[clang] Add support for __declspec(no_init_all) (#116847)

In MSVC, when `/d1initall` is enabled, `__declspec(no_init_all)` can be
applied to a type to suppress auto-initialization for all instances of
that type or to a function to suppress auto-initialization for all
locals within that function.

This change does the same for Clang, except that it applies to the
`-ftrivial-auto-var-init` flag instead.

NOTE: I did not add a Clang-specific spelling for this but would be
happy to make a followup PR if folks are interested in that.

Author: dpaoliello

clang/include/clang/Basic/Attr.td

@@ -4888,3 +4888,10 @@ def ClspvLibclcBuiltin: InheritableAttr {
   let Documentation = [ClspvLibclcBuiltinDoc];
   let SimpleHandler = 1;
 }
+
+def NoTrivialAutoVarInit: InheritableAttr {
+  let Spellings = [Declspec<"no_init_all">];
+  let Subjects = SubjectList<[Function, Tag]>;
+  let Documentation = [NoTrivialAutoVarInitDocs];
+  let SimpleHandler = 1;
+}

clang/include/clang/Basic/AttrDocs.td

@@ -8760,6 +8760,18 @@ Attribute used by `clspv`_ (OpenCL-C to Vulkan SPIR-V compiler) to identify func
 }];
 }
 
+def NoTrivialAutoVarInitDocs : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+The ``__declspec(no_init_all)`` attribute disables the automatic initialization that the
+`-ftrivial-auto-var-init`_ flag would have applied to locals in a marked function, or instances of
+a marked type. Note that this attribute has no effect for locals that are automatically initialized
+without the `-ftrivial-auto-var-init`_ flag.
+
+.. _`-ftrivial-auto-var-init`: ClangCommandLineReference.html#cmdoption-clang-ftrivial-auto-var-init
+}];
+}
+
 def DocCatNonBlockingNonAllocating : DocumentationCategory<"Performance Constraint Attributes"> {
   let Content = [{
 The ``nonblocking``, ``blocking``, ``nonallocating`` and ``allocating`` attributes can be attached

clang/lib/CodeGen/CGDecl.cpp

@@ -1899,13 +1899,16 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
   const Address Loc =
       locIsByrefHeader ? emission.getObjectAddress(*this) : emission.Addr;
 
+  auto hasNoTrivialAutoVarInitAttr = [&](const Decl *D) {
+    return D && D->hasAttr<NoTrivialAutoVarInitAttr>();
+  };
   // Note: constexpr already initializes everything correctly.
   LangOptions::TrivialAutoVarInitKind trivialAutoVarInit =
-      (D.isConstexpr()
+      ((D.isConstexpr() || D.getAttr<UninitializedAttr>() ||
+        hasNoTrivialAutoVarInitAttr(type->getAsTagDecl()) ||
+        hasNoTrivialAutoVarInitAttr(CurFuncDecl))
            ? LangOptions::TrivialAutoVarInitKind::Uninitialized
-           : (D.getAttr<UninitializedAttr>()
-                  ? LangOptions::TrivialAutoVarInitKind::Uninitialized
-                  : getContext().getLangOpts().getTrivialAutoVarInit()));
+           : getContext().getLangOpts().getTrivialAutoVarInit());
 
   auto initializeWhatIsTechnicallyUninitialized = [&](Address Loc) {
     if (trivialAutoVarInit ==
@@ -1944,13 +1947,13 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
                                   replaceUndef(CGM, isPattern, constant));
     }
 
-    if (constant && D.getType()->isBitIntType() &&
-        CGM.getTypes().typeRequiresSplitIntoByteArray(D.getType())) {
+    if (constant && type->isBitIntType() &&
+        CGM.getTypes().typeRequiresSplitIntoByteArray(type)) {
       // Constants for long _BitInt types are split into individual bytes.
       // Try to fold these back into an integer constant so it can be stored
       // properly.
-      llvm::Type *LoadType = CGM.getTypes().convertTypeForLoadStore(
-          D.getType(), constant->getType());
+      llvm::Type *LoadType =
+          CGM.getTypes().convertTypeForLoadStore(type, constant->getType());
       constant = llvm::ConstantFoldLoadFromConst(
           constant, LoadType, llvm::APInt::getZero(32), CGM.getDataLayout());
     }
@@ -1967,8 +1970,7 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
       // It may be that the Init expression uses other uninitialized memory,
       // but auto-var-init here would not help, as auto-init would get
       // overwritten by Init.
-      if (!D.getType()->isScalarType() || capturedByInit ||
-          isAccessedBy(D, Init)) {
+      if (!type->isScalarType() || capturedByInit || isAccessedBy(D, Init)) {
         initializeWhatIsTechnicallyUninitialized(Loc);
       }
     }

clang/test/CodeGenCXX/auto-var-init-attr.cpp

@@ -0,0 +1,59 @@
+// RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown -fblocks -fdeclspec -ftrivial-auto-var-init=zero %s -emit-llvm -o - | FileCheck %s
+
+struct S { char c; };
+class C { char c; };
+enum class E { ZERO };
+union U { char c; int i; };
+
+struct __declspec(no_init_all) NoInitS { char c; };
+class __declspec(no_init_all) NoInitC { char c; };
+enum class __declspec(no_init_all) NoInitE { ZERO };
+union __declspec(no_init_all) NoInitU { char c; int i; };
+
+extern "C" {
+  void test_no_attr() {
+    // CHECK-LABEL: @test_no_attr()
+    // CHECK-NEXT:  entry:
+    // CHECK-NEXT:  %s = alloca %struct.S, align 1
+    // CHECK-NEXT:  %c = alloca %class.C, align 1
+    // CHECK-NEXT:  %e = alloca i32, align 4
+    // CHECK-NEXT:  %u = alloca %union.U, align 4
+    // CHECK-NEXT:  call void @llvm.memset.p0.i64(ptr align 1 %s, i8 0, i64 1, i1 false)
+    // CHECK-NEXT:  call void @llvm.memset.p0.i64(ptr align 1 %c, i8 0, i64 1, i1 false)
+    // CHECK-NEXT:  store i32 0, ptr %e, align 4
+    // CHECK-NEXT:  call void @llvm.memset.p0.i64(ptr align 4 %u, i8 0, i64 4, i1 false)
+    // CHECK-NEXT   ret void
+    S s;
+    C c;
+    E e;
+    U u;
+  }
+
+  void __declspec(no_init_all) test_attr_on_function() {
+    // CHECK-LABEL: @test_attr_on_function()
+    // CHECK-NEXT:  entry:
+    // CHECK-NEXT:  %s = alloca %struct.S, align 1
+    // CHECK-NEXT:  %c = alloca %class.C, align 1
+    // CHECK-NEXT:  %e = alloca i32, align 4
+    // CHECK-NEXT:  %u = alloca %union.U, align 4
+    // CHECK-NEXT:  ret void
+    S s;
+    C c;
+    E e;
+    U u;
+  }
+
+  void test_attr_on_decl() {
+    // CHECK-LABEL: @test_attr_on_decl()
+    // CHECK-NEXT:  entry:
+    // CHECK-NEXT:  %s = alloca %struct.NoInitS, align 1
+    // CHECK-NEXT:  %c = alloca %class.NoInitC, align 1
+    // CHECK-NEXT:  %e = alloca i32, align 4
+    // CHECK-NEXT:  %u = alloca %union.NoInitU, align 4
+    // CHECK-NEXT:  ret void
+    NoInitS s;
+    NoInitC c;
+    NoInitE e;
+    NoInitU u;
+  }
+}