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

[Nick Desaulniers via cfe-commits]

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@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -Wunused -x c -verify %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -Wunused -x c++ -verify=expected,cxx %s
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+static int f(void) { return 42; }
+int g(void) __attribute__((alias("f")));
+
+static int foo [] = { 42, 0xDEAD }; // cxx-warning{{variable 'foo' is not needed and will not be emitted}}
+extern typeof(foo) bar __attribute__((unused, alias("foo")));
+
+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; }
+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; }
+int g1(void) __attribute__((alias("_ZL2f1v")));
+}
+
+/// 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; } // cxx-warning{{unused function 'resolver1'}}
+static int (*resolver1(int))() { return f; }
+int ifunc1() __attribute__((ifunc("_ZL9resolver1i")));
+
+/// We should report "unused function" for f3(int).
+namespace ns {
+static int f3(int) { return 42; } // cxx-warning{{unused function 'f3'}}
+static int f3() { return 42; } // cxx-warning{{unused function 'f3'}}
+int g3() __attribute__((alias("_ZN2nsL2f3Ev")));
+}
+
+template <class T>
+static void *f4(T) { return nullptr; }
+static void *f4() { return nullptr; } // cxx-warning{{unused function 'f4'}}
+extern void g4() __attribute__((ifunc("_ZL2f4IiEPvT_")));
+void *use3 = f4(0);
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nickdesaulniers wrote:

I'm curious what happens without the explicit usage. Template instantiation would fail.  Then we'd error that no such function was found? Perhaps consider adding another test case where that happens?  I'd like to ensure folks can't try to hide potential gadgets that way.

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