You should be able to see it. REX prefixes cost a lot less than register spills do. If you mean literally `gcc -S`, -O0 is worse than not optimized and basical

You should be able to see it. REX prefixes cost a lot less than register spills do. If you mean literally `gcc -S`, -O0 is worse than not optimized and basically keeps everything in memory to make it easier to debug. -Os is the one with readable sensible asm.