Introduce GlobalSplit pass.

This pass splits globals into elements using inrange annotations on
getelementptr indices.

Differential Revision: https://reviews.llvm.org/D22295

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287178 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/InitializePasses.h

@@ -145,6 +145,7 @@ void initializeGVNLegacyPassPass(PassRegistry&);
 void initializeGlobalDCELegacyPassPass(PassRegistry&);
 void initializeGlobalMergePass(PassRegistry&);
 void initializeGlobalOptLegacyPassPass(PassRegistry&);
+void initializeGlobalSplitPass(PassRegistry&);
 void initializeGlobalsAAWrapperPassPass(PassRegistry&);
 void initializeGuardWideningLegacyPassPass(PassRegistry&);
 void initializeIPCPPass(PassRegistry&);

include/llvm/Transforms/IPO.h

@@ -225,6 +225,10 @@ ModulePass *createCrossDSOCFIPass();
 /// metadata.
 ModulePass *createWholeProgramDevirtPass();
 
+/// This pass splits globals into pieces for the benefit of whole-program
+/// devirtualization and control-flow integrity.
+ModulePass *createGlobalSplitPass();
+
 //===----------------------------------------------------------------------===//
 // SampleProfilePass - Loads sample profile data from disk and generates
 // IR metadata to reflect the profile.

lib/Transforms/IPO/CMakeLists.txt

@@ -12,6 +12,7 @@ add_llvm_library(LLVMipo
   FunctionImport.cpp
   GlobalDCE.cpp
   GlobalOpt.cpp
+  GlobalSplit.cpp
   IPConstantPropagation.cpp
   IPO.cpp
   InferFunctionAttrs.cpp

