llvm dev - Sep 2014 - [LLVMdev] More FMA folding opportunities

Hi,

I think more opportunities might be added for FMA in the DAG combiner,
please tell me what you think. Right now, those cases are implemented:

fold (fadd (fmul x, y), z) -> (fma x, y, z)
fold (fadd x, (fmul y, z)) -> (fma y, z, x)

When the TLI callback "enableAggressiveFMAFusion" returns true, we
might
also support:

fold (fadd (fma x, y, (fmul u, v)), z) -> (fma x, y (fma u, v, z))
fold (fadd x, (fma y, z, (fmul u, v)) -> (fma y, z (fma u, v, x))

This kind of reassociation generates two FMA for (x^2 + y^2 + z).

Finally, specifically for the PPC target, we could ignore FP_EXTEND in the
patterns above as it will be removed by the Machine Common Subexpression
Elimination pass. For instance:

fold (fadd (fpext (fmul x, y)), z) -> (fma x, y, z)
fold (fadd (fpext (fma x, y, (fmul u, v))), z) -> (fma x, y (fma u, v, z))
...

Thanks for your help.
Olivier
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----- Original Message -----
> From: "Olivier H Sallenave" <ohsallen at us.ibm.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Monday, September 29, 2014 3:34:51 PM
> Subject: [LLVMdev] More FMA folding opportunities
> 
> Hi,
> 
> I think more opportunities might be added for FMA in the DAG
> combiner, please tell me what you think. Right now, those cases are
> implemented:
> 
> fold (fadd (fmul x, y), z) -> (fma x, y, z)
> fold (fadd x, (fmul y, z)) -> (fma y, z, x)
> 
> When the TLI callback "enableAggressiveFMAFusion" returns true,
we
> might also support:
> 
> fold (fadd (fma x, y, (fmul u, v)), z) -> (fma x, y (fma u, v, z))
> fold (fadd x, (fma y, z, (fmul u, v)) -> (fma y, z (fma u, v, x))
> 
> This kind of reassociation generates two FMA for (x^2 + y^2 + z).
> 
> Finally, specifically for the PPC target, we could ignore FP_EXTEND
> in the patterns above as it will be removed by the Machine Common
> Subexpression Elimination pass. For instance:
> 
> fold (fadd (fpext (fmul x, y)), z) -> (fma x, y, z)
> fold (fadd (fpext (fma x, y, (fmul u, v))), z) -> (fma x, y (fma u,
> v, z))
> ...
Yes, this all sounds reasonable.

Thanks again,
Hal
> 
> Thanks for your help.
> Olivier
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory