or
-decrement.

   If a register used as the operand of these expressions is used in
another address in an insn, the original value of the register is used.
Uses of the register outside of an address are not permitted within the
same insn as a use in an embedded side effect expression because such
insns behave differently on different machines and hence must be treated
as ambiguous and disallowed.

   An instruction that can be represented with an embedded side effect
could also be represented using `parallel' containing an additional
`set' to describe how the address register is altered.  This is not
done because machines that allow these operations at all typically
allow them wherever a memory address is called for.  Describing them as
additional parallel stores would require doubling the number of entries
in the machine description.


File: gccint.info,  Node: Assembler,  Next: Insns,  Prev: Incdec,  Up: RTL

Assembler Instructions as Expressions
=====================================

   The RTX code `asm_operands' represents a value produced by a
user-specified assembler instruction.  It is used to represent an `asm'
statement with arguments.  An `asm' statement with a single output
operand, like this:

     asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z));

is represented using a single `asm_operands' RTX which represents the
value that is stored in `outputvar':

     (set RTX-FOR-OUTPUTVAR
          (asm_operands "foo %1,%2,%0" "a" 0
                        [RTX-FOR-ADDITION-RESULT RTX-FOR-*Z]
                        [(asm_input:M1 "g")
                         (asm_input:M2 "di")]))

Here the operands of the `asm_operands' RTX are the assembler template
string, the output-operand's constraint, the index-number of the output
operand among the output operands specified, a vector of input operand
RTX's, and a vector of input-operand modes and constraints.  The mode
M1 is the mode of the sum `x+y'; M2 is that of `*z'.

   When an `asm' statement has multiple output values, its insn has
several such `set' RTX's inside of a `parallel'.  Each `set' contains a
`asm_operands'; all of these share the same assembler template and
vectors, but each contains the constraint for the respective output
operand.  They are also distinguished by the output-operand index
number, which is 0, 1, ... for successive output operands.

