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

[Linus Torvalds]

On Wed, May 6, 2020 at 6:51 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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
>         1:
>                 retq

If you want to just avoid the 'cmov', the best way to do that is to
insert a barrier() on one side of the if-statement.

That breaks the ability to turn the conditional jump into a cmov.

HOWEVER.

It looks like noth clang and gcc will move the indirect jump to the
conditional sites, but then neither of them is smart enough to just
turn the indirect jump into one direct jump.

Strange. So you still get an indirect call for just the "ret" case.
The code looks actively stupid with

gcc:
  .L7:
        movl    $__static_call_nop, %eax
        jmp     *%rax

clang:
  .LBB1_1:
        mov     eax, offset __static_call_nop
        jmp     rax                     # TAILCALL

despite the barrier not being between those two points. The only
difference is the assembler syntax.

Odd. That's such a trivial and obvious optimization. But presumably
it's a pattern that just doesn't happen normally.

                Linus