Message ID | 01ad01d8a663$6e062820$4a127860$@nextmovesoftware.com |
---|---|
State | New |
Headers | show |
Series | [x86] Improved pre-reload split of double word comparison against -1. | expand |
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.
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md index f1158e1..e8f3851 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)