Why does GCC generate 15-20

Question

I first noticed in 2009 that GCC (at least on my projects and on my machines) have the tendency to generate noticeably faster code if I optimize for size (<

Answer 1

I'm adding this post-accept to point out that the effects of alignment on overall performance of programs - including big ones - has been studied. For example, this article (and I believe a version of this also appeared in CACM) shows how link order and OS environment size changes alone were sufficient to shift performance significantly. They attribute this to alignment of "hot loops".

This paper, titled "Producing wrong data without doing anything obviously wrong!" says that inadvertent experimental bias due to nearly uncontrollable differences in program running environments probably renders many benchmark results meaningless.

I think you're encountering a different angle on the same observation.

For performance-critical code, this is a pretty good argument for systems that assess the environment at installation or run time and choose the local best among differently optimized versions of key routines.