[x86] Improved pre-reload split of double word comparison against -1.
Commit Message
This patch adds an extra optimization to *cmp<dwi>_doubleword to improve
the code generated for comparisons against -1. Hypothetically, if a
comparison against -1 reached this splitter we'd currently generate code
that looks like:
notq %rdx ; 3 bytes
notq %rax ; 3 bytes
orq %rdx, %rax ; 3 bytes
setne %al
With this patch we would instead generate the superior:
andq %rdx, %rax ; 3 bytes
cmpq $-1, %rax ; 4 bytes
setne %al
which is both faster and smaller, and also what's currently generated
thanks to the middle-end splitting double word comparisons against
zero and minus one during RTL expansion. Should that change, this would
become a missed-optimization regression, but this patch also (potentially)
helps suitable comparisons created by CSE and combine.
This patch has been tested on x86_64-pc-linux-gnu, on its own and in
combination with a middle-end patch tweaking RTL expansion, both with
and without --target-board=unix{-m32}, with no new failures.
Ok for mainline?
2022-08-02 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* config/i386/i386.md (*cmp<dwi>_doubleword): Add a special case
to split comparisons against -1 using AND and CMP -1 instructions.
Thanks again,
Roger
--
Comments
On Tue, Aug 2, 2022 at 1:31 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch adds an extra optimization to *cmp<dwi>_doubleword to improve
> the code generated for comparisons against -1. Hypothetically, if a
> comparison against -1 reached this splitter we'd currently generate code
> that looks like:
>
> notq %rdx ; 3 bytes
> notq %rax ; 3 bytes
> orq %rdx, %rax ; 3 bytes
> setne %al
>
> With this patch we would instead generate the superior:
>
> andq %rdx, %rax ; 3 bytes
> cmpq $-1, %rax ; 4 bytes
> setne %al
>
> which is both faster and smaller, and also what's currently generated
> thanks to the middle-end splitting double word comparisons against
> zero and minus one during RTL expansion. Should that change, this would
> become a missed-optimization regression, but this patch also (potentially)
> helps suitable comparisons created by CSE and combine.
>
> This patch has been tested on x86_64-pc-linux-gnu, on its own and in
> combination with a middle-end patch tweaking RTL expansion, both with
> and without --target-board=unix{-m32}, with no new failures.
> Ok for mainline?
>
>
> 2022-08-02 Roger Sayle <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386.md (*cmp<dwi>_doubleword): Add a special case
> to split comparisons against -1 using AND and CMP -1 instructions.
OK.
Thanks,
Uros.
@@ -1526,6 +1526,15 @@
operands[i] = force_reg (<MODE>mode, operands[i]);
operands[4] = gen_reg_rtx (<MODE>mode);
+
+ /* Special case comparisons against -1. */
+ if (operands[1] == constm1_rtx && operands[3] == constm1_rtx)
+ {
+ emit_insn (gen_and<mode>3 (operands[4], operands[0], operands[2]));
+ emit_insn (gen_cmp_1 (<MODE>mode, operands[4], constm1_rtx));
+ DONE;
+ }
+
if (operands[1] == const0_rtx)
emit_move_insn (operands[4], operands[0]);
else if (operands[0] == const0_rtx)