gnu /
gcc /
3155d51bfd1de8b6c4645dcb2292248a8d7cc3c9 [PATCH] PR rtl-optimization/46235: Improved use of bt for bit tests on x86_64.
This patch tackles PR46235 to improve the code generated for bit tests
on x86_64 by making more use of the bt instruction. Currently, GCC emits
bt instructions when followed by condition jumps (thanks to Uros' splitters).
This patch adds splitters in i386.md, to catch the cases where bt is followed
by a conditional move (as in the original report), or by a setc/setnc (as in
comment 5 of the Bugzilla PR).
With this patch, the function in the original PR
int foo(int a, int x, int y) {
if (a & (1 << x))
return a;
return 1;
}
which with -O2 on mainline generates:
foo: movl %edi, %eax
movl %esi, %ecx
sarl %cl, %eax
testb $1, %al
movl $1, %eax
cmovne %edi, %eax
ret
now generates:
foo: btl %esi, %edi
movl $1, %eax
cmovc %edi, %eax
ret
Likewise, IsBitSet1 and IsBitSet2 (from comment 5)
bool IsBitSet1(unsigned char byte, int index) {
return (byte & (1<<index)) != 0;
}
bool IsBitSet2(unsigned char byte, int index) {
return (byte >> index) & 1;
}
Before:
movzbl %dil, %eax
movl %esi, %ecx
sarl %cl, %eax
andl $1, %eax
ret
After:
movzbl %dil, %edi
btl %esi, %edi
setc %al
ret
According to Agner Fog, SAR/SHR r,cl takes 2 cycles on skylake,
where BT r,r takes only one, so the performance improvements on
recent hardware may be more significant than implied by just
the reduced number of instructions. I've avoided transforming cases
(such as btsi_setcsi) where using bt sequences may not be a clear
win (over sarq/andl).
2010-06-15 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
PR rtl-optimization/46235
* config/i386/i386.md: New define_split for bt followed by cmov.
(*bt<mode>_setcqi): New define_insn_and_split for bt followed by setc.
(*bt<mode>_setncqi): New define_insn_and_split for bt then setnc.
(*bt<mode>_setnc<mode>): New define_insn_and_split for bt followed
by setnc with zero extension.
gcc/testsuite/ChangeLog
PR rtl-optimization/46235
* gcc.target/i386/bt-5.c: New test.
* gcc.target/i386/bt-6.c: New test.
* gcc.target/i386/bt-7.c: New test.
4 files changed
