https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104574
Bug ID: 104574 Summary: GCC misses basic optimization for restricted pointers Product: gcc Version: 12.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: m...@nieper-wisskirchen.de Target Milestone: --- The following TU void g (void); int f (int *restrict p) { p[0] = 7; g (); return p[0]; } is compiled by GCC at -O3 to f: pushq %rbx movq %rdi, %rbx movl $7, (%rdi) call g movl (%rbx), %eax popq %rbx ret Obviously, GCC seems to think that g may modify the data reachable through p. However, if g did that it would cause undefined behavior anyway. So GCC misses a simple optimization opportunity here; it doesn't have to reload the memory contents after the call to g; in fact, the function will always return 7. For comparison, Clang compiles the TU to f: pushq %rax movl $7, (%rdi) callq g movl $7, %eax popq %rcx retq