Assume __cxa_allocate_exception returns an under-aligned memory on

Darwin if the version of libc++abi isn't new enough to include the fix
in r319123

This patch resurrects r264998, which was committed to work around a bug
in libc++abi that was causing _cxa_allocate_exception to return a memory
that wasn't double-word aligned.

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160328/154332.html

In addition, this patch makes clang issue a warning if the type of the
thrown object requires an alignment that is larger than the minimum
guaranteed by the target C++ runtime.

rdar://problem/49864414

Differential Revision: https://reviews.llvm.org/D61667

llvm-svn: 360404

clang/include/clang/AST/ASTContext.h

@@ -2166,6 +2166,13 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// pointers and large arrays get extra alignment.
   CharUnits getDeclAlign(const Decl *D, bool ForAlignof = false) const;
 
+  /// Return the alignment (in bytes) of the thrown exception object. This is
+  /// only meaningful for targets that allocate C++ exceptions in a system
+  /// runtime, such as those using the Itanium C++ ABI.
+  CharUnits getExnObjectAlignment() const {
+    return toCharUnitsFromBits(Target->getExnObjectAlignment());
+  }
+
   /// Get or compute information about the layout of the specified
   /// record (struct/union/class) \p D, which indicates its size and field
   /// position information.

clang/include/clang/Basic/DiagnosticGroups.td

@@ -408,6 +408,7 @@ def ObjCMultipleMethodNames : DiagGroup<"objc-multiple-method-names">;
 def ObjCFlexibleArray : DiagGroup<"objc-flexible-array">;
 def ObjCBoxing : DiagGroup<"objc-boxing">;
 def OpenCLUnsupportedRGBA: DiagGroup<"opencl-unsupported-rgba">;
+def UnderalignedExceptionObject : DiagGroup<"underaligned-exception-object">;
 def DeprecatedObjCIsaUsage : DiagGroup<"deprecated-objc-isa-usage">;
 def ExplicitInitializeCall : DiagGroup<"explicit-initialize-call">;
 def Packed : DiagGroup<"packed">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6584,6 +6584,12 @@ def err_throw_incomplete : Error<
   "cannot throw object of incomplete type %0">;
 def err_throw_incomplete_ptr : Error<
   "cannot throw pointer to object of incomplete type %0">;
+def warn_throw_underaligned_obj : Warning<
+  "underaligned exception object thrown">,
+  InGroup<UnderalignedExceptionObject>;
+def note_throw_underaligned_obj : Note<
+  "required alignment of type %0 (%1 bytes) is larger than the supported "
+  "alignment of C++ exception objects on this target (%2 bytes)">;
 def err_return_in_constructor_handler : Error<
   "return in the catch of a function try block of a constructor is illegal">;
 def warn_cdtor_function_try_handler_mem_expr : Warning<

clang/include/clang/Basic/TargetInfo.h

@@ -637,6 +637,21 @@ class TargetInfo : public virtual TransferrableTargetInfo,
   /// types for the given target.
   unsigned getSimdDefaultAlign() const { return SimdDefaultAlign; }
 
+  /// Return the alignment (in bits) of the thrown exception object. This is
+  /// only meaningful for targets that allocate C++ exceptions in a system
+  /// runtime, such as those using the Itanium C++ ABI.
+  virtual unsigned getExnObjectAlignment() const {
+    // Itanium says that an _Unwind_Exception has to be "double-word"
+    // aligned (and thus the end of it is also so-aligned), meaning 16
+    // bytes.  Of course, that was written for the actual Itanium,
+    // which is a 64-bit platform.  Classically, the ABI doesn't really
+    // specify the alignment on other platforms, but in practice
+    // libUnwind declares the struct with __attribute__((aligned)), so
+    // we assume that alignment here.  (It's generally 16 bytes, but
+    // some targets overwrite it.)
+    return getDefaultAlignForAttributeAligned();
+  }
+
   /// Return the size of intmax_t and uintmax_t for this target, in bits.
   unsigned getIntMaxTWidth() const {
     return getTypeWidth(IntMaxType);

clang/lib/Basic/Targets/OSTargets.h

@@ -133,6 +133,37 @@ class LLVM_LIBRARY_VISIBILITY DarwinTargetInfo : public OSTargetInfo<Target> {
   /// attribute on declarations that can be dynamically replaced.
   bool hasProtectedVisibility() const override { return false; }
 
