Messages in this thread | | | Date | Wed, 6 May 2020 15:51:28 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 14/18] static_call: Add static_cond_call() |
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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.
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