[LLVMdev] Question about FMA formation

[Shuxin Yang]

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).

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 
> <mailto:milseman at apple.com>> wrote:
>
>
>     On Dec 12, 2012, at 3:40 PM, Shuxin Yang <shuxin.llvm at gmail.com
>     <mailto: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
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