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

Peter Zijlstra
2020-05-06T13:51:53Z

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
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.

> Us to remove the conditional check that a volatile read
> wasn't NULL?

No, obviously the load is required, and the READ_ONCE() is so that the
compiler will not emit 2 different loads (just for giggles).

That is:

tmp1 = name.func;
if (!tmp) {
tmp2 = name.func;
tmp2(args);
}

is a valid translation of:

if (!name.func)
name.func(args)

and allows for a NULL dereference (as noted by Rasmus).

What I did do want, per the above, is to avoid the indirect (tail) call.
Because indirect jmp/call are evil and expensive.

> I am simultaneously impressed
> and disgusted by this btw, cool stuff.

Yes, it's nasty, esp the casting of a function pointer like that is
gruesome.