Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs
From: Nick Desaulniers
Date: Wed Jun 26 2019 - 18:14:27 EST
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:
> 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
> .byte STATIC_KEY_INIT_NOP
> 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.