diff --git a/llvm/test/tools/llvm-reduce/remove-bbs-unreachable.ll b/llvm/test/tools/llvm-reduce/remove-bbs-unreachable.ll
@@ -2,7 +2,7 @@
 ; unreachable blocks.
 ;
 ; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=basic-blocks --test FileCheck --test-arg --check-prefixes=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
-; RUN: cat %t | FileCheck %s
+; RUN: FileCheck %s < %t
 
 ; CHECK-INTERESTINGNESS: test
 

diff --git a/llvm/test/tools/llvm-reduce/remove-bbs.ll b/llvm/test/tools/llvm-reduce/remove-bbs.ll
@@ -1,8 +1,8 @@
 ; Test that llvm-reduce can remove uninteresting Basic Blocks, and remove them from instructions (i.e. SwitchInst, BranchInst and IndirectBrInst)
 ; Note: if an uninteresting BB is the default case for a switch, the instruction is removed altogether (since the default case cannot be replaced)
 ;
-; RUN: llvm-reduce --delta-passes=basic-blocks --test %python --test-arg %p/Inputs/remove-bbs.py %s -o %t
-; RUN: cat %t | FileCheck -implicit-check-not=uninteresting %s
+; RUN: llvm-reduce -abort-on-invalid-reduction --delta-passes=basic-blocks --test %python --test-arg %p/Inputs/remove-bbs.py %s -o %t
+; RUN: FileCheck -implicit-check-not=uninteresting %s < %t
 
 define void @main() {
 interesting:

diff --git a/llvm/test/tools/llvm-reduce/remove-call-site-attributes.ll b/llvm/test/tools/llvm-reduce/remove-call-site-attributes.ll
@@ -1,6 +1,6 @@
 ; Test that llvm-reduce can remove uninteresting operand bundles from calls.
 ;
-; RUN: llvm-reduce --delta-passes=operand-bundles,attributes --test FileCheck --test-arg --check-prefixes=CHECK-ALL,CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
+; RUN: llvm-reduce -abort-on-invalid-reduction --delta-passes=operand-bundles,attributes --test FileCheck --test-arg --check-prefixes=CHECK-ALL,CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
 ; RUN: cat %t | FileCheck --check-prefixes=CHECK-ALL,CHECK-FINAL %s
 
 ; CHECK-ALL: declare i32 @f1(i32, i32)

diff --git a/llvm/tools/llvm-reduce/DeltaManager.h b/llvm/tools/llvm-reduce/DeltaManager.h
@@ -1,4 +1,4 @@
-//===- DeltaManager.h - Runs Delta Passes to reduce Input -----------------===//
+//===- DeltaManager.h - Runs Delta Passes to reduce Input -------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/ReducerWorkItem.h b/llvm/tools/llvm-reduce/ReducerWorkItem.h
@@ -1,4 +1,4 @@
-//===- ReducerWorkItem.h - Wrapper for Module and MachineFunction ---------===//
+//===- ReducerWorkItem.h - Wrapper for Module -------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/Delta.h b/llvm/tools/llvm-reduce/deltas/Delta.h
@@ -1,4 +1,4 @@
-//===- Delta.h - Delta Debugging Algorithm Implementation -----------------===//
+//===- Delta.h - Delta Debugging Algorithm Implementation -------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceAliases.h b/llvm/tools/llvm-reduce/deltas/ReduceAliases.h
@@ -1,4 +1,4 @@
-//===- ReduceAliases.h - Specialized Delta Pass ---------------------------===//
+//===- ReduceAliases.h - Specialized Delta Pass -----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceArguments.h b/llvm/tools/llvm-reduce/deltas/ReduceArguments.h
@@ -1,4 +1,4 @@
-//===- ReduceArguments.h - Specialized Delta Pass -------------------------===//
+//===- ReduceArguments.h - Specialized Delta Pass ---------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceAttributes.h b/llvm/tools/llvm-reduce/deltas/ReduceAttributes.h
@@ -1,4 +1,4 @@
-//===- ReduceAttributes.h - Specialized Delta Pass ------------------------===//
+//===- ReduceAttributes.h - Specialized Delta Pass --------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.h b/llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.h
@@ -1,4 +1,4 @@
-//===- ReduceArguments.h - Specialized Delta Pass -------------------------===//
+//===- ReduceArguments.h - Specialized Delta Pass ---------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceFunctionBodies.h b/llvm/tools/llvm-reduce/deltas/ReduceFunctionBodies.h
@@ -1,4 +1,4 @@
-//===- ReduceFunctionBodies.h - Specialized Delta Pass --------------------===//
+//===- ReduceFunctionBodies.h - Specialized Delta Pass ----------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceFunctions.h b/llvm/tools/llvm-reduce/deltas/ReduceFunctions.h
@@ -1,4 +1,4 @@
-//===- ReduceFunctions.h - Specialized Delta Pass -------------------------===//
+//===- ReduceFunctions.h - Specialized Delta Pass ---------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceGlobalObjects.h b/llvm/tools/llvm-reduce/deltas/ReduceGlobalObjects.h
@@ -1,4 +1,4 @@
-//===- ReduceGlobalObjects.h ----------------------------------------------===//
+//===- ReduceGlobalObjects.h ------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceGlobalValues.h b/llvm/tools/llvm-reduce/deltas/ReduceGlobalValues.h
@@ -1,4 +1,4 @@
-//===- ReduceGlobalValues.h - Specialized Delta Pass ----------------------===//
+//===- ReduceGlobalValues.h - Specialized Delta Pass ------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceGlobalVarInitializers.h b/llvm/tools/llvm-reduce/deltas/ReduceGlobalVarInitializers.h
@@ -1,4 +1,4 @@
-//===- reduceGlobalsInitializersDeltaPass.h - Specialized Delta Pass ------===//
+//===- ReduceGlobalVarInitializers.h ----------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceGlobalVars.h b/llvm/tools/llvm-reduce/deltas/ReduceGlobalVars.h
@@ -1,4 +1,4 @@
-//===- ReduceGlobalVars.h - Specialized Delta Pass ------------------------===//
+//===- ReduceGlobalVars.h - Specialized Delta Pass --------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceInstructionFlagsMIR.h b/llvm/tools/llvm-reduce/deltas/ReduceInstructionFlagsMIR.h
@@ -1,4 +1,4 @@
-//===- ReduceInstructionFlagsMIR.h  - Specialized Delta Pass --------------===//
+//===- ReduceInstructionFlagsMIR.h  -----------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceInstructions.h b/llvm/tools/llvm-reduce/deltas/ReduceInstructions.h
@@ -1,4 +1,4 @@
-//===- ReduceArguments.h - Specialized Delta Pass -------------------------===//
+//===- ReduceInstructions.h -------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceInstructionsMIR.h b/llvm/tools/llvm-reduce/deltas/ReduceInstructionsMIR.h
@@ -1,4 +1,4 @@
-//===- ReduceInstructionsMIR.h  - Specialized Delta Pass ------------------===//
+//===- ReduceInstructionsMIR.h  - Specialized Delta Pass --------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceMetadata.h b/llvm/tools/llvm-reduce/deltas/ReduceMetadata.h
@@ -1,4 +1,4 @@
-//===- ReduceMetadata.h - Specialized Delta Pass --------------------------===//
+//===- ReduceMetadata.h - Specialized Delta Pass ----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceModuleData.h b/llvm/tools/llvm-reduce/deltas/ReduceModuleData.h
@@ -1,4 +1,4 @@
-//===- ReduceModuleData.h --------------------------------------------===//
+//===- ReduceModuleData.h ---------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceOperandBundles.h b/llvm/tools/llvm-reduce/deltas/ReduceOperandBundles.h
@@ -1,4 +1,4 @@
-//===- ReduceOperandBundes.h - Specialized Delta Pass ---------------------===//
+//===- ReduceOperandBundes.h ------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceOperands.cpp b/llvm/tools/llvm-reduce/deltas/ReduceOperands.cpp
@@ -22,6 +22,17 @@ extractOperandsFromModule(Oracle &O, Module &Program,
                           function_ref<Value *(Use &)> ReduceValue) {
   for (auto &F : Program.functions()) {
     for (auto &I : instructions(&F)) {
+      if (PHINode *Phi = dyn_cast<PHINode>(&I)) {
+        for (auto &Op : Phi->incoming_values()) {
+          if (!O.shouldKeep()) {
+            if (Value *Reduced = ReduceValue(Op))
+              Phi->setIncomingValueForBlock(Phi->getIncomingBlock(Op), Reduced);
+          }
+        }
+
+        continue;
+      }
+
       for (auto &Op : I.operands()) {
         if (!O.shouldKeep()) {
           if (Value *Reduced = ReduceValue(Op))

