Using and Porting GNU CC
Here is a table of the instruction names that are meaningful in the RTL generation pass of the compiler. Giving one of these names to an instruction pattern tells the RTL generation pass that it can use the pattern in to accomplish a certain task.
movsi' moves full-word data.
If operand 0 is a
subreg with mode m of a register whose
own mode is wider than m, the effect of this instruction is
to store the specified value in the part of the register that corresponds
to mode m. The effect on the rest of the register is undefined.
This class of patterns is special in several ways. First of all, each of these names must be defined, because there is no other way to copy a datum from one place to another.
Second, these patterns are not used solely in the RTL generation pass. Even the reload pass can generate move insns to copy values from stack slots into temporary registers. When it does so, one of the operands is a hard register and the other is an operand that can need to be reloaded into a register.
Therefore, when given such a pair of operands, the pattern must generate
RTL which needs no reloading and needs no temporary registers---no
registers other than the operands. For example, if you support the
pattern with a
define_expand, then in such a case the
define_expand mustn't call
force_reg or any other such
function which might generate new pseudo registers.
This requirement exists even for subword modes on a RISC machine where
fetching those modes from memory normally requires several insns and
some temporary registers. Look in `
spur.md' to see how the
requirement can be satisfied.
During reload a memory reference with an invalid address may be passed
as an operand. Such an address will be replaced with a valid address
later in the reload pass. In this case, nothing may be done with the
address except to use it as it stands. If it is copied, it will not be
replaced with a valid address. No attempt should be made to make such
an address into a valid address and no routine (such as
change_address) that will do so may be called. Note that
general_operand will fail when applied to such an address.
The global variable
reload_in_progress (which must be explicitly
declared if required) can be used to determine whether such special
handling is required.
The variety of operands that have reloads depends on the rest of the machine description, but typically on a RISC machine these can only be pseudo registers that did not get hard registers, while on other machines explicit memory references will get optional reloads.
If a scratch register is required to move an object to or from memory,
it can be allocated using
gen_reg_rtx prior to reload. But this
is impossible during and after reload. If there are cases needing
scratch registers after reload, you must define
SECONDARY_INPUT_RELOAD_CLASS and perhaps also
SECONDARY_OUTPUT_RELOAD_CLASS to detect them, and provide
reload_inm' or `
reload_outm' to handle
them. See Register Classes.
The constraints on a `
movem' must permit moving any hard
register to any other hard register provided that
HARD_REGNO_MODE_OK permits mode m in both registers and
REGISTER_MOVE_COST applied to their classes returns a value of 2.
It is obligatory to support floating point `
instructions into and out of any registers that can hold fixed point
values, because unions and structures (which have modes
DImode) can be in those registers and they may have floating
There may also be a need to support fixed point `
instructions in and out of floating point registers. Unfortunately, I
have forgotten why this was so, and I don't know whether it is still
HARD_REGNO_MODE_OK rejects fixed point values in
floating point registers, then the constraints of the fixed point
movem' instructions must be designed to avoid ever trying to
reload into a floating point register.
movm', but used when a scratch register is required to move between operand 0 and operand 1. Operand 2 describes the scratch register. See the discussion of the
SECONDARY_RELOAD_CLASSmacro in see Register Classes.
movm' except that if operand 0 is a
subregwith mode m of a register whose natural mode is wider, the `
movstrictm' instruction is guaranteed not to alter any of the register except the part which belongs to mode m.
Define this only if the target machine really has such an instruction; do not define this if the most efficient way of loading consecutive registers from memory is to do them one at a time.
On some machines, there are restrictions as to which consecutive
registers can be stored into memory, such as particular starting or
ending register numbers or only a range of valid counts. For those
machines, use a
define_expand (see Expander Definitions)
and make the pattern fail if the restrictions are not met.
Write the generated insn as a
parallel with elements being a
set of one register from the appropriate memory location (you may
clobber elements). Use a
match_parallel (see RTL Template) to recognize the insn. See
a29k.md' and `
rs6000.md' for examples of the use of this insn
load_multiple', but store several consecutive registers into consecutive memory locations. Operand 0 is the first of the consecutive memory locations, operand 1 is the first register, and operand 2 is a constant: the number of consecutive registers.
HImode, and store a
SImodeproduct in operand 0.
For machines with an instruction that produces both a quotient and a
remainder, provide a pattern for `
divmodm4' but do not
provide patterns for `
divm3' and `
allows optimization in the relatively common case when both the quotient
and remainder are computed.
