Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

[Nick Desaulniers]

On Wed, Jun 26, 2019 at 1:49 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 25, 2019 at 11:15:57AM -0700, Nick Desaulniers wrote:
>
> > Unreleased versions of Clang built from source can;
>
> I've bad experiences with using unreleased compilers; life is too short.

Yes; but before release is when they need the help the most in order
for testing to find regressions.

>
> > We're currently planning multiple output constraint support w/ asm
> > goto, and have recently implemented things like
> > __GCC_ASM_FLAG_OUTPUTS__.
>
> That's good to hear.
>
> > If there's other features that we should
> > start implementing, please let us know.
>
> If you've got any ideas on how to make this:
>
>   https://lkml.kernel.org/r/20190621120923.GT3463@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> work, that'd be nice. Basically I wanted the asm goto to emit a 2 or 5
> byte JMP/NOP depending on the displacement size. We can trivially get
> JMP right by using:
>
>         jmp \l_yes
>
> and letting the assembler sort it, but getting the NOP right has so far
> eluded me:
>
> .if \l_yes - (. + 2) < 127
>         .byte 0x66, 0x90
> .else
>         .byte STATIC_KEY_INIT_NOP
> .endif
>
> doesn't work. We can ofcourse unconditionally emit the JMP and then
> rewrite the binary afterward, and replace the emitted jumps with the
> right size NOP, but that's a bit yuck.
>
> Once it emits the variable size instruction consistently, we can update
> the patching side to use the same condition to select the new
> instruction (and fix objtool).

Not sure; the assembler directives and their requirements aren't
something I'm too familiar with.

-- 
Thanks,
~Nick Desaulniers