InstSimplify: If divisor element is undef simplify to undef

Summary:
If any vector divisor element is undef, we can arbitrarily choose it be
zero which would make the div/rem an undef value by definition.

Reviewers: spatel, reames

Reviewed By: spatel

Subscribers: magabari, llvm-commits

Differential Revision: https://reviews.llvm.org/D42485

llvm-svn: 323343

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -868,13 +868,14 @@ static Value *simplifyDivRem(Value *Op0, Value *Op1, bool IsDiv) {
   if (match(Op1, m_Zero()))
     return UndefValue::get(Ty);
 
-  // If any element of a constant divisor vector is zero, the whole op is undef.
+  // If any element of a constant divisor vector is zero or undef, the whole op
+  // is undef.
   auto *Op1C = dyn_cast<Constant>(Op1);
   if (Op1C && Ty->isVectorTy()) {
     unsigned NumElts = Ty->getVectorNumElements();
     for (unsigned i = 0; i != NumElts; ++i) {
       Constant *Elt = Op1C->getAggregateElement(i);
-      if (Elt && Elt->isNullValue())
+      if (Elt && (Elt->isNullValue() || isa<UndefValue>(Elt)))
         return UndefValue::get(Ty);
     }
   }

llvm/test/Transforms/InstSimplify/div.ll

@@ -34,6 +34,22 @@ define <2 x i8> @udiv_zero_elt_vec(<2 x i8> %x) {
   ret <2 x i8> %div
 }
 
+define <2 x i8> @sdiv_undef_elt_vec(<2 x i8> %x) {
+; CHECK-LABEL: @sdiv_undef_elt_vec(
+; CHECK-NEXT:    ret <2 x i8> undef
+;
+  %div = sdiv <2 x i8> %x, <i8 -42, i8 undef>
+  ret <2 x i8> %div
+}
+
+define <2 x i8> @udiv_undef_elt_vec(<2 x i8> %x) {
+; CHECK-LABEL: @udiv_undef_elt_vec(
+; CHECK-NEXT:    ret <2 x i8> undef
+;
+  %div = udiv <2 x i8> %x, <i8 undef, i8 42>
+  ret <2 x i8> %div
+}
+
 ; Division-by-zero is undef. UB in any vector lane means the whole op is undef.
 ; Thus, we can simplify this: if any element of 'y' is 0, we can do anything.
 ; Therefore, assume that all elements of 'y' must be 1.

llvm/test/Transforms/InstSimplify/rem.ll

@@ -35,6 +35,22 @@ define <2 x i8> @urem_zero_elt_vec(<2 x i8> %x) {
   ret <2 x i8> %rem
 }
 
+define <2 x i8> @srem_undef_elt_vec(<2 x i8> %x) {
+; CHECK-LABEL: @srem_undef_elt_vec(
+; CHECK-NEXT:    ret <2 x i8> undef
+;
+  %rem = srem <2 x i8> %x, <i8 -42, i8 undef>
+  ret <2 x i8> %rem
+}
+
+define <2 x i8> @urem_undef_elt_vec(<2 x i8> %x) {
+; CHECK-LABEL: @urem_undef_elt_vec(
+; CHECK-NEXT:    ret <2 x i8> undef
+;
+  %rem = urem <2 x i8> %x, <i8 undef, i8 42>
+  ret <2 x i8> %rem
+}
+
 ; Division-by-zero is undef. UB in any vector lane means the whole op is undef.
 ; Thus, we can simplify this: if any element of 'y' is 0, we can do anything.
 ; Therefore, assume that all elements of 'y' must be 1.