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(gcc-300.info)Costs
Describing Relative Costs of Operations
=======================================
These macros let you describe the relative speed of various
operations on the target machine.
`CONST_COSTS (X, CODE, OUTER_CODE)'
A part of a C `switch' statement that describes the relative costs
of constant RTL expressions. It must contain `case' labels for
expression codes `const_int', `const', `symbol_ref', `label_ref'
and `const_double'. Each case must ultimately reach a `return'
statement to return the relative cost of the use of that kind of
constant value in an expression. The cost may depend on the
precise value of the constant, which is available for examination
in X, and the rtx code of the expression in which it is contained,
found in OUTER_CODE.
CODE is the expression code--redundant, since it can be obtained
with `GET_CODE (X)'.
`RTX_COSTS (X, CODE, OUTER_CODE)'
Like `CONST_COSTS' but applies to nonconstant RTL expressions.
This can be used, for example, to indicate how costly a multiply
instruction is. In writing this macro, you can use the construct
`COSTS_N_INSNS (N)' to specify a cost equal to N fast
instructions. OUTER_CODE is the code of the expression in which X
is contained.
This macro is optional; do not define it if the default cost
assumptions are adequate for the target machine.
`DEFAULT_RTX_COSTS (X, CODE, OUTER_CODE)'
This macro, if defined, is called for any case not handled by the
`RTX_COSTS' or `CONST_COSTS' macros. This eliminates the need to
put case labels into the macro, but the code, or any functions it
calls, must assume that the RTL in X could be of any type that has
not already been handled. The arguments are the same as for
`RTX_COSTS', and the macro should execute a return statement giving
the cost of any RTL expressions that it can handle. The default
cost calculation is used for any RTL for which this macro does not
return a value.
This macro is optional; do not define it if the default cost
assumptions are adequate for the target machine.
`ADDRESS_COST (ADDRESS)'
An expression giving the cost of an addressing mode that contains
ADDRESS. If not defined, the cost is computed from the ADDRESS
expression and the `CONST_COSTS' values.
For most CISC machines, the default cost is a good approximation
of the true cost of the addressing mode. However, on RISC
machines, all instructions normally have the same length and
execution time. Hence all addresses will have equal costs.
In cases where more than one form of an address is known, the form
with the lowest cost will be used. If multiple forms have the
same, lowest, cost, the one that is the most complex will be used.
For example, suppose an address that is equal to the sum of a
register and a constant is used twice in the same basic block.
When this macro is not defined, the address will be computed in a
register and memory references will be indirect through that
register. On machines where the cost of the addressing mode
containing the sum is no higher than that of a simple indirect
reference, this will produce an additional instruction and
possibly require an additional register. Proper specification of
this macro eliminates this overhead for such machines.
Similar use of this macro is made in strength reduction of loops.
ADDRESS need not be valid as an address. In such a case, the cost
is not relevant and can be any value; invalid addresses need not be
assigned a different cost.
On machines where an address involving more than one register is as
cheap as an address computation involving only one register,
defining `ADDRESS_COST' to reflect this can cause two registers to
be live over a region of code where only one would have been if
`ADDRESS_COST' were not defined in that manner. This effect should
be considered in the definition of this macro. Equivalent costs
should probably only be given to addresses with different numbers
of registers on machines with lots of registers.
This macro will normally either not be defined or be defined as a
constant.
`REGISTER_MOVE_COST (MODE, FROM, TO)'
A C expression for the cost of moving data of mode MODE from a
register in class FROM to one in class TO. The classes are
expressed using the enumeration values such as `GENERAL_REGS'. A
value of 2 is the default; other values are interpreted relative to
that.
It is not required that the cost always equal 2 when FROM is the
same as TO; on some machines it is expensive to move between
registers if they are not general registers.
If reload sees an insn consisting of a single `set' between two
hard registers, and if `REGISTER_MOVE_COST' applied to their
classes returns a value of 2, reload does not check to ensure that
the constraints of the insn are met. Setting a cost of other than
2 will allow reload to verify that the constraints are met. You
should do this if the `movM' pattern's constraints do not allow
such copying.
`MEMORY_MOVE_COST (MODE, CLASS, IN)'
A C expression for the cost of moving data of mode MODE between a
register of class CLASS and memory; IN is zero if the value is to
be written to memory, nonzero if it is to be read in. This cost
is relative to those in `REGISTER_MOVE_COST'. If moving between
registers and memory is more expensive than between two registers,
you should define this macro to express the relative cost.
If you do not define this macro, GCC uses a default cost of 4 plus
the cost of copying via a secondary reload register, if one is
needed. If your machine requires a secondary reload register to
copy between memory and a register of CLASS but the reload
mechanism is more complex than copying via an intermediate, define
this macro to reflect the actual cost of the move.
GCC defines the function `memory_move_secondary_cost' if secondary
reloads are needed. It computes the costs due to copying via a
secondary register. If your machine copies from memory using a
secondary register in the conventional way but the default base
value of 4 is not correct for your machine, define this macro to
add some other value to the result of that function. The
arguments to that function are the same as to this macro.
`BRANCH_COST'
A C expression for the cost of a branch instruction. A value of 1
is the default; other values are interpreted relative to that.
Here are additional macros which do not specify precise relative
costs, but only that certain actions are more expensive than GCC would
ordinarily expect.
