On Mon, Jan 24, 2022 at 7:32 PM Thiago Macieira <[email protected]> wrote:
> Today, only in the two files with "_avx2" in the name, unless you raise > the > CPU architecture yourself while configuring Qt. My proposal is to get that > on > Linux by default with the v3 extra build. > Yes, this I got from the discussion. However the code in question isn't in *_avx2. In any case I don't have an objection or an opinion even (about the original discussion), that default should work for my case, though. For *your* code, you need to explicitly enable FMA yourself. The changes > proposed in this thread do not affect user code. > *My* code, is supposed to fix Qt code at some point, so that's why I'm asking. I know I have to do it myself for user code, that's not an issue. > Both glibc, macOS libSystem, and Windows' ucrtbase have implementations > using > the FMA instruction and will detect at runtime which one to use (this was > the > *only* place in ucrtbase I found AVX2/FMA code). You're out of luck if > you're > using MinGW, though. > I suppose that could work, but I'd rather avoid doing the call if I can help it. > All compilers tested will use the FMA instruction if you enabled the CPU > feature, otherwise they'll make a call to the math lbrary's fma() function. Compare: https://gcc.godbolt.org/z/c5385d5GY (no FMA) > to: https://gcc.godbolt.org/z/jf3zhsjPf (with FMA) > Yes, I already looked at the assembly, which is exactly why I'm asking about it.
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