https://bugs.llvm.org/show_bug.cgi?id=43897
Bug ID: 43897
Summary: LLVM should not remove branches if not profitable
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: david.bolvan...@gmail.com
CC: llvm-bugs@lists.llvm.org
void foo(char **d, char **s, int n, int m) {
for (int i = 0; i < n; ++i) {
if (m == 128) // m is usually 128, hot path
__builtin_memcpy(d[i], s[i], m);
else
__builtin_memcpy(d[i], s[i], m);
}
}
LLVM (Simplify CFG?) removes branch and leaves '__builtin_memcpy(d[i], s[i],
m);'. But here, LLVM was too smart and ignored what I wanted to do - "inlined"
version of memcpy.
If I use:
if (m == 128)
__builtin_memcpy(d[i], s[i], 128);
else
__builtin_memcpy(d[i], s[i], m);
Everything is OK. Simplify CFG should not merge branches if it can propagate
constant from condition to one branch.
https://godbolt.org/z/GFfPrb
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