[Clang] Mark this pointer in destructors dead_on_return

clang/docs/ReleaseNotes.rst

@@ -43,6 +43,9 @@ C/C++ Language Potentially Breaking Changes
 C++ Specific Potentially Breaking Changes
 -----------------------------------------
 
+- Clang now more aggressively optimizes away stores to objects after they are
+  dead. This behavior can be disabled with ``-fno-lifetime-dse``.
+
 ABI Changes in This Version
 ---------------------------
 

clang/include/clang/Basic/CodeGenOptions.def

@@ -191,6 +191,8 @@ CODEGENOPT(EnableSegmentedStacks , 1, 0, Benign) ///< Set when -fsplit-stack is
 CODEGENOPT(StackClashProtector, 1, 0, Benign) ///< Set when -fstack-clash-protection is enabled.
 CODEGENOPT(NoImplicitFloat   , 1, 0, Benign) ///< Set when -mno-implicit-float is enabled.
 CODEGENOPT(NullPointerIsValid , 1, 0, Benign) ///< Assume Null pointer deference is defined.
+CODEGENOPT(StrictLifetimes , 1, 0, Benign) ///< Assume an object is dead
+                                           ///< after its destructor returns.
 CODEGENOPT(OpenCLCorrectlyRoundedDivSqrt, 1, 0, Benign) ///< -cl-fp32-correctly-rounded-divide-sqrt
 CODEGENOPT(HIPCorrectlyRoundedDivSqrt, 1, 1, Benign) ///< -fno-hip-fp32-correctly-rounded-divide-sqrt
 CODEGENOPT(DisableBlockSignatureString, 1, 0, Benign) ///< Set when -fdisable-block-signature-string is enabled.

clang/include/clang/Options/Options.td

@@ -2965,6 +2965,20 @@ attribute as undefined behavior. (And, thus the optimizer may assume that any
 pointer used in such a way must not have been null and optimize away the
 branches accordingly.) On by default.}]>;
 
+defm lifetime_dse
+    : BoolFOption<"lifetime-dse", CodeGenOpts<"StrictLifetimes">, DefaultTrue,
+                  NegFlag<SetFalse, [], [ClangOption, CC1Option],
+                          "Do not aggressively delete stores after the end of "
+                          "an object's lifetime">,
+                  PosFlag<SetTrue, [], [ClangOption, CC1Option],
+                          "Aggressively delete stores after the end of an "
+                          "object's lifetime">,
+                  BothFlags<[], [ClangOption, CLOption]>>,
+      DocBrief<
+          [{When enabled, mark an object dead after its destructor returns which allows the
+the optimizer to perform more aggressive optimizations in some cases, particularly dead
+store elimination.}]>;
+
 defm use_line_directives : BoolFOption<"use-line-directives",
   PreprocessorOutputOpts<"UseLineDirectives">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option],

clang/lib/CodeGen/CGCall.cpp

@@ -2780,7 +2780,8 @@ void CodeGenModule::ConstructAttributeList(StringRef Name,
   }
 
   // Apply `nonnull`, `dereferenceable(N)` and `align N` to the `this` argument,
-  // unless this is a thunk function.
+  // unless this is a thunk function. Add dead_on_return to the `this` argument
+  // in base class destructors to aid in DSE.
   // FIXME: fix this properly, https://reviews.llvm.org/D100388
   if (FI.isInstanceMethod() && !IRFunctionArgs.hasInallocaArg() &&
       !FI.arg_begin()->type->isVoidPointerType() && !IsThunk) {
@@ -2813,6 +2814,21 @@ void CodeGenModule::ConstructAttributeList(StringRef Name,
             .getAsAlign();
     Attrs.addAlignmentAttr(Alignment);
 
