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 make 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
[I'd originally tried fixing this by adding a ix86_insn_cost target
hook, but the testsuite is very sensitive to the costing of insns].
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures. Ok for mainline?
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.
Thanks in advance,
Roger
--
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index b4838b7..b4a9519 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -21569,7 +21569,7 @@ 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;
+ *total = COSTS_N_INSNS (1) + 1;
return true;
case CONST_DOUBLE:
