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| author | Roger Sayle <roger@nextmovesoftware.com> | 2024-05-22 16:45:48 +0100 |
|---|---|---|
| committer | Roger Sayle <roger@nextmovesoftware.com> | 2024-05-22 16:45:48 +0100 |
| commit | a3b16e73a2d5b2d4d20ef6f2fd164cea633bbec8 (patch) | |
| tree | a019997a59b4280823969eacf0c36fbe35b02852 /gcc | |
| parent | 26df7b4684e201e66c09dd018603a248ddc5f437 (diff) | |
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i386: Correct insn_cost of movabsq.
This single line patch fixes a strange quirk/glitch in i386's rtx_costs,
which considers an instruction loading a 64-bit constant to be significantly
cheaper than loading a 32-bit (or smaller) constant.
Consider the two functions:
unsigned long long foo() { return 0x0123456789abcdefULL; }
unsigned int bar() { return 10; }
and the corresponding lines from combine's dump file:
insn_cost 1 for #: r98:DI=0x123456789abcdef
insn_cost 4 for #: ax:SI=0xa
The same issue can be seen in -dP assembler output.
movabsq $81985529216486895, %rax # 5 [c=1 l=10] *movdi_internal/4
The problem is that pattern_costs interpretation of rtx_costs contains
"return cost > 0 ? cost : COSTS_N_INSNS (1)" where a zero value (for
example a register or small immediate constant) is considered special,
and equivalent to a single instruction, but all other values are treated
as verbatim. Hence to x86_64's 10-byte long movabsq instruction slightly
more expensive than a simple constant, rtx_costs needs to return
COSTS_N_INSNS(1)+1 and not 1. With this change, the insn_cost of
movabsq is the intended value 5:
insn_cost 5 for #: r98:DI=0x123456789abcdef
and
movabsq $81985529216486895, %rax # 5 [c=5 l=10] *movdi_internal/4
2024-05-22 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* config/i386/i386.cc (ix86_rtx_costs) <case CONST_INT>:
A CONST_INT that isn't x86_64_immediate_operand requires an extra
(expensive) movabsq insn to load, so return COSTS_N_INSNS (1) + 1.
Diffstat (limited to 'gcc')
| -rw-r--r-- | gcc/config/i386/i386.cc | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc index 69cd4ae..3e2a3a1 100644 --- a/gcc/config/i386/i386.cc +++ b/gcc/config/i386/i386.cc @@ -21562,7 +21562,8 @@ ix86_rtx_costs (rtx x, machine_mode mode, int outer_code_i, int opno, if (x86_64_immediate_operand (x, VOIDmode)) *total = 0; else - *total = 1; + /* movabsq is slightly more expensive than a simple instruction. */ + *total = COSTS_N_INSNS (1) + 1; return true; case CONST_DOUBLE: |