Don't fold a binop into phi having undef values (#1591)

lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -752,6 +752,15 @@ Instruction *InstCombiner::FoldOpIntoSelect(Instruction &Op, SelectInst *SI) {
   return nullptr;
 }
 
+// HLSL change begins
+static bool HasUndefValue(PHINode *PHI) {
+  for (unsigned i = 0; i < PHI->getNumIncomingValues(); i++)
+    if (isa<UndefValue>(PHI->getIncomingValue(i)))
+      return true;
+  return false;
+}
+// HLSL change ends
+
 /// FoldOpIntoPhi - Given a binary operator, cast instruction, or select which
 /// has a PHI node as operand #0, see if we can fold the instruction into the
 /// PHI (which is only possible if all operands to the PHI are constants).
@@ -762,6 +771,12 @@ Instruction *InstCombiner::FoldOpIntoPhi(Instruction &I) {
   if (NumPHIValues == 0)
     return nullptr;
 
+  // HLSL Change begins
+  // Don't fold a binary op into PHI having undef value(s).
+  if (HasUndefValue(PN))
+    return nullptr;
+  // HLSL Change ends
+
   // We normally only transform phis with a single use.  However, if a PHI has
   // multiple uses and they are all the same operation, we can fold *all* of the
   // uses into the PHI.

tools/clang/test/CodeGenHLSL/quick-test/check_inst_combine_phi_with_undef.hlsl

@@ -0,0 +1,22 @@
+// RUN: %dxc -E main -T ps_6_0 %s  | FileCheck %s
+
+// CHECK: define void @main()
+// CHECK: ret void
+
+float main(int a
+           : A) : SV_Target {
+  float y;
+  [branch] if (a > 0) {
+    y = a;
+  }
+  else {
+    a = a * a;
+  }
+
+  // Here "y" will appear as undef in the phi node when "else" block is taken.
+  // The instcombine pass should not fold the below mul instruction to phi
+  // node if phi has undef values.
+  float z = y * 3.3f;
+  z = z / a;
+  return z;
+}