diff options
| author | Roger Sayle <roger@nextmovesoftware.com> | 2022-08-03 09:03:17 +0100 |
|---|---|---|
| committer | Roger Sayle <roger@nextmovesoftware.com> | 2022-08-03 09:03:17 +0100 |
| commit | e6b011bcfd52c245978ccd540e3f929571c59471 (patch) | |
| tree | e21eecf84eda974e76f8737f77a801c3edb15269 /gcc | |
| parent | 7baed397dd0c220a72589c435e7db1676aced01e (diff) | |
Improved pre-reload split of double word comparison against -1 on x86.
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.
2022-08-03 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.
Diffstat (limited to 'gcc')
| -rw-r--r-- | gcc/config/i386/i386.md | 9 |
1 files changed, 9 insertions, 0 deletions
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md index f1158e1356b..e8f3851be01 100644 --- a/gcc/config/i386/i386.md +++ b/gcc/config/i386/i386.md @@ -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) |