[llvm-commits] [llvm] r154265 - in /llvm/trunk: lib/CodeGen/SelectionDAG/DAGCombiner.cpp test/CodeGen/ARM/vdiv_combine.ll test/CodeGen/X86/fdiv.ll

[Duncan Sands]

Hi Benjamin,

>> --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Sat Apr  7 15:04:00 2012
>> @@ -5725,6 +5725,19 @@
>>    if (N0CFP&&  N1CFP&&  VT != MVT::ppcf128)
>>      return DAG.getNode(ISD::FDIV, N->getDebugLoc(), VT, N0, N1);
>>
>> +  // fold (fdiv X, c2) ->  fmul X, 1/c2 if there is no precision loss or if
>> +  // losing precision is acceptable.
>> +  if (N1CFP&&  VT != MVT::ppcf128) {
>> +    // Compute the reciprocal 1.0 / c2.
>> +    APFloat N1APF = N1CFP->getValueAPF();
>> +    APFloat Recip(N1APF.getSemantics(), 1); // 1.0
>> +    APFloat::opStatus st = Recip.divide(N1APF, APFloat::rmNearestTiesToEven);
>> +    // Only do the transform if the reciprocal is not too horrible (eg not NaN).
>> +    if (st == APFloat::opOK || (st == APFloat::opInexact&&
>> +                                DAG.getTarget().Options.UnsafeFPMath))
>> +      return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT, N0,
>> +                         DAG.getConstantFP(Recip, VT));
>> +  }
>
> One thing to keep in mind is that we don't want to turn fdivs into fmuls with a denormal number. Denormals may be disabled on the CPU (ops always return 0.0), in other cases they are just painfully slow.

how to tell if the reciprocal is denormal?  I don't immediately see an APFloat
method for that.

> I have a bad feeling about allowing any reciprocal in safe math mode if the division was exact, this is only safe for powers of two in general (see APFloat::getExactInverse).

Are you saying that forming 1/c may return opOK rather than opInexact even if
the division is not exact?  If so, what's the point of the status flags?  That
said, since the exact version is done at the IR level how about I just remove
it here?

Ciao, Duncan.