[Sema] Mark alias/ifunc targets used and consider mangled names

https://reviews.llvm.org/D54188 marked "alias" targets as used in C to
fix -Wunused false positives. This patch extends the approach to handle
mangled names to support global scope names in C++ and the
`overloadable` attribute in C.

(Note: we should skip `UsingShadowDecl`, which would trigger an
assertion failure in `ItaniumMangleContextImpl::mangleCXXName`.
See regression test added by commit 1c2afba.)

In addition, we mark ifunc targets as used to fix #63957 (temporarily
used by xz; ifunc was removed by
tukaani-project/xz@689ae24)

While our approach has false negatives for namespace scope names, the
majority of alias/ifunc C++ uses (global scope with no overloads) are
handled.

Note: The following function with internal linkage but C language
linkage type is mangled in Clang but not in GCC. This inconsistency
makes alias/ifunc difficult to use in C++ with portability (#88593).
```
extern "C" {
static void f0() {}
// GCC: void g0() __attribute__((alias("_ZL2f0v")));
// Clang: void g0() __attribute__((alias("f0")));
}
```

Pull Request: #87130

Author: MaskRay

clang/lib/Sema/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(LLVM_LINK_COMPONENTS
   Core
+  Demangle
   FrontendHLSL
   FrontendOpenMP
   MC

clang/lib/Sema/SemaDeclAttr.cpp

@@ -45,6 +45,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/STLForwardCompat.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/Demangle/Demangle.h"
 #include "llvm/IR/Assumptions.h"
 #include "llvm/MC/MCSectionMachO.h"
 #include "llvm/Support/Error.h"
@@ -1983,6 +1984,38 @@ static void handleWeakRefAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (S.Context) WeakRefAttr(S.Context, AL));
 }
 
+// Mark alias/ifunc target as used. Due to name mangling, we look up the
+// demangled name ignoring parameters (not supported by microsoftDemangle
+// https://github.com/llvm/llvm-project/issues/88825). This should handle the
+// majority of use cases while leaving namespace scope names unmarked.
+static void markUsedForAliasOrIfunc(Sema &S, Decl *D, const ParsedAttr &AL,
+                                    StringRef Str) {
+  std::unique_ptr<char, llvm::FreeDeleter> Demangled;
+  if (S.getASTContext().getCXXABIKind() != TargetCXXABI::Microsoft)
+    Demangled.reset(llvm::itaniumDemangle(Str, /*ParseParams=*/false));
+  std::unique_ptr<MangleContext> MC(S.Context.createMangleContext());
+  SmallString<256> Name;
+
+  const DeclarationNameInfo Target(
+      &S.Context.Idents.get(Demangled ? Demangled.get() : Str), AL.getLoc());
+  LookupResult LR(S, Target, Sema::LookupOrdinaryName);
+  if (S.LookupName(LR, S.TUScope)) {
+    for (NamedDecl *ND : LR) {
+      if (!isa<FunctionDecl>(ND) && !isa<VarDecl>(ND))
+        continue;
+      if (MC->shouldMangleDeclName(ND)) {
+        llvm::raw_svector_ostream Out(Name);
+        Name.clear();
+        MC->mangleName(GlobalDecl(ND), Out);
+      } else {
+        Name = ND->getIdentifier()->getName();
+      }
+      if (Name == Str)
+        ND->markUsed(S.Context);
+    }
+  }
+}
+
 static void handleIFuncAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   StringRef Str;
   if (!S.checkStringLiteralArgumentAttr(AL, 0, Str))
@@ -1995,6 +2028,7 @@ static void handleIFuncAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     return;
   }
 
+  markUsedForAliasOrIfunc(S, D, AL, Str);
   D->addAttr(::new (S.Context) IFuncAttr(S.Context, AL, Str));
 }
 
@@ -2029,17 +2063,7 @@ static void handleAliasAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     }
   }
 
