[Clang] Add fake use emission to Clang with -fextend-lifetimes

Following the previous patch which adds the "extend lifetimes" flag
without (almost) any functionality, this patch adds the real feature by
allowing Clang to emit fake uses. These are emitted as a new form of cleanup,
set for variable addresses, which just emits a fake use intrinsic when the
variable falls out of scope. The code for achieving this is simple, with most
of the logic centered on determining whether to emit a fake use for a given
address, and on ensuring that fake uses are ignored in a few cases.

Author: SLTozer

clang/lib/CodeGen/CGCall.cpp

@@ -3595,15 +3595,26 @@ static llvm::StoreInst *findDominatingStoreToReturnValue(CodeGenFunction &CGF) {
     llvm::BasicBlock *IP = CGF.Builder.GetInsertBlock();
     if (IP->empty()) return nullptr;
 
-    // Look at directly preceding instruction, skipping bitcasts and lifetime
-    // markers.
+    // Look at directly preceding instruction, skipping bitcasts, lifetime
+    // markers, and fake uses and their operands.
+    const llvm::Instruction *LoadIntoFakeUse = nullptr;
     for (llvm::Instruction &I : make_range(IP->rbegin(), IP->rend())) {
+      // Ignore instructions that are just loads for fake uses; the load should
+      // immediately precede the fake use, so we only need to remember the
+      // operand for the last fake use seen.
+      if (LoadIntoFakeUse == &I)
+        continue;
       if (isa<llvm::BitCastInst>(&I))
         continue;
-      if (auto *II = dyn_cast<llvm::IntrinsicInst>(&I))
+      if (auto *II = dyn_cast<llvm::IntrinsicInst>(&I)) {
         if (II->getIntrinsicID() == llvm::Intrinsic::lifetime_end)
           continue;
 
+        if (II->getIntrinsicID() == llvm::Intrinsic::fake_use) {
+          LoadIntoFakeUse = dyn_cast<llvm::Instruction>(II->getArgOperand(0));
+          continue;
+        }
+      }
       return GetStoreIfValid(&I);
     }
     return nullptr;

clang/lib/CodeGen/CGCleanup.cpp

@@ -112,11 +112,11 @@ void EHScopeStack::deallocate(size_t Size) {
   StartOfData += llvm::alignTo(Size, ScopeStackAlignment);
 }
 
-bool EHScopeStack::containsOnlyLifetimeMarkers(
+bool EHScopeStack::containsOnlyNoopCleanups(
     EHScopeStack::stable_iterator Old) const {
   for (EHScopeStack::iterator it = begin(); stabilize(it) != Old; it++) {
     EHCleanupScope *cleanup = dyn_cast<EHCleanupScope>(&*it);
-    if (!cleanup || !cleanup->isLifetimeMarker())
+    if (!cleanup || !(cleanup->isLifetimeMarker() || cleanup->isFakeUse()))
       return false;
   }
 
@@ -154,6 +154,7 @@ void *EHScopeStack::pushCleanup(CleanupKind Kind, size_t Size) {
   bool IsNormalCleanup = Kind & NormalCleanup;
   bool IsEHCleanup = Kind & EHCleanup;
   bool IsLifetimeMarker = Kind & LifetimeMarker;
+  bool IsFakeUse = Kind & FakeUse;
 
   // Per C++ [except.terminate], it is implementation-defined whether none,
   // some, or all cleanups are called before std::terminate. Thus, when
@@ -176,6 +177,8 @@ void *EHScopeStack::pushCleanup(CleanupKind Kind, size_t Size) {
     InnermostEHScope = stable_begin();
   if (IsLifetimeMarker)
     Scope->setLifetimeMarker();
+  if (IsFakeUse)
+    Scope->setFakeUse();
 
   // With Windows -EHa, Invoke llvm.seh.scope.begin() for EHCleanup
   // If exceptions are disabled/ignored and SEH is not in use, then there is no

clang/lib/CodeGen/CGCleanup.h

@@ -87,6 +87,10 @@ class EHScope {
     LLVM_PREFERRED_TYPE(bool)
     unsigned IsLifetimeMarker : 1;
 
