[clang] [Sema] Mark alias/ifunc targets used and consider mangled names (PR #87130)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/87130

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++. We fix false
positives while causing false negatives when overloads are present.

In addition, we mark ifunc targets as used to fix #63957.

While our approach has false negatives and does not handle namespace
scope names, the majority of alias/ifunc C++ uses (global scope with no
overloads) are handled.


>From 23422a0b3af3e070fed5ae86ed0f67acec066c0a Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Fri, 29 Mar 2024 17:48:14 -0700
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Created using spr 1.3.5-bogner
---
 clang/lib/Sema/CMakeLists.txt         |  1 +
 clang/lib/Sema/SemaDeclAttr.cpp       | 31 +++++++++++++++++----------
 clang/test/AST/ast-dump-attr-json.cpp |  1 +
 clang/test/Sema/alias-unused.c        | 30 +++++++++++++++++++++++++-
 4 files changed, 51 insertions(+), 12 deletions(-)

diff --git a/clang/lib/Sema/CMakeLists.txt b/clang/lib/Sema/CMakeLists.txt
index e8bff07ced0cfa..7657536fabd1eb 100644
--- a/clang/lib/Sema/CMakeLists.txt
+++ b/clang/lib/Sema/CMakeLists.txt
@@ -1,5 +1,6 @@
 set(LLVM_LINK_COMPONENTS
   Core
+  Demangle
   FrontendHLSL
   FrontendOpenMP
   MC
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index f25f3afd0f4af2..8ed5242f325e5e 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -42,6 +42,7 @@
 #include "clang/Sema/SemaInternal.h"
 #include "llvm/ADT/STLExtras.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"
@@ -1980,6 +1981,23 @@ 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. For C++, we look up the demangled name
+// ignoring parameters. This should handle the majority of use cases while
+// leaveing false positives for namespace scope names and false negatives in
+// the presence of overloads.
+static void markUsedForAliasOrIfunc(Sema &S, Decl *D, const ParsedAttr &AL,
+                                    StringRef Str) {
+  char *Demangled = llvm::itaniumDemangle(Str, /*ParseParams=*/false);
+  if (Demangled)
+    Str = Demangled;
+  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);
+  free(Demangled);
+}
+
 static void handleIFuncAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   StringRef Str;
   if (!S.checkStringLiteralArgumentAttr(AL, 0, Str))
@@ -1992,6 +2010,7 @@ 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));
 }
 
@@ -2026,17 +2045,7 @@ 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));
 }
 
diff --git a/clang/test/AST/ast-dump-attr-json.cpp b/clang/test/AST/ast-dump-attr-json.cpp
index 051c2956abfdf7..883e584bfedf07 100644
--- a/clang/test/AST/ast-dump-attr-json.cpp
+++ b/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": {
diff --git a/clang/test/Sema/alias-unused.c b/clang/test/Sema/alias-unused.c
index de9fc8cc737662..030b5737a93a21 100644
--- a/clang/test/Sema/alias-unused.c
+++ b/clang/test/Sema/alias-unused.c
@@ -1,7 +1,35 @@
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -Wunneeded-internal-declaration -x c -verify %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -Wunused -x c -verify %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -Wunused -x c++ -verify %s
+
+#ifdef __cplusplus
+extern "C" {
+#else
 // expected-no-diagnostics
+#endif
 static int f(void) { return 42; }
 int g(void) __attribute__((alias("f")));
 
 static int foo [] = { 42, 0xDEAD };
 extern typeof(foo) bar __attribute__((unused, alias("foo")));
+
+static int (*resolver(void))(void) { return f; }
+int ifunc(void) __attribute__((ifunc("resolver")));
+
+#ifdef __cplusplus
+}
+
+/// We demangle alias/ifunc target and mark all found functions as used.
+static int f1(int) { return 42; }
+static int f1(void) { return 42; }
+int g1(void) __attribute__((alias("_ZL2f1v")));
+
+static int (*resolver1(void))(void) { return f; }
+static int (*resolver1(int))(void) { return f; }
+int ifunc1(void) __attribute__((ifunc("_ZL9resolver1v")));
+
+namespace ns {
+static int f2(int) { return 42; } // expected-warning{{unused function 'f2'}}
+static int f2(void) { return 42; } // expected-warning{{unused function 'f2'}}
+int g2(void) __attribute__((alias("_ZN2nsL2f2Ev")));
+}
+#endif