-  // Mark target used to prevent unneeded-internal-declaration warnings.
-  if (!S.LangOpts.CPlusPlus) {
-    // FIXME: demangle Str for C++, as the attribute refers to the mangled
-    // linkage name, not the pre-mangled identifier.
-    const DeclarationNameInfo target(&S.Context.Idents.get(Str), AL.getLoc());
-    LookupResult LR(S, target, Sema::LookupOrdinaryName);
-    if (S.LookupQualifiedName(LR, S.getCurLexicalContext()))
-      for (NamedDecl *ND : LR)
-        ND->markUsed(S.Context);
-  }
-
+  markUsedForAliasOrIfunc(S, D, AL, Str);
   D->addAttr(::new (S.Context) AliasAttr(S.Context, AL, Str));
 }
 

clang/test/AST/ast-dump-attr-json.cpp

@@ -46,6 +46,7 @@ __thread __attribute__ ((tls_model ("local-exec"))) int tls_model_var;
 // CHECK-NEXT:    "tokLen": 11
 // CHECK-NEXT:   }
 // CHECK-NEXT:  },
+// CHECK-NEXT:  "isUsed": true,
 // CHECK-NEXT:  "name": "global_decl",
 // CHECK-NEXT:  "mangledName": "global_decl",
 // CHECK-NEXT:  "type": {

clang/test/Sema/alias-unused-win.cpp

@@ -7,7 +7,7 @@ extern "C" {
 static int f(void) { return 42; } // cxx-warning{{unused function 'f'}}
 int g(void) __attribute__((alias("f")));
 
-static int foo [] = { 42, 0xDEAD }; // cxx-warning{{variable 'foo' is not needed and will not be emitted}}
+static int foo [] = { 42, 0xDEAD };
 extern typeof(foo) bar __attribute__((unused, alias("foo")));
 
 static int __attribute__((overloadable)) f0(int x) { return x; } // expected-warning{{unused function 'f0'}}

clang/test/Sema/alias-unused.cpp

@@ -14,24 +14,26 @@ extern typeof(foo) bar __attribute__((unused, alias("foo")));
 /// We report a warning in C++ mode because the internal linkage `resolver` gets
 /// mangled as it does not have a language linkage. GCC does not mangle
 /// `resolver` or report a warning.
-static int (*resolver(void))(void) { return f; } // expected-warning{{unused function 'resolver'}}
+static int (*resolver(void))(void) { return f; } // cxx-warning{{unused function 'resolver'}}
 int ifunc(void) __attribute__((ifunc("resolver")));
 
-static int __attribute__((overloadable)) f0(int x) { return x; } // expected-warning{{unused function 'f0'}}
+static int __attribute__((overloadable)) f0(int x) { return x; }
 static float __attribute__((overloadable)) f0(float x) { return x; } // expected-warning{{unused function 'f0'}}
 int g0(void) __attribute__((alias("_ZL2f0i")));
 
 #ifdef __cplusplus
-static int f1() { return 42; } // expected-warning{{unused function 'f1'}}
+static int f1() { return 42; }
 int g1(void) __attribute__((alias("_ZL2f1v")));
 }
 
-static int f2(int) { return 42; } // expected-warning{{unused function 'f2'}}
-static int f2() { return 42; } // expected-warning{{unused function 'f2'}}
+/// We demangle alias/ifunc target and mark all found functions as used.
+
+static int f2(int) { return 42; } // cxx-warning{{unused function 'f2'}}
+static int f2() { return 42; }
 int g2() __attribute__((alias("_ZL2f2v")));
 
-static int (*resolver1())() { return f; } // expected-warning{{unused function 'resolver1'}}
-static int (*resolver1(int))() { return f; } // expected-warning{{unused function 'resolver1'}}
+static int (*resolver1())() { return f; } // cxx-warning{{unused function 'resolver1'}}
+static int (*resolver1(int))() { return f; }
 int ifunc1() __attribute__((ifunc("_ZL9resolver1i")));
 
 /// TODO: We should report "unused function" for f3(int).

utils/bazel/llvm-project-overlay/clang/BUILD.bazel

@@ -1136,6 +1136,7 @@ cc_library(
         "//llvm:AllTargetsAsmParsers",
         "//llvm:AllTargetsCodeGens",
         "//llvm:Core",
+        "//llvm:Demangle",
         "//llvm:FrontendHLSL",
         "//llvm:FrontendOpenMP",
         "//llvm:MC",