lib/Transforms/IPO/GlobalSplit.cpp

@@ -0,0 +1,164 @@
+//===- GlobalSplit.cpp - global variable splitter -------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass uses inrange annotations on GEP indices to split globals where
+// beneficial. Clang currently attaches these annotations to references to
+// virtual table globals under the Itanium ABI for the benefit of the
+// whole-program virtual call optimization and control flow integrity passes.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
+#include "llvm/Pass.h"
+
+#include <set>
+
+using namespace llvm;
+
+namespace {
+
+bool splitGlobal(GlobalVariable &GV) {
+  // If the address of the global is taken outside of the module, we cannot
+  // apply this transformation.
+  if (!GV.hasLocalLinkage())
+    return false;
+
+  // We currently only know how to split ConstantStructs.
+  auto *Init = dyn_cast_or_null<ConstantStruct>(GV.getInitializer());
+  if (!Init)
+    return false;
+
+  // Verify that each user of the global is an inrange getelementptr constant.
+  // From this it follows that any loads from or stores to that global must use
+  // a pointer derived from an inrange getelementptr constant, which is
+  // sufficient to allow us to apply the splitting transform.
+  for (User *U : GV.users()) {
+    if (!isa<Constant>(U))
+      return false;
+
+    auto *GEP = dyn_cast<GEPOperator>(U);
+    if (!GEP || !GEP->getInRangeIndex() || *GEP->getInRangeIndex() != 1 ||
+        !isa<ConstantInt>(GEP->getOperand(1)) ||
+        !cast<ConstantInt>(GEP->getOperand(1))->isZero() ||
+        !isa<ConstantInt>(GEP->getOperand(2)))
+      return false;
+  }
+
+  SmallVector<MDNode *, 2> Types;
+  GV.getMetadata(LLVMContext::MD_type, Types);
+
+  const DataLayout &DL = GV.getParent()->getDataLayout();
+  const StructLayout *SL = DL.getStructLayout(Init->getType());
+
+  IntegerType *Int32Ty = Type::getInt32Ty(GV.getContext());
+
+  std::vector<GlobalVariable *> SplitGlobals(Init->getNumOperands());
+  for (unsigned I = 0; I != Init->getNumOperands(); ++I) {
+    // Build a global representing this split piece.
+    auto *SplitGV =
+        new GlobalVariable(*GV.getParent(), Init->getOperand(I)->getType(),
+                           GV.isConstant(), GlobalValue::PrivateLinkage,
+                           Init->getOperand(I), GV.getName() + "." + utostr(I));
+    SplitGlobals[I] = SplitGV;
+
+    unsigned SplitBegin = SL->getElementOffset(I);
+    unsigned SplitEnd = (I == Init->getNumOperands() - 1)
+                            ? SL->getSizeInBytes()
+                            : SL->getElementOffset(I + 1);
+
+    // Rebuild type metadata, adjusting by the split offset.
+    // FIXME: See if we can use DW_OP_piece to preserve debug metadata here.
+    for (MDNode *Type : Types) {
+      uint64_t ByteOffset = cast<ConstantInt>(
+              cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
+              ->getZExtValue();
+      if (ByteOffset < SplitBegin || ByteOffset >= SplitEnd)
+        continue;
+      SplitGV->addMetadata(
+          LLVMContext::MD_type,
+          *MDNode::get(GV.getContext(),
+                       {ConstantAsMetadata::get(
+                            ConstantInt::get(Int32Ty, ByteOffset - SplitBegin)),
+                        Type->getOperand(1)}));
+    }
+  }
+
+  for (User *U : GV.users()) {
+    auto *GEP = cast<GEPOperator>(U);
+    unsigned I = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
+    if (I >= SplitGlobals.size())
+      continue;
+
+    SmallVector<Value *, 4> Ops;
+    Ops.push_back(ConstantInt::get(Int32Ty, 0));
+    for (unsigned I = 3; I != GEP->getNumOperands(); ++I)
+      Ops.push_back(GEP->getOperand(I));
+
+    auto *NewGEP = ConstantExpr::getGetElementPtr(
+        SplitGlobals[I]->getInitializer()->getType(), SplitGlobals[I], Ops,
+        GEP->isInBounds());
+    GEP->replaceAllUsesWith(NewGEP);
+  }
+
+  // Finally, remove the original global. Any remaining uses refer to invalid
+  // elements of the global, so replace with undef.
+  if (!GV.use_empty())
+    GV.replaceAllUsesWith(UndefValue::get(GV.getType()));
+  GV.eraseFromParent();
+  return true;
+}
+
+bool splitGlobals(Module &M) {
+  // First, see if the module uses either of the llvm.type.test or
+  // llvm.type.checked.load intrinsics, which indicates that splitting globals
+  // may be beneficial.
+  Function *TypeTestFunc =
+      M.getFunction(Intrinsic::getName(Intrinsic::type_test));
+  Function *TypeCheckedLoadFunc =
+      M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
+  if ((!TypeTestFunc || TypeTestFunc->use_empty()) &&
+      (!TypeCheckedLoadFunc || TypeCheckedLoadFunc->use_empty()))
+    return false;
+
+  bool Changed = false;
+  for (auto I = M.global_begin(); I != M.global_end();) {
+    GlobalVariable &GV = *I;
+    ++I;
+    Changed |= splitGlobal(GV);
+  }
+  return Changed;
+}
+
+struct GlobalSplit : public ModulePass {
+  static char ID;
+  GlobalSplit() : ModulePass(ID) {
+    initializeGlobalSplitPass(*PassRegistry::getPassRegistry());
+  }
+  bool runOnModule(Module &M) {
+    if (skipModule(M))
+      return false;
+
+    return splitGlobals(M);
+  }
+};
+
+}
+
+INITIALIZE_PASS(GlobalSplit, "globalsplit", "Global splitter", false, false)
+char GlobalSplit::ID = 0;
+
+ModulePass *llvm::createGlobalSplitPass() {
+  return new GlobalSplit;
+}

lib/Transforms/IPO/IPO.cpp

@@ -32,6 +32,7 @@ void llvm::initializeIPO(PassRegistry &Registry) {
   initializeForceFunctionAttrsLegacyPassPass(Registry);
   initializeGlobalDCELegacyPassPass(Registry);
   initializeGlobalOptLegacyPassPass(Registry);
+  initializeGlobalSplitPass(Registry);
   initializeIPCPPass(Registry);
   initializeAlwaysInlinerLegacyPassPass(Registry);
   initializeSimpleInlinerPass(Registry);

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -692,6 +692,11 @@ void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
   PM.add(createPostOrderFunctionAttrsLegacyPass());
   PM.add(createReversePostOrderFunctionAttrsPass());
 
