[llvm-reduce] Add reduction for aliases.

This patch adds a new reduction pass that tries to remove aliases.
It runs early, as most of those likely can be removed up-front in
practice.

This substantially improves llvm-reduce for IR generated by the swift
compiler, which can generate a lot of aliases which lead to lots of
invalid reductions.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D90260

test/Reduce/remove-alias.ll

@@ -8,20 +8,20 @@
 ; CHECK-FINAL: @g1 = global
 ; CHECK-FINAL: @g2 = global
 
-; CHECK-FINAL: $a1
-; CHECK-FINAL: $a2
-; CHECK-FINAL: $a3
-; CHECK-FINAL: $a4
+; CHECK-FINAL-NOT: $a1
+; CHECK-FINAL-NOT: $a2
+; CHECK-FINAL-NOT: $a3
+; CHECK-FINAL-NOT: $a4
 ; CHECK-FINAL: $a5
 ; CHECK-FINAL: $a6
 
 ; CHECK-FINAL-NOT: @llvm.used
 ; CHECK-FINAL-NOT: @llvm.compiler.used
 
-; CHECK-FINAL: define void @fn1
-; CHECK-FINAL: define void @fn2
+; CHECK-FINAL-NOT: define void @fn1
+; CHECK-FINAL-NOT: define void @fn2
 ; CHECK-FINAL: define void @fn3
-; CHECK-FINAL: define void @fn4
+; CHECK-FINAL-NOT: define void @fn4
 
 @g1 = global [ 4 x i32 ] zeroinitializer
 @g2 = global [ 4 x i32 ] zeroinitializer

tools/llvm-reduce/CMakeLists.txt

@@ -13,6 +13,7 @@ set(LLVM_LINK_COMPONENTS
 add_llvm_tool(llvm-reduce
   TestRunner.cpp
   deltas/Delta.cpp
+  deltas/ReduceAliases.cpp
   deltas/ReduceArguments.cpp
   deltas/ReduceAttributes.cpp
   deltas/ReduceBasicBlocks.cpp

tools/llvm-reduce/DeltaManager.h

@@ -13,6 +13,7 @@
 
 #include "TestRunner.h"
 #include "deltas/Delta.h"
+#include "deltas/ReduceAliases.h"
 #include "deltas/ReduceArguments.h"
 #include "deltas/ReduceAttributes.h"
 #include "deltas/ReduceBasicBlocks.h"
@@ -27,6 +28,7 @@ namespace llvm {
 
 // TODO: Add CLI option to run only specified Passes (for unit tests)
 inline void runDeltaPasses(TestRunner &Tester) {
+  reduceAliasesDeltaPass(Tester);
   reduceFunctionBodiesDeltaPass(Tester);
   reduceFunctionsDeltaPass(Tester);
   reduceBasicBlocksDeltaPass(Tester);

tools/llvm-reduce/deltas/ReduceAliases.cpp

@@ -0,0 +1,53 @@
+//===- ReduceAliases.cpp - Specialized Delta Pass -------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a function which calls the Generic Delta pass in order
+// to reduce aliases in the provided Module.
+//
+//===----------------------------------------------------------------------===//
+
+#include "ReduceAliases.h"
+#include "Delta.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/GlobalValue.h"
+
+using namespace llvm;
+
+/// Removes all aliases aren't inside any of the
+/// desired Chunks.
+static void extractAliasesFromModule(const std::vector<Chunk> &ChunksToKeep,
+                                     Module *Program) {
+  Oracle O(ChunksToKeep);
+
+  for (auto &GA : make_early_inc_range(Program->aliases())) {
+    if (!O.shouldKeep()) {
+      GA.replaceAllUsesWith(GA.getAliasee());
+      GA.eraseFromParent();
+    }
+  }
+}
+
+/// Counts the amount of aliases and prints their respective name & index.
+static int countAliases(Module *Program) {
+  // TODO: Silence index with --quiet flag
+  errs() << "----------------------------\n";
+  errs() << "Aliases Index Reference:\n";
+  int Count = 0;
+  for (auto &GA : Program->aliases())
+    errs() << "\t" << ++Count << ": " << GA.getName() << "\n";
+
+  errs() << "----------------------------\n";
+  return Count;
+}
+
+void llvm::reduceAliasesDeltaPass(TestRunner &Test) {
+  errs() << "*** Reducing Aliases ...\n";
+  int Functions = countAliases(Test.getProgram());
+  runDeltaPass(Test, Functions, extractAliasesFromModule);
+  errs() << "----------------------------\n";
+}

tools/llvm-reduce/deltas/ReduceAliases.h

@@ -0,0 +1,18 @@
+//===- ReduceAliases.h - Specialized Delta Pass ---------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a function which calls the Generic Delta pass in order
+// to reduce aliases in the provided Module.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Delta.h"
+
+namespace llvm {
+void reduceAliasesDeltaPass(TestRunner &Test);
+} // namespace llvm