http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47949
Summary: Missed optimization for -Os using xchg instead of mov. Product: gcc Version: 4.6.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: target AssignedTo: unassig...@gcc.gnu.org ReportedBy: svfue...@gmail.com Target: x86 / amd64 xchg %eax, reg is a one-byte instruction. If reg is dead, this instruction could replace the two-byte mov reg, %eax for a one-byte savings. ie: int foo(int x) { return x; } currently compiles to mov %edi,%eax retq with -Os, whereas the following may be better: xchg %eax, %edi retq (Similar cases exist with mov reg, %rax; mov reg, %ax; and mov reg, %al) Note that xchg is slower than mov, so this is only an optimization when size is more important than speed.