https://bugs.llvm.org/show_bug.cgi?id=46053
Bug ID: 46053
Summary: Loop counting top bits set badly vectorized on X86
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedbugs at nondot.org
Reporter: nikita.ppv at gmail.com
CC: llvm-bugs at lists.llvm.org, llvm-dev at redking.me.uk,
spatel+llvm at rotateright.com
Originally reported at: https://github.com/rust-lang/rust/issues/72355.
A loop counting the number of bytes with the top bit set:
#include <stddef.h>
#include <stdint.h>
uint64_t count_non_ascii(unsigned *buffer, size_t len) {
uint64_t count = 0;
for (size_t i = 0; i < len; i++) {
count += buffer[i] >= 0x80;
}
return count;
}
Gets vectorized to some complicated code: https://c.godbolt.org/z/jTU3rX
While this could be just movmsk + popcnt.
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