Re: [PATCH v4 14/18] static_call: Add static_cond_call()

Peter Zijlstra
2020-05-06T16:00:50Z

On Wed, May 06, 2020 at 03:51:28PM +0200, Peter Zijlstra wrote:
> On Tue, May 05, 2020 at 11:13:53AM -0700, Nick Desaulniers wrote:
> > On Tue, May 5, 2020 at 2:36 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > >
> > > HJ, Nick,
> > >
> > > Any chance any of you can see a way to make your respective compilers
> > > not emit utter junk for this?
> > >
> > > On Mon, May 04, 2020 at 10:14:45PM +0200, Peter Zijlstra wrote:
> > >
> > > > https://godbolt.org/z/SDRG2q
> >
> > Woah, a godbolt link! Now we're speaking the same language. What were
> > you expecting?
>
> Given the output for x86-64 clang (trunk)
>
> bar: # @bar
> movl %edi, .L_x$local(%rip)
> retq
> ponies: # @ponies
> movq .Lfoo$local(%rip), %rax
> testq %rax, %rax
> movl $__static_call_nop, %ecx
> cmovneq %rax, %rcx
> jmpq *%rcx # TAILCALL
> __static_call_nop: # @__static_call_nop
> retq
> _x:
> .L_x$local:
> .long 0 # 0x0
>
> foo:
> .Lfoo$local:
> .zero 8
>
>
> I was hoping for:
>
> bar: # @bar
> movl %edi, .L_x$local(%rip)
> retq
> ponies: # @ponies
> movq .Lfoo$local(%rip), %rax
> testq %rax, %rax
> jz 1f
> jmpq *%rcx # TAILCALL

Obviously this then wants to be *%rax.

> 1:
> retq
>
> That avoids the indirect call (possible retpoline) and does an immediate
> return.
>
> So it does 2 things different:
>
> - it realizes the NULL case is a constant and uses an
> immediate call and avoids the indirect call/jmp.
>
> - it realizes __static_call_nop() is a no-op and avoids the call
> entirely and does an immediate return.

IOW, have it inline __static_call_nop().