+    const auto *DD = dyn_cast_if_present<CXXDestructorDecl>(
+        CalleeInfo.getCalleeDecl().getDecl());
+    if (DD && CodeGenOpts.StrictLifetimes) {
+      const CXXRecordDecl *ClassDecl =
+          dyn_cast<CXXRecordDecl>(DD->getDeclContext());
+      // TODO(boomanaiden154): We are being intentionally conservative here
+      // as we gain experience with this optimization. We should remove the
+      // condition for non-virtual bases after more testing. We cannot add
+      // dead_on_return if we have virtual base classes because they will
+      // generally still be live after the base object destructor.
+      if (ClassDecl->getNumBases() == 0 && ClassDecl->getNumVBases() == 0)
+        Attrs.addDeadOnReturnAttr(llvm::DeadOnReturnInfo(
+            getMinimumClassObjectSize(ClassDecl).getQuantity()));
+    }
+
     ArgAttrs[IRArgs.first] = llvm::AttributeSet::get(getLLVMContext(), Attrs);
   }
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5681,6 +5681,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.addOptOutFlag(CmdArgs, options::OPT_fdelete_null_pointer_checks,
                      options::OPT_fno_delete_null_pointer_checks);
 
+  Args.addOptOutFlag(CmdArgs, options::OPT_flifetime_dse,
+                     options::OPT_fno_lifetime_dse);
+
   // LLVM Code Generator Options.
 
   if (Arg *A = Args.getLastArg(options::OPT_mabi_EQ_quadword_atomics)) {

clang/test/CodeGenCXX/destructor-dead-on-return.cpp

@@ -0,0 +1,72 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --filter "define" --version 6
+// Check that we appropriately annotate destructors with a dead_on_return
+// attribute.
+//
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fno-lifetime-dse -emit-llvm %s -o - | FileCheck %s --check-prefix NO-LIFETIME-DSE
+
+void opaque(char*);
+
+class Foo {
+public:
+  ~Foo();
+
+private:
+  char FooVar[4];
+};
+
+// CHECK-LABEL: define dso_local void @_ZN3FooD1Ev(
+// CHECK-SAME: ptr noundef nonnull align 1 dead_on_return(4) dereferenceable(4) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+// NO-LIFETIME-DSE-LABEL: define dso_local void @_ZN3FooD1Ev(
+// NO-LIFETIME-DSE-SAME: ptr noundef nonnull align 1 dereferenceable(4) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+Foo::~Foo() {
+  opaque(FooVar);
+}
+
+class Bar {
+public:
+  ~Bar();
+
+private:
+  char BarVar[8];
+};
+
+// CHECK-LABEL: define dso_local void @_ZN3BarD1Ev(
+// CHECK-SAME: ptr noundef nonnull align 1 dead_on_return(8) dereferenceable(8) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+// NO-LIFETIME-DSE-LABEL: define dso_local void @_ZN3BarD1Ev(
+// NO-LIFETIME-DSE-SAME: ptr noundef nonnull align 1 dereferenceable(8) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+Bar::~Bar() {
+  opaque(BarVar);
+}
+
+class BarInheritsFoo : public Foo {
+public:
+  ~BarInheritsFoo();
+
+private:
+  char BarVar[12];
+};
+
+// CHECK-LABEL: define dso_local void @_ZN14BarInheritsFooD1Ev(
+// CHECK-SAME: ptr noundef nonnull align 1 dereferenceable(16) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+// NO-LIFETIME-DSE-LABEL: define dso_local void @_ZN14BarInheritsFooD1Ev(
+// NO-LIFETIME-DSE-SAME: ptr noundef nonnull align 1 dereferenceable(16) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+BarInheritsFoo::~BarInheritsFoo() {
+  opaque(BarVar);
+}
+
+class BarVirtualInheritsFoo : public virtual Foo {
+public:
+  ~BarVirtualInheritsFoo();
+
+private:
+  char BarVar[16];
+};
+
+// CHECK-LABEL: define dso_local void @_ZN21BarVirtualInheritsFooD1Ev(
+// CHECK-SAME: ptr noundef nonnull align 8 dereferenceable(24) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+// NO-LIFETIME-DSE-LABEL: define dso_local void @_ZN21BarVirtualInheritsFooD1Ev(
+// NO-LIFETIME-DSE-SAME: ptr noundef nonnull align 8 dereferenceable(24) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+BarVirtualInheritsFoo::~BarVirtualInheritsFoo() {
+  opaque(BarVar);
+}