Index: lib/Target/ARM/ARMCodeGenPrepare.cpp
===================================================================
--- lib/Target/ARM/ARMCodeGenPrepare.cpp
+++ lib/Target/ARM/ARMCodeGenPrepare.cpp
@@ -165,6 +165,9 @@
   if (!isa<Instruction>(V))
     return false;
 
+  if (isa<SExtInst>(V))
+    return true;
+
   unsigned Opc = cast<Instruction>(V)->getOpcode();
   return Opc == Instruction::AShr || Opc == Instruction::SDiv ||
          Opc == Instruction::SRem;
@@ -201,17 +204,10 @@
 static bool isSource(Value *V) {
   if (!isa<IntegerType>(V->getType()))
     return false;
-  // TODO Allow zext to be sources.
-  if (isa<Argument>(V))
-    return true;
-  else if (isa<LoadInst>(V))
-    return true;
-  else if (isa<BitCastInst>(V))
+  else if (isa<Argument>(V) || isa<LoadInst>(V) || isa<CallInst>(V))
     return true;
-  else if (auto *Call = dyn_cast<CallInst>(V))
-    return Call->hasRetAttr(Attribute::AttrKind::ZExt);
-  else if (auto *Trunc = dyn_cast<TruncInst>(V))
-    return isSupportedType(Trunc);
+  else if (isa<CastInst>(V))
+    return isSupportedType(V);
   return false;
 }
 
@@ -515,31 +511,22 @@
 /// return value is zeroext. We don't allow opcodes that can introduce sign
 /// bits.
 bool ARMCodeGenPrepare::isSupportedValue(Value *V) {
-  if (isa<ICmpInst>(V))
-    return true;
+  if (generateSignBits(V))
+    return false;
 
-  // Memory instructions
-  if (isa<StoreInst>(V) || isa<GetElementPtrInst>(V))
-    return true;
+  // Disallow for simplicity.
+  if (isa<ConstantExpr>(V))
+    return false;
 
-  // Branches and targets.
-  if( isa<BranchInst>(V) || isa<SwitchInst>(V) || isa<BasicBlock>(V))
+  // Special case because they generate an i1, which we don't generally
+  // support.
+  if (isa<ICmpInst>(V))
     return true;
 
-  // Non-instruction values that we can handle.
-  if ((isa<Constant>(V) && !isa<ConstantExpr>(V)) || isa<Argument>(V))
-    return isSupportedType(V);
-
-  if (isa<PHINode>(V) || isa<SelectInst>(V) || isa<ReturnInst>(V) ||
-      isa<LoadInst>(V))
-    return isSupportedType(V);
-
-  // Truncs can be either sources or sinks.
-  if (auto *Trunc = dyn_cast<TruncInst>(V))
-    return isSupportedType(Trunc) || isSupportedType(Trunc->getOperand(0));
-
-  if (isa<CastInst>(V) && !isa<SExtInst>(V))
-    return isSupportedType(cast<CastInst>(V)->getOperand(0));
+  // Both ZExts and Truncs can be either sources and sinks. BitCasts are also
+  // sources and SExts are disallowed through their sign bit generation.
+  if (auto *Cast = dyn_cast<CastInst>(V))
+    return isSupportedType(Cast) || isSupportedType(Cast->getOperand(0));
 
   // Special cases for calls as we need to check for zeroext
   // TODO We should accept calls even if they don't have zeroext, as they can
@@ -548,17 +535,7 @@
     return isSupportedType(Call) &&
            Call->hasRetAttr(Attribute::AttrKind::ZExt);
 
-  if (!isa<BinaryOperator>(V))
-    return false;
-
-  if (!isSupportedType(V))
-    return false;
-
-  if (generateSignBits(V)) {
-    LLVM_DEBUG(dbgs() << "ARM CGP: No, instruction can generate sign bits.\n");
-    return false;
-  }
-  return true;
+  return isSupportedType(V);
 }
 
 /// Check that the type of V would be promoted and that the original type is
Index: test/CodeGen/ARM/arm-cgp-casts.ll
===================================================================
--- test/CodeGen/ARM/arm-cgp-casts.ll
+++ test/CodeGen/ARM/arm-cgp-casts.ll
@@ -85,6 +85,40 @@
   ret i8 %2
 }
 
+; TODO: We should be able to remove both extends from this example, by looking
+; back through the zexts and accepting a type size <= than the icmp.
+; CHECK-COMMON-LABEL: icmp_i16_zext
+; CHECK-COMMON: uxt
+; CHECK-COMMON: uxt
+define i8 @icmp_i16_zext(i8* %ptr) {
+entry:
+  %gep = getelementptr inbounds i8, i8* %ptr, i32 0
+  %0 = load i8, i8* %gep, align 1
+  %1 = sub nuw nsw i8 %0, 1
+  %conv44 = zext i8 %0 to i16
+  br label %preheader
+
+preheader:
+  br label %body
+
+body:
+  %2 = phi i8 [ %1, %preheader ], [ %3, %if.end ]
+  %si.0274 = phi i16 [ %conv44, %preheader ], [ %inc, %if.end ]
+  %conv51266 = zext i8 %2 to i16
+  %cmp52267 = icmp eq i16 %si.0274, %conv51266
+  br i1 %cmp52267, label %if.end, label %exit
+
+if.end:
+  %inc = add nuw i16 %si.0274, 1
+  %conv = zext i16 %inc to i32
+  %gep1 = getelementptr inbounds i8, i8* %ptr, i32 %conv
+  %3 = load i8, i8* %gep1, align 1
+  br label %body
+
+exit:
+  ret i8 %2
+}
+
 ; Won't don't handle sext
 ; CHECK-COMMON-LABEL: icmp_sext_zext_store_i8_i16
 ; CHECK-COMMON: ldrb