Messages in this thread | | | From | Saleem Abdulrasool <> | Date | Fri, 16 Dec 2022 12:56:23 -0800 | Subject | Re: [PATCH] riscv: avoid enabling vectorized code generation |
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On Fri, Dec 16, 2022 at 11:54 AM Palmer Dabbelt <palmer@dabbelt.com> wrote: > > On Fri, 16 Dec 2022 11:45:21 PST (-0800), ben.dooks@codethink.co.uk wrote: > > > > > > On 2022-12-16 18:50, Saleem Abdulrasool wrote: > >> The compiler is free to generate vectorized operations for zero'ing > >> memory. The kernel does not use the vector unit on RISCV, similar to > >> architectures such as x86 where we use `-mno-mmx` et al to prevent the > >> implicit vectorization. Perform a similar check for > >> `-mno-implicit-float` to avoid this on RISC-V targets. > > > > I'm not sure if we should be emitting either of the vector or floating > > point instrucitons in the kernel without explicitly marking the section > > of code which is using them such as specific accelerator blocks. > > Yep, we can't let the compiler just blindly enable V or F/D. V would > very much break things as we have no support, but even when that's in > we'll we at roughly the same spot as F/D are now where we need to handle > the lazy save/restore bits. > > This looks like an LLVM-only option, I see at least some handling here > > https://github.com/llvm/llvm-project/blob/a72883b7612f5c00b592da85ed2f1fd81258cc08/clang/lib/Driver/ToolChains/Clang.cpp#L2098 > > but I don't really know LLVM enough to understand if there's some > default for `-mimplicit-float` and I can't find anything in the docs. > If it can be turned on by default and that results in F/D/V instructions > then we'll need to explicitly turn it off, and that would need to be > backported.
Yes, this is an LLVM option, but I think that the `cc-option` wrapping should help ensure that we do not break the gcc build. This only recently was added to clang, so an older clang would also miss this flag. The `-mimplicit-float` is the default AFAIK, which is why we needed to add this flag in the first place. Enabling V exposed this, which is why the commit message mentions vector.
> > Maybe Nick or Nathan knows what's up here?
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