Using and Porting GNU CC

16.13: 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.

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 (from, to)

A C expression for the cost of moving data 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 4 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 (m)

A C expression for the cost of moving data of mode m between a register and memory. A value of 2 is the default; 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.

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 GNU CC 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

Define this macro to be the value 1 if unaligned accesses have a cost many times greater than aligned accesses, for example if they are emulated in a trap handler.

When this macro is non-zero, the compiler will act as if STRICT_ALIGNMENT were non-zero when generating code for block moves. This can cause significantly more instructions to be produced. Therefore, do not set this macro non-zero 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.

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 number of scalar move insns which 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.

If you don't define this, a reasonable default is used.

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.