Simplify demanded bits of select sources where the condition is a constant vector

llvm-svn: 160835

Author: cooperp

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -899,5 +899,16 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
     return &SI;
   }
 
+  if (VectorType* VecTy = dyn_cast<VectorType>(SI.getType())) {
+    unsigned VWidth = VecTy->getNumElements();
+    APInt UndefElts(VWidth, 0);
+    APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
+    if (Value *V = SimplifyDemandedVectorElts(&SI, AllOnesEltMask, UndefElts)) {
+      if (V != &SI)
+        return ReplaceInstUsesWith(SI, V);
+      return &SI;
+    }
+  }
+
   return 0;
 }

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -989,6 +989,29 @@ Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts,
     }
     break;
   }
+  case Instruction::Select: {
+    APInt LeftDemanded(DemandedElts), RightDemanded(DemandedElts);
+    if (ConstantVector* CV = dyn_cast<ConstantVector>(I->getOperand(0))) {
+      for (unsigned i = 0; i < VWidth; i++) {
+        if (CV->getAggregateElement(i)->isNullValue())
+          LeftDemanded.clearBit(i);
+        else
+          RightDemanded.clearBit(i);
+      }
+    }
+
+    TmpV = SimplifyDemandedVectorElts(I->getOperand(1), LeftDemanded,
+                                      UndefElts, Depth+1);
+    if (TmpV) { I->setOperand(1, TmpV); MadeChange = true; }
+
+    TmpV = SimplifyDemandedVectorElts(I->getOperand(2), RightDemanded,
+                                      UndefElts2, Depth+1);
+    if (TmpV) { I->setOperand(2, TmpV); MadeChange = true; }
+      
+    // Output elements are undefined if both are undefined.
+    UndefElts &= UndefElts2;
+    break;
+  }
   case Instruction::BitCast: {
     // Vector->vector casts only.
     VectorType *VTy = dyn_cast<VectorType>(I->getOperand(0)->getType());

llvm/test/Transforms/InstCombine/vec_demanded_elts.ll

@@ -190,4 +190,23 @@ define <2 x double> @test_fpext(float %f) {
   ret <2 x double> %ret
 }
 
+define <4 x float> @test_select(float %f, float %g) {
+; CHECK: @test_select
+; CHECK: %a0 = insertelement <4 x float> undef, float %f, i32 0
+; CHECK-NOT: insertelement
+; CHECK: %a3 = insertelement <4 x float> %a0, float 3.000000e+00, i32 3
+; CHECK-NOT: insertelement
+; CHECK: %ret = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <4 x float> %a3, <4 x float> <float undef, float 4.000000e+00, float 5.000000e+00, float undef>
+  %a0 = insertelement <4 x float> undef, float %f, i32 0
+  %a1 = insertelement <4 x float> %a0, float 1.000000e+00, i32 1
+  %a2 = insertelement <4 x float> %a1, float 2.000000e+00, i32 2
+  %a3 = insertelement <4 x float> %a2, float 3.000000e+00, i32 3
+  %b0 = insertelement <4 x float> undef, float %g, i32 0
+  %b1 = insertelement <4 x float> %b0, float 4.000000e+00, i32 1
+  %b2 = insertelement <4 x float> %b1, float 5.000000e+00, i32 2
+  %b3 = insertelement <4 x float> %b2, float 6.000000e+00, i32 3
+  %ret = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <4 x float> %a3, <4 x float> %b3
+  ret <4 x float> %ret
+}
+