[LLVMdev] Problems expanding fcmp to a libcall

Evan Cheng evan.cheng at apple.com
Thu Jun 26 15:16:52 PDT 2008

On Jun 25, 2008, at 5:13 AM, Richard Osborne wrote:

> Evan Cheng wrote:
>> On Jun 23, 2008, at 5:35 AM, Richard Osborne wrote:
>>> I'm trying to write a backend for a target with no hardware floating
>>> point support. I've added a single i32 register class. I'm wanting  
>>> all
>>> floating point operations to be lowered to library function calls.  
>>> For
>>> the most part LLVM seems to get this right. For example
>>> define double @div(double %a, double %b) {
>>> %result = fdiv double %a, %b
>>> ret double %result
>>> }
>>> is expanded to a ISD::CALL of __divdf3 which is then lowered via the
>>> LowerOperation hook of my backend.
>>> However I run into problems with fcmp. With the following code:
>>> define i1 @fmp(double %a) {
>>> %result = fcmp uno double %a, 0.000000e+00
>>> ret i1 %result
>>> }
>>> the fcmp is expanded to the a call to __unorddf2 which is then
>>> lowered via the LowerOperation hook of my backend. However for some
>>> reason
>>> there remains a ISD::CALL node with __unorddf2 in the DAG after
>>> legalization. This
>>> then causes selection to fail with
>>> Cannot yet select: 0x13b7cc0: i32,i32,ch = call 0x13b76e0,  
>>> 0x13b7800,
>>> 0x13b7800, 0x13b7800, 0x13b77a0, 0x13b78f0, 0x13b79a0, 0x13b80d0,
>>> 0x13b7a00, 0x13b78f0, 0x13b79a0, 0x13b80d0, 0x13b7a00
>>> Are there any additional steps I need to take in my target, or could
>>> this be a bug in the Legalization phase?
>> This sounds like a bug in your target. Why not custom lower the f32
>> setcc nodes directly to the desired target nodes rather than doing
>> this  two stage lowering?
>> Evan
> At the moment I'm not doing any custom lowering in my target - the
> lowering I was describing was what I observed the SectionDAG was  
> doing.
> I was under the impression that LLVM's soft float support meant that  
> if
> I didn't call addRegisterClass() with any FP types then floating point
> operations would be expanded into libcalls and it would all just
> work(tm). And for the most part it does work - addition, division, etc
> on floating point types are all lower correctly by the SelectionDAG
> without any further intervention.
> However it fails fcmp. I was wanting to understand if this was  
> expected
> and if so what I should do about it. It sounds like I need to custom
> lower the nodes directly. I would certainly be nice if this wasn't
> necessary.

Ok, I am not sure I understand your original question then. Legalizer  
is converting the setcc node into a call to __unorddf2. Is that what  
you want? But you also stated:

>>> he fcmp is expanded to the a call to __unorddf2 which is then
>>> lowered via the LowerOperation hook of my backend.

Does that mean you are then lowering the call to some other  
operations?  That means your lowering code is somehow not removing the  
call code. Perhaps you are not updating all the uses. Nevertheless  
this is not the right approach, you should instead custom lower the  
setcc node directly to the target specific node.


> I tried the same code with the MIPS target and it also fails in the  
> same
> manner - a ISD::CALL node remains in the DAG after legalization and  
> the
> instruction selection fails.
> Richard
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