https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114760
Bug ID: 114760
Summary: traling zero count detection failure
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jiangning.liu at amperecomputing dot com
Target Milestone: ---
For this small case, gcc failed to detect trailing zero count calculation, so
the x86 instruction tzcnt cannot be generated, but clang can generate it.
unsigned ntz32_6a(unsigned x) {
int n;
n = 32;
while (x != 0) {
n = n - 1;
x = x + x;
}
return n;
}
If we slightly change "x = x + x" to "x = x << 1", the optimization will just
work.
unsigned ntz32_6a(unsigned x) {
int n;
n = 32;
while (x != 0) {
n = n - 1;
x = x << 1;
}
return n;
}
It seems number_of_iterations_cltz/number_of_iterations_cltz_complement in
tree-ssa-loop-niter.cc or somewhere else need to be enhanced.
