On Wed, Jun 26, 2019 at 1:49 AM Peter Zijlstra <pet...@infradead.org> 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.gt3...@hirez.programming.kicks-ass.net > > 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
