Messages in this thread | | | From | Nick Desaulniers <> | Date | Fri, 4 Aug 2023 09:15:54 -0700 | Subject | Re: [PATCH] LoongArch: Drop -ffreestanding from CFLAGS |
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On Fri, Aug 4, 2023 at 8:56 AM Xi Ruoyao <xry111@xry111.site> wrote: > > On Fri, 2023-08-04 at 08:46 -0700, Nick Desaulniers wrote: > > > Not all processors support unaligned access, so we need the > > > alternative mechanism to select memset/memcpy/memmove implementations. > > > If remove -ffreestanding, the builtin implementation cannot be used on > > > all hardware. > > > > That sounds like a compiler bug in that compiler's implementation of > > string.h builtins then; it should default to the safest implementation > > (aligned accesses) until instructed otherwise. Have you filed a bug > > against your compiler vendor for which compiler you observe that > > behavior from? > > AFAIK there is no such bug in GCC. > > But GCC indeed has a bug about builtin expansion: it generates really > stupid code for __builtin_memcpy and friends. See > https://gcc.gnu.org/PR109465. The bug is fixed for GCC 14, but GCC 14 > won't be released soon.
Perhaps -fno-builtin-* flags may be of help here, and more precise an incision than the blunt -ffreestanding.
> > > At the very least, there should be a comment above the addition of > > -ffreestanding justifying why it's being used, probably with a link to > > the above bug report. > > > > I would expect either -mcpu or perhaps some other -m flag to guide the > > compiler when it is safe to emit memcpy (and friends) in terms of > > unaligned access or not. > > It's controlled by -m{no-,}strict-align. LoongArch kernel defaults to - > mstrict-align, with this the compiler should not generate unaligned > access. -mno-strict-align is hidden behind CONFIG_EXPERT (FWIW I > personally dislike the decision to hide it).
If GCC generates unaligned accesses in its implementation of __builtin_memcpy when -mstrict-align is set, then that is a bug in GCC and should be reported.
Clang users should not have to pay for such mistakes.
> > -- > Xi Ruoyao <xry111@xry111.site> > School of Aerospace Science and Technology, Xidian University
-- Thanks, ~Nick Desaulniers
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