llvm dev - Dec 2012 - [LLVMdev] Question about FMA formation

On Dec 12, 2012, at 4:49 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
> Hi, Eli, Mike and Lang:
> 
>    Thank you all for the input. This is one e.g which might be difficult
for isel:
>   a*b + c*d + e => a*b + (c*d + e). 
> 
You hit send right when I did!
For your example, do you mean that it's grouped like:
(fadd (fadd (fmul a b) (fmul c d)) e)

How would your pass go about handling these patterns and is that something that
would be too complicated for fast-isel to do on the fly?
> Thanks
> Shuxin
> 
> On 12/12/12 4:43 PM, Lang Hames wrote:
>> A little background:
>> 
>> The fmuladd intrinsic was introduced to support the FP_CONTRACT pragma
in C. llvm.fmuladd.* is generated by clang when it sees an expression of the
form  'a * b + c' within a single source statement.
>> 
>> If you want to opportunistically form FMA target instructions my
inclination would be to skip llvm.fmuladd.* and just form them from a*b+c
expressions at isel time. I don't see any fundamental problem with forming
llvm.fmuladd.* to model FMA formation opportunities in an IR pass though.
>> 
>> - Lang.
>> 
>> 
>> On Wed, Dec 12, 2012 at 4:11 PM, Michael Ilseman <milseman at
apple.com> wrote:
>> 
>> On Dec 12, 2012, at 3:40 PM, Shuxin Yang <shuxin.llvm at
gmail.com> wrote:
>> 
>>> Hi, Dear All:
>>> 
>>>   I'm going implement FMA formation. On some architectures,
"FMA a, b, c" is more precise than
>>> "a * b + c".  I'm wondering if FMA could be less
precise. In the former case, can we enable FMA
>>> formation despite restrictive FP mode?
>>> 
>> 
>> I believe that a pass to form fmuladd[1] intrinsic calls would be very
useful! The fmuladd intrinsic is defined such that its formation should be
isolated from worries about strictness. It simply means "a * b + c"
and leaves the decision of whether or not to fuse up to the code generator. Of
course, one probably would only run your pass if they wanted the code generator
to fuse it, but the pass itself should be valid.
>> 
>> Someone please correct me if I misunderstand this intrinsic. 
>> 
>> [1] http://llvm.org/docs/LangRef.html#llvm-fmuladd-intrinsic
>> 
>>> Thanks
>>> Shuxin
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> 
>> 
>> _______________________________________________
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>> 
>> 
> 
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You hit send right when I did!
> For your example, do you mean that it's grouped like:
> (fadd (fadd (fmul a b) (fmul c d)) e)
>
> How would your pass go about handling these patterns and is that something
> that would be too complicated for fast-isel to do on the fly?
>
>
Depends on how they're grouped, but if the formation happens prior to
codegen then fast-isel will just handle whatever new instruction you've
got. An example of IR would be useful though :)

-eric
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Right now we're shying towards having a re-association helper in
codegen-prepare that will re-associate expressions (if allowed). This would
allow fast-isel to more easily spot FMA opportunities, and form better code.

On Dec 12, 2012, at 5:11 PM, Eric Christopher <echristo at gmail.com>
wrote:
> 
> 
> 
> You hit send right when I did!
> For your example, do you mean that it's grouped like:
> (fadd (fadd (fmul a b) (fmul c d)) e)
> 
> How would your pass go about handling these patterns and is that something
that would be too complicated for fast-isel to do on the fly?
> 
> 
> Depends on how they're grouped, but if the formation happens prior to
codegen then fast-isel will just handle whatever new instruction you've got.
An example of IR would be useful though :)
> 
> -eric
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