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* [gcc r13-1945] PR target/47949: Use xchg to move from/to AX_REG with -Oz on x86.
@ 2022-08-03 8:11 Roger Sayle
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From: Roger Sayle @ 2022-08-03 8:11 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:fc6ef90173478521982e9df3831a06ea85b4f41e
commit r13-1945-gfc6ef90173478521982e9df3831a06ea85b4f41e
Author: Roger Sayle <roger@nextmovesoftware.com>
Date: Wed Aug 3 09:07:36 2022 +0100
PR target/47949: Use xchg to move from/to AX_REG with -Oz on x86.
This patch adds a peephole2 to i386.md to implement the suggestion in
PR target/47949, of using xchg instead of mov for moving values to/from
the %rax/%eax register, controlled by -Oz, as the xchg instruction is
one byte shorter than the move it is replacing.
The new test case is taken from the PR:
int foo(int x) { return x; }
where previously we'd generate:
foo: mov %edi,%eax // 2 bytes
ret
but with this patch, using -Oz, we generate:
foo: xchg %eax,%edi // 1 byte
ret
On the CSiBE benchmark, this saves a total of 10238 bytes (reducing
the -Oz total from 3661796 bytes to 3651558 bytes, a 0.28% saving).
Interestingly, some modern architectures (such as Zen 3) implement
xchg using zero latency register renaming (just like mov), so in theory
this transformation could be enabled when optimizing for speed, if
benchmarking shows the improved code density produces consistently
better performance. However, this is architecture dependent, and
there may be interactions using xchg (instead a single_set) in the
late RTL passes (such as cprop_hardreg), so for now I've restricted
this to -Oz.
2022-08-03 Roger Sayle <roger@nextmovesoftware.com>
Uroš Bizjak <ubizjak@gmail.com>
gcc/ChangeLog
PR target/47949
* config/i386/i386.md (peephole2): New peephole2 to convert
SWI48 moves to/from %rax/%eax where the src is dead to xchg,
when optimizing for minimal size with -Oz.
gcc/testsuite/ChangeLog
PR target/47949
* gcc.target/i386/pr47949.c: New test case.
Diff:
---
gcc/config/i386/i386.md | 12 ++++++++++++
gcc/testsuite/gcc.target/i386/pr47949.c | 15 +++++++++++++++
2 files changed, 27 insertions(+)
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index e8f3851be01..298e4b30348 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -3027,6 +3027,18 @@
[(parallel [(set (match_dup 1) (match_dup 2))
(set (match_dup 2) (match_dup 1))])])
+;; Convert moves to/from AX_REG into xchg with -Oz.
+(define_peephole2
+ [(set (match_operand:SWI48 0 "general_reg_operand")
+ (match_operand:SWI48 1 "general_reg_operand"))]
+ "optimize_size > 1
+ && (REGNO (operands[0]) == AX_REG
+ || REGNO (operands[1]) == AX_REG)
+ && optimize_insn_for_size_p ()
+ && peep2_reg_dead_p (1, operands[1])"
+ [(parallel [(set (match_dup 0) (match_dup 1))
+ (set (match_dup 1) (match_dup 0))])])
+
(define_expand "movstrict<mode>"
[(set (strict_low_part (match_operand:SWI12 0 "register_operand"))
(match_operand:SWI12 1 "general_operand"))]
diff --git a/gcc/testsuite/gcc.target/i386/pr47949.c b/gcc/testsuite/gcc.target/i386/pr47949.c
new file mode 100644
index 00000000000..a0524b1f00d
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr47949.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-Oz" } */
+/* { dg-additional-options "-mregparm=2" { target ia32 } } */
+
+int foo(int x, int y)
+{
+ return y;
+}
+
+long bar(long x, long y)
+{
+ return y;
+}
+
+/* { dg-final { scan-assembler-times "xchg" 2 } } */
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