[PATCH] D107883: [RISCV] Select vector mul by 2 to a vector add.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

DAGCombine should turn mul by 2 into shl. Except that it can't for scalable vectors because isKnownToBeAPowerOfTwo doesn't work for scalable vectors. We should just make DAG combine work even if we have just have to special case SPLAT_VECTOR.

  // fold (mul x, (1 << c)) -> x << c                                                                                                                                                                     
  if (isConstantOrConstantVector(N1, /*NoOpaques*/ true) &&                                                                                                                                               
      DAG.isKnownToBeAPowerOfTwo(N1) &&                                                                                                                                                                   
      (!VT.isVector() || Level <= AfterLegalizeVectorOps)) {                                                                                                                                              
    SDLoc DL(N);                                                                                                                                                                                          
    SDValue LogBase2 = BuildLogBase2(N1, DL);                                                                                                                                                             
    EVT ShiftVT = getShiftAmountTy(N0.getValueType());                                                                                                                                                    
    SDValue Trunc = DAG.getZExtOrTrunc(LogBase2, DL, ShiftVT);                                                                                                                                            
    return DAG.getNode(ISD::SHL, DL, VT, N0, Trunc);                                                                                                                                                      
  }   


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D107883/new/

https://reviews.llvm.org/D107883