[LLVMdev] More FMA folding opportunities

[Olivier H Sallenave]

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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