public inbox for gcc-cvs@sourceware.org
help / color / mirror / Atom feed
* [gcc r13-1944] Improved pre-reload split of double word comparison against -1 on x86.
@ 2022-08-03 8:05 Roger Sayle
0 siblings, 0 replies; only message in thread
From: Roger Sayle @ 2022-08-03 8:05 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:e6b011bcfd52c245978ccd540e3f929571c59471
commit r13-1944-ge6b011bcfd52c245978ccd540e3f929571c59471
Author: Roger Sayle <roger@nextmovesoftware.com>
Date: Wed Aug 3 09:03:17 2022 +0100
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.
Diff:
---
gcc/config/i386/i386.md | 9 +++++++++
1 file changed, 9 insertions(+)
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)
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2022-08-03 8:05 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-08-03 8:05 [gcc r13-1944] Improved pre-reload split of double word comparison against -1 on x86 Roger Sayle
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).