If an instruction that just produces a quotient or just a remainder
exists and is more efficient than the instruction that produces both,
write the output routine of `
divmodm4' to call
find_reg_note and look for a
REG_UNUSED note on the
quotient or remainder and generate the appropriate instruction.
sqrt built-in function of C always uses the mode which
corresponds to the C data type
ffs built-in function of C always uses the mode which
corresponds to the C data type
(set (cc0) (compare (match_operand:m 0 ...) (match_operand:m 1 ...)))
(set (cc0) (match_operand:m 0 ...))
tstm' patterns should not be defined for machines that do
(cc0). Doing so would confuse the optimizer since it
would no longer be clear which
set operations were comparisons.
cmpm' patterns should be used instead.
Pmode. The number of bytes to move is the third operand, in mode m.
The fourth operand is the known shared alignment of the source and
destination, in the form of a
const_int rtx. Thus, if the
compiler knows that both source and destination are word-aligned,
it may provide the value 4 for this operand.
These patterns need not give special consideration to the possibility that the source and destination strings might overlap.
movstrm'. The two memory blocks specified are compared byte by byte in lexicographic order. The effect of the instruction is to store a value in operand 0 whose sign indicates the result of the comparison.
Compute the length of a string, with three operands.
Operand 0 is the result (of mode m), operand 1 is
mem referring to the first character of the string,
operand 2 is the character to search for (normally zero),
and operand 3 is a constant describing the known alignment
of the beginning of the string.
fixmn2' but works for any floating point value of mode m by converting the value to an integer.
fixunsmn2' but works for any floating point value of mode m by converting the value to an integer.
word_mode. Operand 1 may have mode
word_modeis allowed only for registers. Operands 2 and 3 must be valid for
The RTL generation pass generates this instruction only with constants for operands 2 and 3.
The bit-field value is sign-extended to a full word integer before it is stored in operand 0.
extv' except that the bit-field value is zero-extended.
word_mode) into a bit field in operand 0, where operand 1 specifies the width in bits and operand 2 the starting bit. Operand 0 may have mode
word_modeis allowed only for registers. Operands 1 and 2 must be valid for
The RTL generation pass generates this instruction only with constants for operands 1 and 2.
You specify the mode that the operand must have when you write the
match_operand expression. The compiler automatically sees
which mode you have used and supplies an operand of that mode.
The value stored for a true condition must have 1 as its low bit, or
else must be negative. Otherwise the instruction is not suitable and
you should omit it from the machine description. You describe to the
compiler exactly which value is stored by defining the macro
STORE_FLAG_VALUE (see Misc). If a description cannot be
found that can be used for all the `
scond' patterns, you
should omit those operations from the machine description.
These operations may fail, but should do so only in relatively uncommon cases; if they would fail for common cases involving integer comparisons, it is best to omit these patterns.
If these operations are omitted, the compiler will usually generate code
that copies the constant one to the target and branches around an
assignment of zero to the target. If this code is more efficient than
the potential instructions used for the `
followed by those required to convert the result into a 1 or a zero in
SImode, you should omit the `
scond' operations from
the machine description.
label_refthat refers to the label to jump to. Jump if the condition codes meet condition cond.
Some machines do not follow the model assumed here where a comparison
instruction is followed by a conditional branch instruction. In that
case, the `
cmpm' (and `
tstm') patterns should
simply store the operands away and generate all the required insns in a
define_expand (see Expander Definitions) for the conditional
branch operations. All calls to expand `
bcond' patterns are
immediately preceded by calls to expand either a `
pattern or a `
Machines that use a pseudo register for the condition code value, or where the mode used for the comparison depends on the condition being tested, should also use the above mechanism. See Jump Patterns
The above discussion also applies to `
SImode, except it is normally a
const_int); operand 2 is the number of registers used as operands.
On most machines, operand 2 is not actually stored into the RTL pattern. It is supplied for the sake of some RISC machines which need to put this information into the assembler code; they can put it in the RTL instead of operand 1.
Operand 0 should be a
mem RTX whose address is the address of the
function. Note, however, that this address can be a
expression even if it would not be a legitimate memory address on the
target machine. If it is also not a valid argument for a call
instruction, the pattern for this operation should be a
define_expand (see Expander Definitions) that places the
address into a register and uses that register in the call instruction.
call' instruction (but with numbers increased by one).
