Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers

[Uros Bizjak]

On 4. 03. 25 00:59, H. Peter Anvin wrote:
> On March 3, 2025 2:45:48 PM PST, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > On Mon, Mar 03, 2025 at 02:31:50PM -0800, H. Peter Anvin wrote:
> > > > +#ifdef CONFIG_UNWINDER_FRAME_POINTER
> > > > #define ASM_CALL_CONSTRAINT "r" (__builtin_frame_address(0))
> > > > +#else
> > > > +#define ASM_CALL_CONSTRAINT
> > > > +#endif
> > > >
> > > > #endif /* __ASSEMBLY__ */
> > >
> > > Wait, why was this changed? I actually tested this form at least once
> > > and found that it didn't work under all circumstances...
> >
> > Do you have any more details about where this didn't work?  I tested
> > with several configs and it seems to work fine.  Objtool will complain
> > if it doesn't work.
> >
> > See here for the justification (the previous version was producing crap
> > code in Clang):
> >
> >   https://lore.kernel.org/dbea2ae2fb39bece21013f939ddeb15507baa7d3.1740964309.git.jpoimboe@xxxxxxxxxx
>
> I need to dig it up, but I had a discussion about this with some gcc people about a year ago.

It was discussed here [1].

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117311

Uros.