https://bugs.llvm.org/show_bug.cgi?id=43574
Bug ID: 43574
Summary: Vectorized fp min/max reduction loop uses
minnum/maxnum for horizontal reduction, but not loop
body
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: craig.top...@gmail.com
CC: llvm-bugs@lists.llvm.org
For a min/max reduction loop, the vectorizer adds fast math flags to the
fcmp/select for the horizontal reduction after the loop. But the selects in the
loop don't have fastmath flags. This leads to InstCombine creating
fmaxnum/fminnum intrinsics for the horizontal reduction. X86 codegens both the
loop body and the horizontal reduction to the FP min/max instructions so the
difference didn't end up mattering. It just looked inconsistent.
clang -Ofast -march=skylake-avx512
float a[1024];
float foo() {
float min = 0.0;
for (int i = 0; i != 1024; ++i)
min = min < a[i] ? min : a[i];
return min;
}
https://godbolt.org/z/AMWYWm
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