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

[Peter Zijlstra]

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.

> 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).