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