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): 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 _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
