https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120427
--- Comment #10 from H.J. Lu <hjl.tools at gmail dot com> --- (In reply to Harald van Dijk from comment #9) > (In reply to H.J. Lu from comment #2) > > We shouldn't transform "mov $-1,reg" to "push $-1; pop reg". > > We should transform "mov $-1,reg" to "or $-1,reg" instead. > > Is that really an improvement? For 32-bit registers, sure, it's fine, but > for 64-bit registers this increases code size, does it not? orq $-1, %rax is > four bytes, when pushq $-1; popq %rax is only three. By default, GCC optimizes for performance. "mov $-1,reg" is write. Other encodings are write after read and maybe more. If you want the smallest code size, please use -Oz.
