https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123762
Bug ID: 123762
Summary: missed optimization for exact integer division by a
constant
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vincent-gcc at vinc17 dot net
Target Milestone: ---
Consider the following code.
unsigned long f (unsigned long i)
{
if (i % 3)
__builtin_unreachable ();
return i / 3;
}
With gcc (Debian 20260121-1) 16.0.1 20260121 (experimental) [trunk
r16-6952-g14cd2833b27], using -O3, I get the following code on x86_64:
movabsq $-6148914691236517205, %rax
mulq %rdi
movq %rdx, %rax
shrq %rax
ret
However, since
if (i % 3)
__builtin_unreachable ();
ensures that i is a multiple of 3, the division by 3 could be done just by
multiplying by the inverse of 3 mod 2^64.
