https://bugs.llvm.org/show_bug.cgi?id=50140
Bug ID: 50140
Summary: UBSan safe leading/trailing zero counts can't simplify
to zero-input correct variants
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
https://simd.godbolt.org/z/KrahKsdhe
clang -g0 -O3 -march=haswell
#include <cstdint>
#include <climits>
auto lz(unsigned x) {
int c = __builtin_clz( x );
return x ? c : sizeof(x) * CHAR_BIT;
}
auto tz(unsigned x) {
int c = __builtin_ctz( x );
return x ? c : sizeof(x) * CHAR_BIT;
}
lz:
lzcntl %edi, %eax
retq
tz:
tzcntl %edi, %eax
retq
auto lz_ubsan(unsigned x) {
int c = __builtin_clz( x ? x : ~0U );
return x ? c : sizeof(x) * CHAR_BIT;
}
auto tz_ubsan(unsigned x) {
int c = __builtin_ctz( x ? x : ~0U );
return x ? c : sizeof(x) * CHAR_BIT;
}
lz_ubsan:
xorl %eax, %eax
cmpl $1, %edi
sbbl %eax, %eax
orl %edi, %eax
lzcntl %eax, %ecx
testl %edi, %edi
movl $32, %eax
cmovnel %ecx, %eax
retq
tz_ubsan:
xorl %eax, %eax
cmpl $1, %edi
sbbl %eax, %eax
orl %edi, %eax
tzcntl %eax, %ecx
testl %edi, %edi
movl $32, %eax
cmovnel %ecx, %eax
retq
To prevent ubsan warnings, we mustn't call the ctz/clz builtins with a
zero-value. So we typically insert an all-bits value to make it shutup
Unfortunately this conflicts with the post-combine that recognise cases where
the zero-value input should be correctly handled (e.g tzcnt/lzcnt x86
instructions).
Funnily enough, gcc has better code from the ubsan-safe cases.....
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