https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98425
Bug ID: 98425 Summary: Superfluous sign-extend for constrained integer Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: rtl-optimization Assignee: unassigned at gcc dot gnu.org Reporter: koenigni at gcc dot gnu.org Target Milestone: --- Hello everyone, A small missed optimization I noticed while toying around with the difference between signed and unsigned integers. The following code int baz(int *p, int i) { int j; if (i >= 0) { j = i + 4; return p[j]; } else __builtin_unreachable(); } is compiled with `gcc -O3 -S` to baz: addl $4, %esi movslq %esi, %rsi movl (%rdi,%rsi,4), %eax ret The movslq instruction is unnecessary since i is constrained to never be negative and therefore no sign extension is needed. This probably also prevents the addl instructions to be removed and the offset being put into the movl. For comparison, clang (with the same options) compiles the code to baz: movl %esi, %eax movl 16(%rdi, %rax, 4), %eax ret Optimal would probably be baz: movl 16(%rdi, %rsi, 4), %eax ret