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Re: Disappeared flag: -maes on -march=ivybridge, present in -march=native


On Monday, 29 July 2019, Marc Glisse wrote:
This is a repeat of commit r263989, which commit r264052 accidentally reverted.
(...) As you can see, this is very much on purpose. See https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01940.html for the explanation that came with the patch.

D'oh! I shouldn't have investigated this in the middle of the night,
apparently. I grepped the history for `r263989`, but the commit doesn't
contain that string, but instead
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263989
(so without the `r` prefix). I didn't find the commit so I discarded the
idea of trying to find it way too quickly. Especially given the fact, that the repeated commit was found by the bisect and not the original one, I randomly decided to stop looking for it. Should've investigated this further after starting the day clear-headed!

On Monday, 29 July 2019 07:47 CEST Thiago Macieira wrote:
Older versions of GCC turned AES on for -march=westmere and up, but not anymore. The commit you found changed that.
Ok, gotcha! I didn't know that not all machines with those platforms
have AES.

if you ran GCC [...] you'd get a fully working build for your CPU using all CPU features you can use.

That's different from LLVM, which attempts to find which CPU most closely matches yours. In that VM scenario, it would either disable some features you could use or enable ones you can't.

Thanks for the interesting pointers, and both of your answers!

--
Kevin Weidemann


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