Re: AMDGPU and 16B stack alignment

[Alex Deucher]

On Tue, Oct 15, 2019 at 2:07 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Tue, Oct 15, 2019 at 12:19 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >
> > On Tue, Oct 15, 2019 at 9:08 AM S, Shirish <sshankar@xxxxxxx> wrote:
> > > On 10/15/2019 3:52 AM, Nick Desaulniers wrote:
> >
> > > My gcc build fails with below errors:
> > >
> > > dcn_calcs.c:1:0: error: -mpreferred-stack-boundary=3 is not between 4 and 12
> > >
> > > dcn_calc_math.c:1:0: error: -mpreferred-stack-boundary=3 is not between 4 and 12
>
> I was able to reproduce this failure on pre-7.1 versions of GCC.  It
> seems that when:
> 1. code is using doubles
> 2. setting -mpreferred-stack-boundary=3 -mno-sse2, ie. 8B stack alignment
> than GCC produces that error:
> https://godbolt.org/z/7T8nbH
>
> That's already a tall order of constraints, so it's understandable
> that the compiler would just error likely during instruction
> selection, but was eventually taught how to solve such constraints.
>
> > >
> > > While GPF observed on clang builds seem to be fixed.
>
> Thanks for the report.  Your testing these patches is invaluable, Shirish!
>
> >
> > Ok, so it seems that gcc insists on having at least 2^4 bytes stack
> > alignment when
> > SSE is enabled on x86-64, but does not actually rely on that for
> > correct operation
> > unless it's using sse2. So -msse always has to be paired with
> >  -mpreferred-stack-boundary=3.
>
> Seemingly only for older versions of GCC, pre 7.1.
>
> >
> > For clang, it sounds like the opposite is true: when passing 16 byte
> > stack alignment
> > and having sse/sse2 enabled, it requires the incoming stack to be 16
> > byte aligned,
>
> I don't think it requires the incoming stack to be 16B aligned for
> sse2, I think it requires the incoming and current stack alignment to
> match. Today it does not, which is why we observe GPFs.
>
> > but passing 8 byte alignment makes it do the right thing.
> >
> > So, should we just always pass $(call cc-option, -mpreferred-stack-boundary=4)
> > to get the desired outcome on both?
>
> Hmmm...I would have liked to remove it outright, as it is an ABI
> mismatch that is likely to result in instability and non-fun-to-debug
> runtime issues in the future.  I suspect my patch does work for GCC
> 7.1+.  The question is: Do we want to either:
> 1. mark AMDGPU broken for GCC < 7.1, or
> 2. continue supporting it via stack alignment mismatch?
>
> 2 is brittle, and may break at any point in the future, but if it's
> working for someone it does make me feel bad to outright disable it.
> What I'd image 2 looks like is (psuedo code in a Makefile):

Well, it's been working as is for years now, at least with gcc, so I'd
hate to break that.

Alex

>
> if CC_IS_GCC && GCC_VERSION < 7.1:
>   set stack alignment to 16B and hope for the best
>
> So my diff would be amended to keep the stack alignment flags, but
> only to support GCC < 7.1.  And that assumes my change compiles with
> GCC 7.1+. (Looks like it does for me locally with GCC 8.3, but I would
> feel even more confident if someone with hardware to test on and GCC
> 7.1+ could boot test).
> --
> Thanks,
> ~Nick Desaulniers
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