https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86923
Bug ID: 86923
Summary: Missed optimization performing consecutive integer
sum, loop not removed
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: nightstrike at gmail dot com
Target Milestone: ---
unsigned long f(unsigned int n) {
unsigned long total = 0;
for (unsigned int i = 0; i < n; ++i)
total += i;
return total;
}
It may not be terribly useful to report this, as I can (and arguably should)
write n(n-1)/2 manually, but clang does successfully transform this into:
lea eax, [rdi - 1]
add edi, -2
imul rdi, rax
shr rdi
add rdi, rax
whereas GCC tries instead to vectorize and process the loop as written.
Obviously, O(1) is preferable to O(n) here.
This applies to both C and C++.
