https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838
Bug ID: 90838
Summary: Detect table-based ctz implementation
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
531.deepsjeng_r has a hot function doing something like:
static const unsigned long long magic = 0x03f08c5392f756cdULL;
static const int table[64] = {
0, 1, 12, 2, 13, 22, 17, 3,
14, 33, 23, 36, 18, 58, 28, 4,
62, 15, 34, 26, 24, 48, 50, 37,
19, 55, 59, 52, 29, 44, 39, 5,
63, 11, 21, 16, 32, 35, 57, 27,
61, 25, 47, 49, 54, 51, 43, 38,
10, 20, 31, 56, 60, 46, 53, 42,
9, 30, 45, 41, 8, 40, 7, 6,
};
int
myctz (unsigned long long b) {
unsigned long long lsb = b & -b;
return table[(lsb * magic) >> 58];
}
This is equivalent to __builtin_ctzl (b).
Would it be possible to match this in GCC?
