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Re: Disappeared flag: -maes on -march=ivybridge, present in -march=native
- From: Kevin Weidemann <kwe-lnx at postn dot eu>
- To: Marc Glisse <marc dot glisse at inria dot fr>, Thiago Macieira <thiago dot macieira at intel dot com>
- Cc: gcc at gcc dot gnu dot org
- Date: Thu, 1 Aug 2019 11:47:41 +0200
- Subject: Re: Disappeared flag: -maes on -march=ivybridge, present in -march=native
- References: <firstname.lastname@example.org> <alpine.DEB.email@example.com>
On Monday, 29 July 2019, Marc Glisse wrote:
This is a repeat of commit r263989, which commit r264052
(...) 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
(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
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!