Re: AMDGPU and 16B stack alignment

[Nick Desaulniers]

On Tue, Oct 15, 2019 at 11:30 AM Alex Deucher <alexdeucher@xxxxxxxxx> wrote:
>
> 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.

Ok, I'm happy to leave that as is for GCC, then.  Would you prefer I
modify it for GCC >7.1 or just leave it alone (maybe I'll add a
comment about *why* it's done for GCC)? Would you prefer 1 patch or 4?

>
> Alex
>
> >
> > if CC_IS_GCC && GCC_VERSION < 7.1:
> >   set stack alignment to 16B and hope for the best

Ie, this ^

> >
> > 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