llvm dev - Jan 2011 - [LLVMdev] X86 -tailcallopt and C calling conversion

Happy 2011, everybody!

It seems -tailcallopt prevents tailcall optimization when both caller
and callee have ccc,
even when it is optimized without an option -tailcallopt.
Is it intended or misoptimized?

In X86ISelLowering.cpp:X86TargetLowering::IsEligibleForTailCallOptimization():

  if (GuaranteedTailCallOpt) {
    if (IsTailCallConvention(CalleeCC) && CCMatch)
      return true;
    return false;
  }

I know -tailcallopt changes calling conversion of fastcc to callee-pop.

ps. I am tweaking tailcallopt on Win64.

...Takumi

On Jan 1, 2011, at 4:20 PM, NAKAMURA Takumi wrote:
> Happy 2011, everybody!
> 
> It seems -tailcallopt prevents tailcall optimization when both caller
> and callee have ccc,
> even when it is optimized without an option -tailcallopt.
Sorry, I don't understand your question. What do you mean by both caller and
callee have ccc?

Evan
> Is it intended or misoptimized?
> 
> In
X86ISelLowering.cpp:X86TargetLowering::IsEligibleForTailCallOptimization():
> 
>  if (GuaranteedTailCallOpt) {
>    if (IsTailCallConvention(CalleeCC) && CCMatch)
>      return true;
>    return false;
>  }
> 
> I know -tailcallopt changes calling conversion of fastcc to callee-pop.
> 
> ps. I am tweaking tailcallopt on Win64.
> 
> ...Takumi
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hello Evan,

I am sorry for my poor question.
s/ccc/default calling conversion/g ;)
("ccc" should be accepted as calling conversion specifier)

When "-tailcallopt" is specified (GuaranteedTailCallOpt),
tailcall optimizer would not touch functions of default calling conversion.


; for example
define void @caller_c() nounwind {
entry:
  tail call void @callee_c()
  ret void
}
declare void @callee_c() nounwind

define fastcc void @caller_f() nounwind {
entry:
  tail call fastcc void @callee_f()
  ret void
}
declare fastcc void @callee_f() nounwind


On {i686|x86_64}-linux without -tailcallopt,
caller_c:
        jmp     callee_c                # TAILCALL
caller_f:
        jmp     callee_f                # TAILCALL


With -tailcallopt, (on x86-64. simila on i686 too)
caller_c:
        pushq   %rax
        callq   callee_c
        popq    %rax
        ret
caller_f:
        pushq   %rax
        popq    %rax
        jmp     callee_f                # TAILCALL


...Takumi