`SLOW_BYTE_ACCESS'
Define this macro as a C expression which is nonzero if accessing
less than a word of memory (i.e. a `char' or a `short') is no
faster than accessing a word of memory, i.e., if such access
require more than one instruction or if there is no difference in
cost between byte and (aligned) word loads.
When this macro is not defined, the compiler will access a field by
finding the smallest containing object; when it is defined, a
fullword load will be used if alignment permits. Unless bytes
accesses are faster than word accesses, using word accesses is
preferable since it may eliminate subsequent memory access if
subsequent accesses occur to other fields in the same word of the
structure, but to different bytes.
`SLOW_ZERO_EXTEND'
Define this macro if zero-extension (of a `char' or `short' to an
`int') can be done faster if the destination is a register that is
known to be zero.
If you define this macro, you must have instruction patterns that
recognize RTL structures like this:
(set (strict_low_part (subreg:QI (reg:SI ...) 0)) ...)
and likewise for `HImode'.
`SLOW_UNALIGNED_ACCESS (MODE, ALIGNMENT)'
Define this macro to be the value 1 if memory accesses described
by the MODE and ALIGNMENT parameters have a cost many times greater
than aligned accesses, for example if they are emulated in a trap
handler.
When this macro is nonzero, the compiler will act as if
`STRICT_ALIGNMENT' were nonzero when generating code for block
moves. This can cause significantly more instructions to be
produced. Therefore, do not set this macro nonzero if unaligned
accesses only add a cycle or two to the time for a memory access.
If the value of this macro is always zero, it need not be defined.
If this macro is defined, it should produce a nonzero value when
`STRICT_ALIGNMENT' is nonzero.
`DONT_REDUCE_ADDR'
Define this macro to inhibit strength reduction of memory
addresses. (On some machines, such strength reduction seems to do
harm rather than good.)
`MOVE_RATIO'
The threshold of number of scalar memory-to-memory move insns,
_below_ which a sequence of insns should be generated instead of a
string move insn or a library call. Increasing the value will
always make code faster, but eventually incurs high cost in
increased code size.
Note that on machines where the corresponding move insn is a
`define_expand' that emits a sequence of insns, this macro counts
the number of such sequences.
If you don't define this, a reasonable default is used.
`MOVE_BY_PIECES_P (SIZE, ALIGNMENT)'
A C expression used to determine whether `move_by_pieces' will be
used to copy a chunk of memory, or whether some other block move
mechanism will be used. Defaults to 1 if `move_by_pieces_ninsns'
returns less than `MOVE_RATIO'.
`MOVE_MAX_PIECES'
A C expression used by `move_by_pieces' to determine the largest
unit a load or store used to copy memory is. Defaults to
`MOVE_MAX'.
`USE_LOAD_POST_INCREMENT (MODE)'
A C expression used to determine whether a load postincrement is a
good thing to use for a given mode. Defaults to the value of
`HAVE_POST_INCREMENT'.
`USE_LOAD_POST_DECREMENT (MODE)'
A C expression used to determine whether a load postdecrement is a
good thing to use for a given mode. Defaults to the value of
`HAVE_POST_DECREMENT'.
`USE_LOAD_PRE_INCREMENT (MODE)'
A C expression used to determine whether a load preincrement is a
good thing to use for a given mode. Defaults to the value of
`HAVE_PRE_INCREMENT'.
`USE_LOAD_PRE_DECREMENT (MODE)'
A C expression used to determine whether a load predecrement is a
good thing to use for a given mode. Defaults to the value of
`HAVE_PRE_DECREMENT'.
`USE_STORE_POST_INCREMENT (MODE)'
A C expression used to determine whether a store postincrement is
a good thing to use for a given mode. Defaults to the value of
`HAVE_POST_INCREMENT'.
`USE_STORE_POST_DECREMENT (MODE)'
A C expression used to determine whether a store postdecrement is
a good thing to use for a given mode. Defaults to the value of
`HAVE_POST_DECREMENT'.
`USE_STORE_PRE_INCREMENT (MODE)'
This macro is used to determine whether a store preincrement is a
good thing to use for a given mode. Defaults to the value of
`HAVE_PRE_INCREMENT'.
`USE_STORE_PRE_DECREMENT (MODE)'
This macro is used to determine whether a store predecrement is a
good thing to use for a given mode. Defaults to the value of
`HAVE_PRE_DECREMENT'.
`NO_FUNCTION_CSE'
Define this macro if it is as good or better to call a constant
function address than to call an address kept in a register.
`NO_RECURSIVE_FUNCTION_CSE'
Define this macro if it is as good or better for a function to call
itself with an explicit address than to call an address kept in a
register.
`ADJUST_COST (INSN, LINK, DEP_INSN, COST)'
A C statement (sans semicolon) to update the integer variable COST
based on the relationship between INSN that is dependent on
DEP_INSN through the dependence LINK. The default is to make no
adjustment to COST. This can be used for example to specify to
the scheduler that an output- or anti-dependence does not incur
the same cost as a data-dependence.
`ADJUST_PRIORITY (INSN)'
A C statement (sans semicolon) to update the integer scheduling
priority `INSN_PRIORITY(INSN)'. Reduce the priority to execute
the INSN earlier, increase the priority to execute INSN later.
Do not define this macro if you do not need to adjust the
scheduling priorities of insns.
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