+  unsigned getExnObjectAlignment() const override {
+    // Older versions of libc++abi guarantee an alignment of only 8-bytes for
+    // exception objects because of a bug in __cxa_exception that was
+    // eventually fixed in r319123.
+    llvm::VersionTuple MinVersion;
+    const llvm::Triple &T = this->getTriple();
+
+    // Compute the earliest OS versions that have the fix to libc++abi.
+    switch (T.getOS()) {
+    case llvm::Triple::Darwin:
+    case llvm::Triple::MacOSX: // Earliest supporting version is 10.14.
+      MinVersion = llvm::VersionTuple(10U, 14U);
+      break;
+    case llvm::Triple::IOS:
+    case llvm::Triple::TvOS: // Earliest supporting version is 12.0.0.
+      MinVersion = llvm::VersionTuple(12U);
+      break;
+    case llvm::Triple::WatchOS: // Earliest supporting version is 5.0.0.
+      MinVersion = llvm::VersionTuple(5U);
+      break;
+    default:
+      llvm_unreachable("Unexpected OS");
+    }
+
+    unsigned Major, Minor, Micro;
+    T.getOSVersion(Major, Minor, Micro);
+    if (llvm::VersionTuple(Major, Minor, Micro) < MinVersion)
+      return 64;
+    return OSTargetInfo<Target>::getExnObjectAlignment();
+  }
+
   TargetInfo::IntType getLeastIntTypeByWidth(unsigned BitWidth,
                                              bool IsSigned) const final {
     // Darwin uses `long long` for `int_least64_t` and `int_fast64_t`.

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -154,19 +154,6 @@ class ItaniumCXXABI : public CodeGen::CGCXXABI {
                                Address Ptr, QualType ElementType,
                                const CXXDestructorDecl *Dtor) override;
 
-  /// Itanium says that an _Unwind_Exception has to be "double-word"
-  /// aligned (and thus the end of it is also so-aligned), meaning 16
-  /// bytes.  Of course, that was written for the actual Itanium,
-  /// which is a 64-bit platform.  Classically, the ABI doesn't really
-  /// specify the alignment on other platforms, but in practice
-  /// libUnwind declares the struct with __attribute__((aligned)), so
-  /// we assume that alignment here.  (It's generally 16 bytes, but
-  /// some targets overwrite it.)
-  CharUnits getAlignmentOfExnObject() {
-    auto align = CGM.getContext().getTargetDefaultAlignForAttributeAligned();
-    return CGM.getContext().toCharUnitsFromBits(align);
-  }
-
   void emitRethrow(CodeGenFunction &CGF, bool isNoReturn) override;
   void emitThrow(CodeGenFunction &CGF, const CXXThrowExpr *E) override;
 
@@ -1191,7 +1178,7 @@ void ItaniumCXXABI::emitThrow(CodeGenFunction &CGF, const CXXThrowExpr *E) {
   llvm::CallInst *ExceptionPtr = CGF.EmitNounwindRuntimeCall(
       AllocExceptionFn, llvm::ConstantInt::get(SizeTy, TypeSize), "exception");
 
-  CharUnits ExnAlign = getAlignmentOfExnObject();
+  CharUnits ExnAlign = CGF.getContext().getExnObjectAlignment();
   CGF.EmitAnyExprToExn(E->getSubExpr(), Address(ExceptionPtr, ExnAlign));
 
   // Now throw the exception.

clang/lib/Sema/SemaExprCXX.cpp

@@ -941,6 +941,21 @@ bool Sema::CheckCXXThrowOperand(SourceLocation ThrowLoc,
     }
   }
 
+  // Under the Itanium C++ ABI, memory for the exception object is allocated by
+  // the runtime with no ability for the compiler to request additional
+  // alignment. Warn if the exception type requires alignment beyond the minimum
+  // guaranteed by the target C++ runtime.
+  if (Context.getTargetInfo().getCXXABI().isItaniumFamily()) {
+    CharUnits TypeAlign = Context.getTypeAlignInChars(Ty);
+    CharUnits ExnObjAlign = Context.getExnObjectAlignment();
+    if (ExnObjAlign < TypeAlign) {
+      Diag(ThrowLoc, diag::warn_throw_underaligned_obj);
+      Diag(ThrowLoc, diag::note_throw_underaligned_obj)
+          << Ty << (unsigned)TypeAlign.getQuantity()
+          << (unsigned)ExnObjAlign.getQuantity();
+    }
+  }
+
   return false;
 }
 

clang/test/CodeGenCXX/eh.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -triple x86_64-apple-darwin -std=c++11 -emit-llvm %s -o - \
-// RUN:   | FileCheck %s
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -triple x86_64-apple-macosx10.13.99 -std=c++11 -emit-llvm -o - %s | FileCheck --check-prefix=CHECK --check-prefix=UNALIGNED %s
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -triple x86_64-apple-macosx10.14 -std=c++11 -emit-llvm -o - %s | FileCheck --check-prefix=CHECK --check-prefix=ALIGNED %s
 
