https://bugs.llvm.org/show_bug.cgi?id=46434
Bug ID: 46434
Summary: Failure to recognize table-based ctz implementation
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: gabrav...@gmail.com
CC: llvm-bugs@lists.llvm.org
int f(unsigned x)
{
static const char table[32] = {0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15,
25, 17, 4, 8, 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9};
return table[((unsigned)((x & -x) * 0x077CB531U)) >> 27];
}
This can be optimized to `__builtin_ctz(x);`. This transformation is done by
GCC (or at least https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838 claims to
do so, though I can't get it to trigger), but not by LLVM.
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