Subroutines that return
BLKmode objects use the `
call' and `
call_value', except used if defined and if
RETURN_POPS_ARGSis non-zero. They should emit a
parallelthat contains both the function call and a
setto indicate the adjustment made to the frame pointer.
For machines where
RETURN_POPS_ARGS can be non-zero, the use of these
patterns increases the number of functions for which the frame pointer
can be eliminated, if desired.
parallelexpression where each element is a
setexpression that indicates the saving of a function return value into the result block.
This instruction pattern should be defined to support
__builtin_apply on machines where special instructions are needed
to call a subroutine with arbitrary arguments or to save the value
returned. This instruction pattern is required on machines that have
multiple registers that can hold a return value (i.e.
FUNCTION_VALUE_REGNO_P is true for more than one register).
Like the `
movm' patterns, this pattern is also used after the
RTL generation phase. In this case it is to support machines where
multiple instructions are usually needed to return from a function, but
some class of functions only requires one instruction to implement a
return. Normally, the applicable functions are those which do not need
to save any registers or allocate stack space.
For such machines, the condition specified in this pattern should only
be true when
reload_completed is non-zero and the function's
epilogue would only be a single instruction. For machines with register
windows, the routine
leaf_function_p may be used to determine if
a register window push is required.
Machines that have conditional return instructions should define patterns such as
(define_insn "" [(set (pc) (if_then_else (match_operator 0 "comparison_operator" [(cc0) (const_int 0)]) (return) (pc)))] "condition" "...")
where condition would normally be the same condition specified on the
__builtin_returnon machines where special instructions are needed to return a value of any type.
Operand 0 is a memory location where the result of calling a function
__builtin_apply is stored; operand 1 is a
expression where each element is a
set expression that indicates
the restoring of a function return value from the result block.
(const_int 0)will do as an RTL pattern.
CASE_DROPS_THROUGHis defined, then an out-of-bounds index drops through to the code following the jump table instead of jumping to this label. In that case, this label is not actually used by the `
casesi' instruction, but it is always provided as an operand.)
The table is a
addr_diff_vec inside of a
jump_insn. The number of elements in the table is one plus the
difference between the upper bound and the lower bound.
This pattern requires two operands: the address or offset, and a label
which should immediately precede the jump table. If the macro
CASE_VECTOR_PC_RELATIVE is defined then the first operand is an
offset which counts from the address of the table; otherwise, it is an
absolute address to jump to. In either case, the first operand has
tablejump' insn is always the last insn before the jump
table it uses. Its assembler code normally has no need to use the
second operand, but you should incorporate it in the RTL pattern so
that the jump optimizer will not delete the table as unreachable code.
Pmode. Do not define these patterns on such machines.
Some machines require special handling for stack pointer saves and
restores. On those machines, define the patterns corresponding to the
non-standard cases by using a
define_expand (see Expander Definitions) that produces the required insns. The three types of
saves and restores are:
save_stack_block' saves the stack pointer at the start of a block that allocates a variable-sized object, and `
restore_stack_block' restores the stack pointer when the block is exited.
save_stack_function' and `
restore_stack_function' do a similar job for the outermost block of a function and are used when the function allocates variable-sized objects or calls
alloca. Only the epilogue uses the restored stack pointer, allowing a simpler save or restore sequence on some machines.
save_stack_nonlocal' is used in functions that contain labels branched to by nested functions. It saves the stack pointer in such a way that the inner function can use `
restore_stack_nonlocal' to restore the stack pointer. The compiler generates code to restore the frame and argument pointer registers, but some machines require saving and restoring additional data such as register window information or stack backchains. Place insns in these patterns to save and restore any such required data.
When saving the stack pointer, operand 0 is the save area and operand 1
is the stack pointer. The mode used to allocate the save area is the
mode of operand 0. You must specify an integral mode, or
VOIDmode if no save area is needed for a particular type of save
(either because no save is needed or because a machine-specific save
area can be used). Operand 0 is the stack pointer and operand 1 is the
save area for restore operations. If `
defined, operand 0 must not be
VOIDmode since these saves can be
A save area is a
mem that is at a constant offset from
virtual_stack_vars_rtx when the stack pointer is saved for use by
nonlocal gotos and a
reg in the other two cases.
STACK_GROWS_DOWNWARDis undefined) operand 0 from the stack pointer to create space for dynamically allocated data.
Do not define this pattern if all that must be done is the subtraction. Some machines require other operations such as stack probes or maintaining the back chain. Define this pattern to emit those operations in addition to updating the stack pointer.