[x86 PATCH] Improve ix86_expand_int_movcc to allow condition (mask) sharing.
Roger Sayle
roger@nextmovesoftware.com
Mon Jan 2 10:50:38 GMT 2023
This patch modifies the way that ix86_expand_int_movcc generates RTL,
to allow the condition mask to be shared/reused between multiple
conditional move sequences. Such redundancy is common when RTL
if-conversion transforms non-trivial basic blocks.
As a motivating example, consider the new test case:
int a, b, c, d;
int foo(int x)
{
if (x == 0) {
a = 3;
b = 1;
c = 4;
d = 1;
} else {
a = 5;
b = 9;
c = 2;
d = 7;
}
return x;
}
This is currently compiled, with -O2, to:
foo: cmpl $1, %edi
movl %edi, %eax
sbbl %edi, %edi
andl $-2, %edi
addl $5, %edi
cmpl $1, %eax
sbbl %esi, %esi
movl %edi, a(%rip)
andl $-8, %esi
addl $9, %esi
cmpl $1, %eax
sbbl %ecx, %ecx
movl %esi, b(%rip)
andl $2, %ecx
addl $2, %ecx
cmpl $1, %eax
sbbl %edx, %edx
movl %ecx, c(%rip)
andl $-6, %edx
addl $7, %edx
movl %edx, d(%rip)
ret
Notice that the if-then-else blocks have been if-converted into four
conditional move sequences/assignments, each consisting of cmpl, sbbl,
andl and addl. However, as the conditions are the same, the cmpl and
sbbl instructions used to generate the mask could be shared by CSE.
This patch enables that, so we now generate:
foo: cmpl $1, %edi
movl %edi, %eax
sbbl %edx, %edx
movl %edx, %edi
movl %edx, %esi
movl %edx, %ecx
andl $-6, %edx
andl $-2, %edi
andl $-8, %esi
andl $2, %ecx
addl $7, %edx
addl $5, %edi
addl $9, %esi
addl $2, %ecx
movl %edx, d(%rip)
movl %edi, a(%rip)
movl %esi, b(%rip)
movl %ecx, c(%rip)
ret
Notice, the code now contains only a single cmpl and a single sbbl,
with their result being shared (via movl).
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32},
with no new failures. Ok for mainline?
2023-01-02 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* config/i386/i386-expand.cc (ix86_expand_int_movcc): Rewrite
RTL expansion to allow condition (mask) to be shared/reused,
by avoiding overwriting pseudos and adding REG_EQUAL notes.
gcc/testsuite/ChangeLog
* gcc.target/i386/cmov10.c: New test case.
Thanks in advance,
Roger
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