[llvm-commits] [llvm] r95195 - in /llvm/trunk: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp lib/Target/X86/X86ISelLowering.cpp test/CodeGen/X86/tailcall2.ll test/CodeGen/X86/tailcallfp2.ll

[Dan Gohman]

On Feb 2, 2010, at 7:28 PM, Evan Cheng wrote:

> Author: evancheng
> Date: Tue Feb  2 21:28:02 2010
> New Revision: 95195
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=95195&view=rev
> Log:
> Allow all types of callee's to be tail called. But avoid automatic tailcall if the callee is a result of bitcast to avoid losing necessary zext / sext etc.

Hi Evan,

I believe the real problem here is not bitcasted callees (which
can't be reliably detected, since the bitcast could happen in code
that isn't visible to the compiler). I believe the real problem
here is with the signext attribute.

In this code, with a direct call:

declare signext i16 @foo(i32) nounwind

define signext i16 @t7() nounwind {
entry:
  %0 = tail call signext i16 @foo(i32 0) nounwind
  ret i16 %0
}

LLVM without the tailcall optimization emits this:

	callq	_foo
	movswl	%ax, %ecx

This means that even though the return types and attributes
match, CodeGen still believes it needs to do caller-side sign
extension. I don't know the details of why that is, but it
says that it's not safe to do a tail call with a  signext
return attribute, regardless of what the callee is.

The same appears to apply to the zeroext attribute also.

Also, I've confirmed that GCC also uses caller-side casts and
suppresses tail calls for both signed short and unsigned short.

Dan