On Thu, May 26, 2022 at 3:12 AM Andrew Haley via Gcc-help < gcc-help@xxxxxxxxxxx> wrote: > On 5/25/22 19:58, Tom Kacvinsky via Gcc-help wrote: > > Does anyone have a quick rundown on what would have changed between GCC > > 8.3.0 and 12.1.0 in terms of what -mtune=generic does for optimizations? > > There was a vast amount of work done on many things, so we can't easily > say what affected you. Generally speaking GCC gets better release to > release, > but bear in mind that compiler optimizations are often based on heuristics. > That is to say, they make code better on average, but sometimes they might > make things worse. If your degradation is slight overall, there might be an > actual regression in the compiler, but chances are you just got unlucky. > I did have an idea of diffing the gcc/config/i386 folder between the 8.3 and 12.1 release tags, but that gave a _lot_ of stuff about support for AVX512. A colleague of mine reminded me of this thread: slowdown with -std=gnu18 with respect to -std=c99 So I checked what the default C standard for GCC 8.3.0 is (gnu11) and 12.1.0 (gnu17). So I used -std=c99 with the GCC 12.1 build of our code, and things went back to normal (and sometimes a teensy bit better). For what it's worth, the performance issue was death by a thousand cuts - there was no glaring hotspot, and least by looking at callgrind output. Tom