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceOperands.h b/llvm/tools/llvm-reduce/deltas/ReduceOperands.h
@@ -1,4 +1,4 @@
-//===----------------------------------------------------------------------===//
+//===------------------------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceOperandsSkip.cpp b/llvm/tools/llvm-reduce/deltas/ReduceOperandsSkip.cpp
@@ -212,8 +212,12 @@ static void extractOperandsFromModule(Oracle &O, Module &Program) {
       }
     });
 
-    for (std::pair<Use *, Value *> P : Replacements)
-      P.first->set(P.second);
+    for (std::pair<Use *, Value *> P : Replacements) {
+      if (PHINode *Phi = dyn_cast<PHINode>(P.first->getUser()))
+        Phi->setIncomingValueForBlock(Phi->getIncomingBlock(*P.first), P.second);
+      else
+        P.first->set(P.second);
+    }
   }
 }
 

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceOperandsSkip.h b/llvm/tools/llvm-reduce/deltas/ReduceOperandsSkip.h
@@ -1,4 +1,4 @@
-//===----------------------------------------------------------------------===//
+//===------------------------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceOperandsToArgs.h b/llvm/tools/llvm-reduce/deltas/ReduceOperandsToArgs.h
@@ -1,4 +1,4 @@
-//===----------------------------------------------------------------------===//
+//===------------------------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceSpecialGlobals.h b/llvm/tools/llvm-reduce/deltas/ReduceSpecialGlobals.h
@@ -1,4 +1,4 @@
-//===- ReduceSpecialGlobals.h - Specialized Delta Pass --------------------===//
+//===- ReduceSpecialGlobals.h - Specialized Delta Pass ----------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/ReduceUsingSimplifyCFG.h b/llvm/tools/llvm-reduce/deltas/ReduceUsingSimplifyCFG.h
@@ -1,4 +1,4 @@
-//===- ReduceUsingSimplifyCFG.h - Specialized Delta Pass ------------------===//
+//===- ReduceUsingSimplifyCFG.h - Specialized Delta Pass --------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/RunIRPasses.h b/llvm/tools/llvm-reduce/deltas/RunIRPasses.h
@@ -1,4 +1,4 @@
-//===- RunIRPasses.h ------------------------------------------------------===//
+//===- RunIRPasses.h --------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

diff --git a/llvm/tools/llvm-reduce/deltas/Utils.h b/llvm/tools/llvm-reduce/deltas/Utils.h
@@ -1,4 +1,4 @@
-//===- Utils.h - llvm-reduce utility functions ----------------------------===//
+//===- Utils.h - llvm-reduce utility functions ------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.