https://gcc.gnu.org/g:a3b16e73a2d5b2d4d20ef6f2fd164cea633bbec8
commit r15-775-ga3b16e73a2d5b2d4d20ef6f2fd164cea633bbec8
Author: Roger Sayle <ro...@nextmovesoftware.com>
Date: Wed May 22 16:45:48 2024 +0100
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 <ro...@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.
Diff:
---
gcc/config/i386/i386.cc | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 69cd4ae05a7..3e2a3a194f1 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:
