https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105521
Bug ID: 105521
Summary: missed optimization in modulo arithmetic
Product: gcc
Version: 11.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: goswin-v-b at web dot de
Target Milestone: ---
I'm trying to compute (a*a)%n for uint64_t types on x86_64 using "gcc -O2 -W
-Wall" like this:
#include <stdint.h>
#include <assert.h>
uint64_t sqrmod(uint64_t a, uint64_t n) {
assert(a < n);
unsigned __int128 x = a;
x *= a;
return x % n;
}
I expected to get the following code:
sqrmod:
cmpq %rsi, %rdi
jnb .L13 // assert(a < n) failure
movq %rdi, %rax
mul %rdi
div %rsi
movq %rdx, %rax
ret
The compiler does get the "mul" right but instead of the "div" it throws in a
call to "__umodti3". The "__umodti3" function is horribly long code that will
be worlds slower than a simple div.
Note: The "asset(a < n);" should tell the compiler that the "div" instruction
can not overflow and will not cause a #DivisionError. Without the assert the
compiler could (conditionally) add "a %= n;" for the same effect.
https://godbolt.org/z/cd57Wd4oo
