[LLVMdev] RFC: Add ISD nodes for mad

[Matt Arsenault]

Hi,

I would like to add an ISD node for an fmad operation (named either FMAD 
or FMULADD). It will have the semantics of returning the same result as 
the separate multiply and add with the intermediate rounding step, and 
not a differently rounded result.  The motivation is to share code with 
the various FMA forming DAG combines, but will be generally more useful 
on targets that support it since it is OK to form without non-default 
-fp-contract modes.

This would also be associated with a minor change to the description of 
the llvm.fmuladd intrinsic. The fmuladd intrinsic currently has what I 
think are poorly worded semantics for how it should behave:

"...is equivalent to the expression a * b + c, except that rounding will 
not be performed between the multiplication and addition steps if the 
code generator fuses the operations. Fusion is not guaranteed, even if 
the target platform supports it."

This is odd because it doesn't guarantee fusion, but if fusion does 
happen, the intermediate rounding is eliminated. This is an odd 
constraint to put on the backend that in my reading doesn't allow ever 
forming a mad with the same intermediate rounding. This is not helpful 
since the fmuladd intrinsic expands to the fmul and fadd in 
SelectionDAGBuilder unless fp-contraction is globally enabled. According 
to this constraint, you could never form a mad because if it was 
expanded from the fmuladd intrinsic, fusion is not allowed. I don't 
think this arbitrary constraint on the backend makes sense, so the 
semantics of fmuladd would be changed to mean that the intermediate 
rounding may or may not happen, but not constrain the backend on only 
fusing if the intermediate rounding is eliminated.

-Matt