+    /// Whether this cleanup is a fake use
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned IsFakeUse : 1;
+
     /// Whether the normal cleanup should test the activation flag.
     LLVM_PREFERRED_TYPE(bool)
     unsigned TestFlagInNormalCleanup : 1;
@@ -352,6 +356,7 @@ class alignas(8) EHCleanupScope : public EHScope {
     CleanupBits.IsEHCleanup = isEH;
     CleanupBits.IsActive = true;
     CleanupBits.IsLifetimeMarker = false;
+    CleanupBits.IsFakeUse = false;
     CleanupBits.TestFlagInNormalCleanup = false;
     CleanupBits.TestFlagInEHCleanup = false;
     CleanupBits.CleanupSize = cleanupSize;
@@ -384,6 +389,9 @@ class alignas(8) EHCleanupScope : public EHScope {
   bool isLifetimeMarker() const { return CleanupBits.IsLifetimeMarker; }
   void setLifetimeMarker() { CleanupBits.IsLifetimeMarker = true; }
 
+  bool isFakeUse() const { return CleanupBits.IsFakeUse; }
+  void setFakeUse() { CleanupBits.IsFakeUse = true; }
+
   bool hasActiveFlag() const { return ActiveFlag.isValid(); }
   Address getActiveFlag() const {
     return ActiveFlag;

clang/lib/CodeGen/CGDecl.cpp

@@ -1352,6 +1352,14 @@ void CodeGenFunction::EmitLifetimeEnd(llvm::Value *Size, llvm::Value *Addr) {
   C->setDoesNotThrow();
 }
 
+void CodeGenFunction::EmitFakeUse(Address Addr) {
+  auto NL = ApplyDebugLocation::CreateEmpty(*this);
+  llvm::Value *V = Builder.CreateLoad(Addr, "fake.use");
+  llvm::CallInst *C = Builder.CreateCall(CGM.getLLVMFakeUseFn(), {V});
+  C->setDoesNotThrow();
+  C->setTailCallKind(llvm::CallInst::TCK_NoTail);
+}
+
 void CodeGenFunction::EmitAndRegisterVariableArrayDimensions(
     CGDebugInfo *DI, const VarDecl &D, bool EmitDebugInfo) {
   // For each dimension stores its QualType and corresponding
@@ -1411,6 +1419,39 @@ void CodeGenFunction::EmitAndRegisterVariableArrayDimensions(
   }
 }
 
+/// Return the maximum size of an aggregate for which we generate a fake use
+/// intrinsic when -fextend-lifetimes is in effect.
+static uint64_t maxFakeUseAggregateSize(const ASTContext &C) {
+  return 4 * C.getTypeSize(C.UnsignedIntTy);
+}
+
+// Helper function to determine whether a variable's or parameter's lifetime
+// should be extended.
+static bool extendLifetime(const ASTContext &Context, const Decl *FuncDecl,
+                           const VarDecl &D,
+                           ImplicitParamDecl *CXXABIThisDecl) {
+  // When we're not inside a valid function it is unlikely that any
+  // lifetime extension is useful.
+  if (!FuncDecl)
+    return false;
+  if (FuncDecl->isImplicit())
+    return false;
+  // Do not extend compiler-created variables except for the this pointer.
+  if (D.isImplicit() && &D != CXXABIThisDecl)
+    return false;
+  QualType Ty = D.getType();
+  // No need to extend volatiles, they have a memory location.
+  if (Ty.isVolatileQualified())
+    return false;
+  // Don't extend variables that exceed a certain size.
+  if (Context.getTypeSize(Ty) > maxFakeUseAggregateSize(Context))
+    return false;
+  // Do not extend variables in nodebug functions.
+  if (FuncDecl->hasAttr<NoDebugAttr>())
+    return false;
+  return true;
+}
+
 /// EmitAutoVarAlloca - Emit the alloca and debug information for a
 /// local variable.  Does not emit initialization or destruction.
 CodeGenFunction::AutoVarEmission
@@ -1663,6 +1704,17 @@ CodeGenFunction::EmitAutoVarAlloca(const VarDecl &D) {
                                          emission.getOriginalAllocatedAddress(),
                                          emission.getSizeForLifetimeMarkers());
 
