Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 9 Apr 2024 08:37:10 -0700 | Subject | Re: More annoying code generation by clang |
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On Tue, Apr 9, 2024 at 3:45 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Apr 08, 2024 at 12:42:31PM -0700, Linus Torvalds wrote: > > > Actually, one of the github issues pages has more of an explanation > > (and yes, it's tied to impedance issues between the inline asm syntax > > and how clang works): > > > > https://github.com/llvm/llvm-project/issues/20571#issuecomment-980933442 > > > > So that same issue seems to suggest Nick is actually working on this and > got stuff merged. Nick, what is the status of your efforts and should we > indeed do the below as Linus suggests or should he upgrade his compiler?
Sorry, I'm no longer working on the issue. I should mark that as such.
The feature got hung up on rewriting one of the register allocation frameworks in llvm. https://github.com/llvm/llvm-project/pull/74344
I have a new set of responsibilities at work so I probably wont be working on that issue any time soon.
> > > diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h > > index 49feac0162a5..0dee061fd7a6 100644 > > --- a/include/linux/compiler-clang.h > > +++ b/include/linux/compiler-clang.h > > @@ -118,3 +118,15 @@ > > > > #define __diag_ignore_all(option, comment) \ > > __diag_clang(13, ignore, option) > > + > > +/* > > + * clang has horrible behavior with "g" or "rm" constraints for asm > > + * inputs, turning them into something worse than "m". Avoid using > > + * constraints with multiple possible uses (but "ir" seems to be ok): > > + * > > + * https://github.com/llvm/llvm-project/issues/20571 > > + * https://github.com/llvm/llvm-project/issues/30873 > > + * https://github.com/llvm/llvm-project/issues/34837
20571 is the cannonical bug for this.
> > + */ > > +#define ASM_INPUT_G "ir" > > +#define ASM_INPUT_RM "r" > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h > > index 2abaa3a825a9..e53acd310545 100644 > > --- a/include/linux/compiler_types.h > > +++ b/include/linux/compiler_types.h > > @@ -380,6 +380,15 @@ struct ftrace_likely_data { > > #define asm_goto_output(x...) asm volatile goto(x) > > #endif > > > > +/* > > + * Clang has trouble with constraints with multiple > > + * alternative behaviors (mainly "g" and "rm"). > > + */ > > +#ifndef ASM_INPUT_G > > + #define ASM_INPUT_G "g" > > + #define ASM_INPUT_RM "rm" > > +#endif > > + > > #ifdef CONFIG_CC_HAS_ASM_INLINE > > #define asm_inline asm __inline > > #else > > -- > > 2.44.0.330.g4d18c88175 > > >
-- Thanks, ~Nick Desaulniers
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