https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91881
Bug ID: 91881
Summary: Value range knowledge of higher bits not used in
optimizations
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the example:
unsigned long long sample2(unsigned long long m) {
if (m >= 100) __builtin_unreachable();
m *= 16;
return m >> 3;
}
After the `if` statement we do know that the higher bits are set to 0. So
instead of generating the following assembly:
sample2(unsigned long long):
mov rax, rdi
sal rax, 4
shr rax, 3
ret
A more optimal assembly could be generated:
sample2(unsigned long long):
lea rax, [rdi + rdi]
ret
Godbolt playground: https://godbolt.org/z/1iSpTh
P.S.: that optimization is important for std::to_chars(..., double) like
functions, where a significant of a double is extracted into an unsigned long
long variable, so its upper bits are always zero.