+  // Analogous to lifetime markers, we use a 'cleanup' to emit fake.use
+  // calls for local variables. We are exempting volatile variables and
+  // non-scalars larger than 4 times the size of an unsigned int (32 bytes).
+  // Larger non-scalars are often allocated in memory and may create unnecessary
+  // overhead.
+  if (CGM.getCodeGenOpts().ExtendLifetimes) {
+    if (extendLifetime(getContext(), CurCodeDecl, D, CXXABIThisDecl))
+      EHStack.pushCleanup<FakeUse>(NormalFakeUse,
+                                   emission.getAllocatedAddress());
+  }
+
   return emission;
 }
 
@@ -2524,6 +2576,14 @@ llvm::Function *CodeGenModule::getLLVMLifetimeEndFn() {
   return LifetimeEndFn;
 }
 
+/// Lazily declare the @llvm.fake.use intrinsic.
+llvm::Function *CodeGenModule::getLLVMFakeUseFn() {
+  if (!FakeUseFn)
+    FakeUseFn = llvm::Intrinsic::getDeclaration(&getModule(),
+                                                llvm::Intrinsic::fake_use);
+  return FakeUseFn;
+}
+
 namespace {
   /// A cleanup to perform a release of an object at the end of a
   /// function.  This is used to balance out the incoming +1 of a
@@ -2717,6 +2777,15 @@ void CodeGenFunction::EmitParmDecl(const VarDecl &D, ParamValue Arg,
 
   setAddrOfLocalVar(&D, DeclPtr);
 
+  // Push a FakeUse 'cleanup' object onto the EHStack for the parameter,
+  // which may be the 'this' pointer. This causes the emission of a fake.use
+  // call with the parameter as argument at the end of the function.
+  if (CGM.getCodeGenOpts().ExtendLifetimes ||
+      (CGM.getCodeGenOpts().ExtendThisPtr && &D == CXXABIThisDecl)) {
+    if (extendLifetime(getContext(), CurCodeDecl, D, CXXABIThisDecl))
+      EHStack.pushCleanup<FakeUse>(NormalFakeUse, DeclPtr);
+  }
+
   // Emit debug info for param declarations in non-thunk functions.
   if (CGDebugInfo *DI = getDebugInfo()) {
     if (CGM.getCodeGenOpts().hasReducedDebugInfo() && !CurFuncIsThunk &&

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -403,9 +403,9 @@ void CodeGenFunction::FinishFunction(SourceLocation EndLoc) {
   // important to do this before we enter the return block or return
   // edges will be *really* confused.
   bool HasCleanups = EHStack.stable_begin() != PrologueCleanupDepth;
-  bool HasOnlyLifetimeMarkers =
-      HasCleanups && EHStack.containsOnlyLifetimeMarkers(PrologueCleanupDepth);
-  bool EmitRetDbgLoc = !HasCleanups || HasOnlyLifetimeMarkers;
+  bool HasOnlyNoopCleanups =
+      HasCleanups && EHStack.containsOnlyNoopCleanups(PrologueCleanupDepth);
+  bool EmitRetDbgLoc = !HasCleanups || HasOnlyNoopCleanups;
 
   std::optional<ApplyDebugLocation> OAL;
   if (HasCleanups) {

clang/lib/CodeGen/CodeGenFunction.h

@@ -718,6 +718,20 @@ class CodeGenFunction : public CodeGenTypeCache {
     }
   };
 
+  // We are using objects of this 'cleanup' class to emit fake.use calls
+  // for -fextend-lifetimes and -fextend-this-ptr. They are placed at the end of
+  // a variable's scope analogous to lifetime markers.
+  class FakeUse final : public EHScopeStack::Cleanup {
+    Address Addr;
+
+  public:
+    FakeUse(Address addr) : Addr(addr) {}
+
+    void Emit(CodeGenFunction &CGF, Flags flags) override {
+      CGF.EmitFakeUse(Addr);
+    }
+  };
+
   /// Header for data within LifetimeExtendedCleanupStack.
   struct LifetimeExtendedCleanupHeader {
     /// The size of the following cleanup object.
@@ -4991,6 +5005,8 @@ class CodeGenFunction : public CodeGenTypeCache {
 
   RValue EmitAtomicExpr(AtomicExpr *E);
 
+  void EmitFakeUse(Address Addr);
+
   //===--------------------------------------------------------------------===//
   //                         Annotations Emission
   //===--------------------------------------------------------------------===//

clang/lib/CodeGen/CodeGenModule.h

@@ -603,6 +603,9 @@ class CodeGenModule : public CodeGenTypeCache {
   /// void @llvm.lifetime.end(i64 %size, i8* nocapture <ptr>)
   llvm::Function *LifetimeEndFn = nullptr;
 
