X cannot launch on r300 GPU and/or AVX-less CPU if Mesa was built with swr enabled
Repeat of bug#107655 which for some reason is absent here, along with whole Mesa bug-list.
So, long story short, swr proved to be nice-to-have alternative to softpipe and llvmpipe for troubles with r600 and amdgpu hardware on my desktop with AVX-capable AMD FX-6100 CPU. However, the same build of Mesa crashes X server on startup on my old AVX-less AMD Athlon 64 X2 TK-57 CPU & r300 GPU laptop. Even if libswrAVX* are packaged separately and not installed, X still crashes. Only when swr is disabled at build, r300 can initialize.
Moreover, for a while building swr was completely broken and even now, when my distro switched to using meson for building Mesa, I have to use
find src/gallium/drivers/swr/rasterizer/jitter -iname "gen_*.hpp" -delete hack in spec-file for it to build. It doesn't seem to matter if Mesa is built with gcc or clang.