 struct test1_D {
   double d;
@@ -13,7 +13,8 @@ void test1() {
 // CHECK:       [[EXNOBJ:%.*]] = call i8* @__cxa_allocate_exception(i64 8)
 // CHECK-NEXT:  [[EXN:%.*]] = bitcast i8* [[EXNOBJ]] to [[DSTAR:%[^*]*\*]]
 // CHECK-NEXT:  [[EXN2:%.*]] = bitcast [[DSTAR]] [[EXN]] to i8*
-// CHECK-NEXT:  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 16 [[EXN2]], i8* align 8 bitcast ([[DSTAR]] @d1 to i8*), i64 8, i1 false)
+// UNALIGNED-NEXT:  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 8 [[EXN2]], i8* align 8 bitcast ([[DSTAR]] @d1 to i8*), i64 8, i1 false)
+// ALIGNED-NEXT:  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 16 [[EXN2]], i8* align 8 bitcast ([[DSTAR]] @d1 to i8*), i64 8, i1 false)
 // CHECK-NEXT:  call void @__cxa_throw(i8* [[EXNOBJ]], i8* bitcast ({ i8*, i8* }* @_ZTI7test1_D to i8*), i8* null) [[NR:#[0-9]+]]
 // CHECK-NEXT:  unreachable
 
@@ -466,7 +467,8 @@ int foo() {
   // CHECK: [[T0:%.*]] = call i8* @__cxa_allocate_exception(i64 16)
   // CHECK-NEXT: [[T1:%.*]] = bitcast i8* [[T0]] to %"class.test17::DerivedException"*
   // CHECK-NEXT: [[T2:%.*]] = bitcast %"class.test17::DerivedException"* [[T1]] to i8*
-  // CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 16 [[T2]], i8 0, i64 16, i1 false)
+  // UNALIGNED-NEXT: call void @llvm.memset.p0i8.i64(i8* align 8 [[T2]], i8 0, i64 16, i1 false)
+  // ALIGNED-NEXT: call void @llvm.memset.p0i8.i64(i8* align 16 [[T2]], i8 0, i64 16, i1 false)
 }
 }
 

clang/test/SemaCXX/warn-overaligned-type-thrown.cpp

@@ -0,0 +1,66 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.13.99 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions -DUNDERALIGNED %s
+// RUN: %clang_cc1 -triple arm64-apple-ios10 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions -DUNDERALIGNED %s
+// RUN: %clang_cc1 -triple arm64-apple-tvos10 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions -DUNDERALIGNED %s
+// RUN: %clang_cc1 -triple arm64-apple-watchos4 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions -DUNDERALIGNED %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.14 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple arm64-apple-ios12 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple arm64-apple-tvos12 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple arm64-apple-watchos5 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple arm-linux-gnueabi -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple aarch64-linux-gnueabi -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple mipsel-linux-gnu -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple mips64el-linux-gnu -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple wasm32-unknown-unknown -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple wasm64-unknown-unknown -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.14 -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions -Wno-underaligned-exception-object -DNODIAG %s
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -verify -fsyntax-only -std=c++11 -fcxx-exceptions -fexceptions -DNODIAG %s
+
+struct S0 {
+  S0();
+  int m;
+};
+
+struct Overaligned1 {
+  Overaligned1();
+  int __attribute__((aligned(16))) m;
+};
+
+struct __attribute__((aligned(16))) Overaligned2 {
+  Overaligned2();
+  int m;
+};
+
+struct Overaligned3 {
+  Overaligned3();
+  int __attribute__((aligned(64))) m;
+};
+
+void test0() {
+  throw S0();
+}
+
+void test1() {
+  throw Overaligned1();
+}
+
+void test2() {
+  throw Overaligned2();
+}
+
+void test3() {
+  throw Overaligned3();
+}
+
+#if defined(NODIAG)
+// expected-no-diagnostics
+#elif defined(UNDERALIGNED)
+// expected-warning@-14 {{underaligned exception object thrown}}
+// expected-note@-15 {{(16 bytes) is larger than the supported alignment of C++ exception objects on this target (8 bytes)}}
+// expected-warning@-12 {{underaligned exception object thrown}}
+// expected-note@-13 {{(16 bytes) is larger than the supported alignment of C++ exception objects on this target (8 bytes)}}
+// expected-warning@-10 {{underaligned exception object thrown}}
+// expected-note@-11 {{(64 bytes) is larger than the supported alignment of C++ exception objects on this target (8 bytes)}}
+#else
+// expected-warning@-13 {{underaligned exception object thrown}}
+// expected-note@-14 {{(64 bytes) is larger than the supported alignment of C++ exception objects on this target (16 bytes)}}
+#endif