[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

[Evan Cheng]

On Feb 3, 2010, at 11:33 AM, Evan Cheng wrote:

> 
> On Feb 3, 2010, at 11:13 AM, Dan Gohman wrote:
> 
>> 
>> 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.
> 
> Yes I am aware this is one of the issues (it caused miscompilation of llvm Module.cpp and others). For now, I think it makes sense to avoid tail calling these because the we can't reasonable claim the caller / callee signatures would match when a bitcast is involved.

Oh you mean we should disable tailcall opt when either the caller or callee result is sext / zext.

Right now we allow tailcall opt when their attributes match. If the calling conventions match I don't see why that's not sufficient. Certainly I am not seeing any evidence that's a problem yet.

Evan

> 
> Evan
> 
>> 
>> 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
>> 
> 
> 
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