[LLVMdev] Multiply-add combining

[Kevin K O'Brien]

Hi Olivier,
      I think we discussed this last Thursday? My feeling is that each use
of the multiply can be considered separately. If it can be combined, then
we should do so. The multiply should be left in place and removed by a dead
code elimination pass sometime later. This is what TOBEY does. If you want
me to explain the XL method in more detail, come talk to me.

              Kevin
----------------------------------------------
Kevin O'Brien
Manager, Advanced Compiler Technology
IBM T.J Watson Research Center, Yorktown Heights, NY


                                                                                                                                         
  From:       Olivier H Sallenave/Watson/IBM                                                                                             
                                                                                                                                         
  To:         llvmdev at cs.uiuc.edu,                                                                                                       
                                                                                                                                         
  Cc:         Samuel F Antao/Watson/IBM at IBMUS, Kevin K O'Brien/Watson/IBM at IBMUS                                                          
                                                                                                                                         
  Date:       08/26/2014 11:12 AM                                                                                                        
                                                                                                                                         
  Subject:    Multiply-add combining                                                                                                     
                                                                                                                                         




Hi,

I tried to compile the following using -ffp-contract=fast:

  %mul = fmul double %sub5, %x
  %add = fadd double %add6, %mul
  %sub = fsub double %sub5, %mul

I expected fadd and fsub to be contracted with fmul, which didn't happen.

When looking in DAGCombiner.cpp, it appears the result of the fmul needs to
be used only once, which isn't the case here as it is used by both the fadd
and the fsub:

    // fold (fadd (fmul x, y), z) -> (fma x, y, z)
    if (N0.getOpcode() == ISD::FMUL && N0.hasOneUse())
      return DAG.getNode(ISD::FMA, SDLoc(N), VT, N0.getOperand(0),
          N0.getOperand(1), N1);

This heuristic looks a little conservative, could we instead check that
every instruction using the result of the fmul are combinable (i.e., they
are either fadd or fsub)?


Thanks in advance,
Olivier
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