+  /// void @llvm.fake.use(...)
+  llvm::Function *FakeUseFn = nullptr;
+
   std::unique_ptr<SanitizerMetadata> SanitizerMD;
 
   llvm::MapVector<const Decl *, bool> DeferredEmptyCoverageMappingDecls;
@@ -1280,6 +1283,7 @@ class CodeGenModule : public CodeGenTypeCache {
 
   llvm::Function *getLLVMLifetimeStartFn();
   llvm::Function *getLLVMLifetimeEndFn();
+  llvm::Function *getLLVMFakeUseFn();
 
   // Make sure that this type is translated.
   void UpdateCompletedType(const TagDecl *TD);

clang/lib/CodeGen/EHScopeStack.h

@@ -87,6 +87,11 @@ enum CleanupKind : unsigned {
 
   LifetimeMarker = 0x8,
   NormalEHLifetimeMarker = LifetimeMarker | NormalAndEHCleanup,
+
+  // FakeUse needs to be recognized as a special cleanup similar to lifetime
+  // markers chiefly to be ignored in most contexts.
+  FakeUse = 0x10,
+  NormalFakeUse = FakeUse | NormalCleanup,
 };
 
 /// A stack of scopes which respond to exceptions, including cleanups
@@ -352,8 +357,8 @@ class EHScopeStack {
   void popTerminate();
 
   // Returns true iff the current scope is either empty or contains only
-  // lifetime markers, i.e. no real cleanup code
-  bool containsOnlyLifetimeMarkers(stable_iterator Old) const;
+  // noop cleanups, i.e. lifetime markers and fake uses.
+  bool containsOnlyNoopCleanups(stable_iterator Old) const;
 
   /// Determines whether the exception-scopes stack is empty.
   bool empty() const { return StartOfData == EndOfBuffer; }

clang/test/CodeGen/extend-liveness1.c

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 %s -O2 -emit-llvm -fextend-lifetimes -o - | FileCheck %s
+// Check that fake use calls are emitted at the correct locations, i.e.
+// at the end of lexical blocks and at the end of the function.
+
+extern int use(int);
+int glob1;
+int glob2;
+float globf;
+
+int foo(int i) {
+  // CHECK: define{{.*}}foo
+  if (i < 4) {
+    int j = i * 3;
+    if (glob1 > 3) {
+      float f = globf;
+      // CHECK: [[SSAVAL:%[a-z0-9]*]] = load float{{.*}}globf
+      j = f;
+      glob2 = j;
+      // CHECK: store{{.*}}glob2
+      // CHECK-NEXT: call void (...) @llvm.fake.use(float [[SSAVAL]])
+    }
+    glob1 = j;
+    // CHECK: store{{.*}}glob1
+    // CHECK-NEXT: call void (...) @llvm.fake.use(i32 %j.
+  }
+  // CHECK: call void (...) @llvm.fake.use(i32 %i)
+  // CHECK-NEXT: ret
+  return 4;
+}

clang/test/CodeGen/extend-liveness2.cpp

@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 %s -O2 -emit-llvm -fextend-lifetimes -fcxx-exceptions -fexceptions -o - | FileCheck %s
+// REQUIRES: x86-registered-target
+// This test checks that the fake_use concept works with exception handling and that we
+// can handle the __int128 data type.
+
+class A {
+public:
+  A(int i) : m_i(i) {}
+  void func(__int128 i128);
+
+  int m_i;
+};
+
+extern int bar();
+extern void foo();
+int glob;
+
+void A::func(__int128 i128) {
+  int j = 4;
+  try {
+    int k = bar();
+    foo();
+    // CHECK: [[SSAVAL:%[a-z0-9]*]] = invoke{{.*}}bar
+    glob = 0;
+    // CHECK: store{{.*}}glob
+    // CHECK-NEXT: call void (...) @llvm.fake.use(i32 [[SSAVAL]])
+  } catch (...) {
+    foo();
+  }
+  // CHECK-LABEL: try.cont:
+  // CHECK-DAG: call void (...) @llvm.fake.use({{.*%this}})
+  // CHECK-DAG: call void (...) @llvm.fake.use(i128 %i128.sroa.0.0.insert.insert)
+  // CHECK: ret void
+}