On Wed, May 22, 2024 at 5:15 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
> 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.
1 of 20,796
[x86_64 PATCH] Correct insn_cost of movabsq.
Inbox
Roger Sayle
5:15 PM (12 minutes ago)
to gcc-patches, me
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
--
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Roger Sayle (nextmovesoftware.com), gcc-patches@gcc.gnu.org
On Wed, May 22, 2024 at 5:15 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
> 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.
OK, with a small comment added.
Thanks,
Uros.
> 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;
Please add a small comment that this cost belongs to movabs.
