configure: fix --cpu=host for Intel Celeron and Pentium #507
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class processors. This is necessary because they don't support AVX, but e.g. gcc's -march=haswell enables AVX instructions; however, the Haswell Pentium G3258 does not support AVX, which -march=native correctly selects. cpu cannot be set to native, since then the conditional checks, for e.g. fast cmov, are not executed.
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Executive summary: It looks good for me. Details: I saved the patch to /etc/portage/patches/media-tv/mythtv and emerged mythtv-31.0_p20210731. [This is a Gentoo-ism for applying patches during a build while continuing to use the package management system.] I'm compiling on a Haswell Celeron. The configuration set HAVE_LRINT correctly (the first symptom of the problem encountered). Running 'ps' during the build, I see -march=native on the gcc command lines, but on the cc1 command lines, I see -march=haswell, but then with tons of explicit flags including -mno-avx, so I think this is just exposing some irrelevant gcc internals, and it's doing the right thing. I didn't do any further code testing, but it appears to me that it solves the problem. (I also didn't closely examine the code changes.) Doing a (More show up as the build progresses.) I don't see anywhere where those defines are used unless I'm missing something, so I assume it's harmless. |
I suspect this has to do with how
moc is the Qt Meta-Object Compiler, so that is probably fine. |
For GCC: x86: "Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set." AArch64: "Additionally on native AArch64 GNU/Linux systems the value ‘native’ tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system." Similar for other architectures. Note: this is equivalent to -mcpu=native for architectures that support that option; notably, for x86, -mcpu is a deprecated synonym of -mtune. However, on x86, -march implies -mtune in this and most cases. For clang: from its command line reference for -mtune=<arg>: "Only supported on X86 and RISC-V. Otherwise accepted for compatibility with GCC."
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I added another commit to set |
class processors.
This is necessary because they don't support AVX, but e.g. gcc's
-march=haswell enables AVX instructions; however, the Haswell
Pentium G3258 does not support AVX, which -march=native correctly
selects.
cpu cannot be set to native, since then the conditional checks,
for e.g. fast cmov, are not executed.
This should fix #505 in master and can be cherry-picked cleanly into fixes/31 and fixes/31.