https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78115
Bug ID: 78115
Summary: Missed optimization for "int modulo 2^31"
Product: gcc
Version: 6.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: tkoeppe at google dot com
Target Milestone: ---
Consider the operation of mapping an int to the unique modular representative
in [0, 2^31).
Readable code:
#include <climits>
int mod31(int num) {
if (num < 0) { num = num + 1 + INT_MAX; }
return num;
}
Paranoid bit-shifter's code:
int mod31shift(int num) {
return static_cast<unsigned int>(num) % (1U << 31);
}
Clang generates the same machine code for both, but GCC does not:
https://godbolt.org/g/2BjNqA
