Index: llvm/trunk/lib/Target/ARM/ARMCodeGenPrepare.cpp
===================================================================
--- llvm/trunk/lib/Target/ARM/ARMCodeGenPrepare.cpp
+++ llvm/trunk/lib/Target/ARM/ARMCodeGenPrepare.cpp
@@ -113,7 +113,11 @@
   SmallPtrSet<Value*, 8> Promoted;
   Module *M = nullptr;
   LLVMContext &Ctx;
+  // The type we promote to: always i32
   IntegerType *ExtTy = nullptr;
+  // The type of the value that the search began from, either i8 or i16.
+  // This defines the max range of the values that we allow in the promoted
+  // tree.
   IntegerType *OrigTy = nullptr;
   SmallPtrSetImpl<Value*> *Visited;
   SmallPtrSetImpl<Value*> *Sources;
@@ -326,7 +330,7 @@
   // - (255 >= 254) == (0xFFFFFFFF >= 254) == true
   //
   // To demonstrate why we can't handle increasing values:
-  // 
+  //
   // %add = add i8 %a, 2
   // %cmp = icmp ult i8 %add, 127
   //
@@ -604,7 +608,7 @@
 
     if (!shouldPromote(I) || SafeToPromote->count(I) || NewInsts.count(I))
       continue;
-  
+
     assert(EnableDSP && "DSP intrinisc insertion not enabled!");
 
     // Replace unsafe instructions with appropriate intrinsic calls.
@@ -685,10 +689,10 @@
   // Some zexts will now have become redundant, along with their trunc
   // operands, so remove them
   for (auto V : *Visited) {
-    if (!isa<CastInst>(V))
+    if (!isa<ZExtInst>(V))
       continue;
 
-    auto ZExt = cast<CastInst>(V);
+    auto ZExt = cast<ZExtInst>(V);
     if (ZExt->getDestTy() != ExtTy)
       continue;
 
@@ -700,9 +704,11 @@
       continue;
     }
 
-    // For any truncs that we insert to handle zexts, we can replace the
-    // result of the zext with the input to the trunc.
-    if (NewInsts.count(Src) && isa<ZExtInst>(V) && isa<TruncInst>(Src)) {
+    // Unless they produce a value that is narrower than ExtTy, we can
+    // replace the result of the zext with the input of a newly inserted
+    // trunc.
+    if (NewInsts.count(Src) && isa<TruncInst>(Src) &&
+        Src->getType() == OrigTy) {
       auto *Trunc = cast<TruncInst>(Src);
       assert(Trunc->getOperand(0)->getType() == ExtTy &&
              "expected inserted trunc to be operating on i32");
Index: llvm/trunk/test/CodeGen/ARM/CGP/arm-cgp-casts.ll
===================================================================
--- llvm/trunk/test/CodeGen/ARM/CGP/arm-cgp-casts.ll
+++ llvm/trunk/test/CodeGen/ARM/CGP/arm-cgp-casts.ll
@@ -588,3 +588,47 @@
   store i8 %cond, i8* @a, align 1
   ret void
 }
+
+; CHECK-LABEL: dont_replace_trunc_1
+; CHECK: cmp
+; CHECK: uxtb
+define void @dont_replace_trunc_1(i8* %a, i16* %b, i16* %c, i32* %d, i8* %e, i32* %f) {
+entry:
+  %0 = load i16, i16* %c, align 2
+  %1 = load i16, i16* %b, align 2
+  %conv = sext i16 %1 to i32
+  store i32 %conv, i32* %f, align 4
+  %2 = trunc i16 %1 to i8
+  %conv1 = and i8 %2, 1
+  store i8 %conv1, i8* %e, align 1
+  %3 = load i8, i8* %a, align 1
+  %narrow = mul nuw i8 %3, %conv1
+  %mul = zext i8 %narrow to i32
+  store i32 %mul, i32* %d, align 4
+  %4 = zext i8 %narrow to i16
+  %conv5 = or i16 %0, %4
+  %tobool = icmp eq i16 %conv5, 0
+  br i1 %tobool, label %if.end, label %for.cond
+
+for.cond:                                         ; preds = %entry, %for.cond
+  br label %for.cond
+
+if.end:                                           ; preds = %entry
+  ret void
+}
+
+; CHECK-LABEL: dont_replace_trunc_2
+; CHECK: cmp
+; CHECK: uxtb
+define i32 @dont_replace_trunc_2(i16* %a, i8* %b) {
+entry:
+  %0 = load i16, i16* %a, align 2
+  %cmp = icmp ugt i16 %0, 8
+  %narrow = select i1 %cmp, i16 %0, i16 0
+  %cond = trunc i16 %narrow to i8
+  %1 = load i8, i8* %b, align 1
+  %or = or i8 %1, %cond
+  store i8 %or, i8* %b, align 1
+  %conv5 = zext i8 %or to i32
+  ret i32 %conv5
+}