+  // Split globals using inrange annotations on GEP indices. This can help
+  // improve the quality of generated code when virtual constant propagation or
+  // control flow integrity are enabled.
+  PM.add(createGlobalSplitPass());
+
   // Apply whole-program devirtualization and virtual constant propagation.
   PM.add(createWholeProgramDevirtPass());
 

test/Transforms/GlobalSplit/basic.ll

@@ -0,0 +1,56 @@
+; RUN: opt -S -globalsplit %s | FileCheck %s
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: @vtt = constant [3 x i8*] [i8* bitcast ([2 x i8* ()*]* @global.0 to i8*), i8* bitcast (i8* ()** getelementptr inbounds ([2 x i8* ()*], [2 x i8* ()*]* @global.0, i32 0, i32 1) to i8*), i8* bitcast ([1 x i8* ()*]* @global.1 to i8*)]
+@vtt = constant [3 x i8*] [
+  i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 0, i32 0) to i8*),
+  i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 0, i32 1) to i8*),
+  i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 1, i32 0) to i8*)
+]
+
+; CHECK-NOT: @global =
+; CHECK: @global.0 = private constant [2 x i8* ()*] [i8* ()* @f1, i8* ()* @f2], !type [[T1:![0-9]+$]]
+; CHECK: @global.1 = private constant [1 x i8* ()*] [i8* ()* @f3], !type [[T2:![0-9]+$]]
+; CHECK-NOT: @global =
+@global = internal constant { [2 x i8* ()*], [1 x i8* ()*] } {
+  [2 x i8* ()*] [i8* ()* @f1, i8* ()* @f2],
+  [1 x i8* ()*] [i8* ()* @f3]
+}, !type !0, !type !1
+
+; CHECK: define i8* @f1()
+define i8* @f1() {
+  ; CHECK-NEXT: ret i8* bitcast ([2 x i8* ()*]* @global.0 to i8*)
+  ret i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 0, i32 0) to i8*)
+}
+
+; CHECK: define i8* @f2()
+define i8* @f2() {
+  ; CHECK-NEXT: ret i8* bitcast (i8* ()** getelementptr inbounds ([2 x i8* ()*], [2 x i8* ()*]* @global.0, i32 0, i32 1) to i8*)
+  ret i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 0, i32 1) to i8*)
+}
+
+; CHECK: define i8* @f3()
+define i8* @f3() {
+  ; CHECK-NEXT: ret i8* bitcast (i8* ()** getelementptr inbounds ([2 x i8* ()*], [2 x i8* ()*]* @global.0, i64 1, i32 0) to i8*)
+  ret i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 0, i32 2) to i8*)
+}
+
+; CHECK: define i8* @f4()
+define i8* @f4() {
+  ; CHECK-NEXT: ret i8* bitcast ([1 x i8* ()*]* @global.1 to i8*)
+  ret i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 1, i32 0) to i8*)
+}
+
+define void @foo() {
+  %p = call i1 @llvm.type.test(i8* null, metadata !"")
+  ret void
+}
+
+declare i1 @llvm.type.test(i8*, metadata) nounwind readnone
+
+; CHECK: [[T1]] = !{i32 8, !"foo"}
+; CHECK: [[T2]] = !{i32 0, !"bar"}
+!0 = !{i32 8, !"foo"}
+!1 = !{i32 16, !"bar"}

test/Transforms/GlobalSplit/non-beneficial.ll

@@ -0,0 +1,24 @@
+; RUN: opt -S -globalsplit %s | FileCheck %s
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: @global =
+@global = internal constant { [2 x i8* ()*], [1 x i8* ()*] } {
+  [2 x i8* ()*] [i8* ()* @f, i8* ()* @g],
+  [1 x i8* ()*] [i8* ()* @h]
+}
+
+define i8* @f() {
+  ret i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 0, i32 0) to i8*)
+}
+
+define i8* @g() {
+  ret i8* null
+}
+
+define i8* @h() {
+  ret i8* null
+}
+
+!0 = !{i32 16}

test/Transforms/GlobalSplit/nonlocal.ll

@@ -0,0 +1,29 @@
+; RUN: opt -S -globalsplit %s | FileCheck %s
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: @global =
+@global = constant { [2 x i8* ()*], [1 x i8* ()*] } {
+  [2 x i8* ()*] [i8* ()* @f, i8* ()* @g],
+  [1 x i8* ()*] [i8* ()* @h]
+}
+
+define i8* @f() {
+  ret i8* bitcast (i8* ()** getelementptr ({ [2 x i8* ()*], [1 x i8* ()*] }, { [2 x i8* ()*], [1 x i8* ()*] }* @global, i32 0, inrange i32 0, i32 0) to i8*)
+}
+
+define i8* @g() {
+  ret i8* null
+}
+
+define i8* @h() {
+  ret i8* null
+}
+
+define void @foo() {
+  %p = call i1 @llvm.type.test(i8* null, metadata !"")
+  ret void
+}
+
+declare i1 @llvm.type.test(i8*, metadata) nounwind readnone