https://llvm.org/bugs/show_bug.cgi?id=28442
Bug ID: 28442
Summary: We should generate the xor idiom instead of movzbl for
zext even if not directly fed by setcc.
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: mku...@google.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Consider:
int foo(int a, int b, int c) {
return (a > 0 && b > 0 && c > 0);
}
Right now, we generate:
testl %edi, %edi
setg %al
testl %esi, %esi
setg %cl
andb %al, %cl
testl %edx, %edx
setg %al
andb %cl, %al
movzbl %al, %eax
retq
This does not get caught by r274692, because the setcc doesn't feed directly
into the zext. This is true starting from the IR level:
%cmp = icmp sgt i32 %a, 0
%cmp1 = icmp sgt i32 %b, 0
%or.cond = and i1 %cmp, %cmp1
%cmp2 = icmp sgt i32 %c, 0
%cmp2. = and i1 %or.cond, %cmp2
%land.ext = zext i1 %cmp2. to i32
ret i32 %land.ext
The performance impact is the same as in the usual "setcc + zext" cases:
int main() {
unsigned x = 0;
unsigned y = 0;
for (unsigned i = 0; i < 2000; ++i) {
for (unsigned j = 0; j < 2000; ++j) {
for (unsigned k = 0; k < 2000; ++k) {
y += ((i >= 10) && (j >= 20) && (k >= 30));
}
}
}
return y;
}
When compiled with clang -O2 -fno-vectorize -fno-unroll-loops runs for 7.8s on
my machine. Using the xor idiom, it runs for 4.9s.
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