[ELF] Suppress "duplicate symbol" when resolving STB_WEAK and STB_GNU_UNIQUE in different COMDATs

```
template <typename T> struct A {
  A() {}
  int value = 0;
};

template <typename Value> struct B {
  static A<int> a;
};

template <typename Value> A<int> B<Value>::a;

inline int foo() {
  return B<int>::a.value;
}
```

```
clang++ -c -fno-pic a.cc -o weak.o
g++ -c -fno-pic a.cc -o unique.o  # --enable-gnu-unique-object

# Duplicate symbol error. In postParse, we do not check `sym.binding`
ld.lld -e 0 weak.o unique.o
```

Mixing GCC and Clang object files in this case is not ideal. .bss._ZGVN1BIiE1aE
has different COMDAT groups. It appears to work in practice because the guard
variable prevents harm due to double initialization.

For the linker, we just stick with the rule that a weak binding does not cause
"duplicate symbol" errors.

Close https://github.com/llvm/llvm-project/issues/58232

Differential Revision: https://reviews.llvm.org/D136381

(cherry picked from commit 0051b6bb78772b0658f28e5f31ddf91c1589aab5)

2 files changed, 43 insertions, 1 deletions

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 927dc272b532..473809b05e9c 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -1157,7 +1157,7 @@ template <class ELFT> void ObjFile<ELFT>::postParse() {
       continue;
     }
 
-    if (binding == STB_WEAK)
+    if (sym.binding == STB_WEAK || binding == STB_WEAK)
       continue;
     std::lock_guard<std::mutex> lock(mu);
     ctx->duplicates.push_back({&sym, this, sec, eSym.st_value});
diff --git a/lld/test/ELF/comdat-binding2.s b/lld/test/ELF/comdat-binding2.s
new file mode 100644
index 000000000000..3ffd7252836c
--- /dev/null
+++ b/lld/test/ELF/comdat-binding2.s
@@ -0,0 +1,42 @@
+# REQUIRES: x86
+## Test we don't report duplicate definition errors when mixing Clang STB_WEAK
+## and GCC STB_GNU_UNIQUE symbols.
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 weak.s -o weak.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 unique.s -o unique.o
+# RUN: ld.lld weak.o unique.o -o weak
+# RUN: llvm-readelf -s weak | FileCheck %s --check-prefix=WEAK
+# RUN: ld.lld unique.o weak.o -o unique
+# RUN: llvm-readelf -s unique | FileCheck %s --check-prefix=UNIQUE
+
+# WEAK:   OBJECT  WEAK   DEFAULT [[#]] _ZN1BIiE1aE
+# UNIQUE: OBJECT  UNIQUE DEFAULT [[#]] _ZN1BIiE1aE
+
+#--- weak.s
+## Clang
+	.type	_ZN1BIiE1aE,@object
+	.section	.bss._ZN1BIiE1aE,"aGwR",@nobits,_ZN1BIiE1aE,comdat
+	.weak	_ZN1BIiE1aE
+_ZN1BIiE1aE:
+	.zero	4
+
+	.type	_ZGVN1BIiE1aE,@object
+	.section	.bss._ZGVN1BIiE1aE,"aGw",@nobits,_ZN1BIiE1aE,comdat
+	.weak	_ZGVN1BIiE1aE
+_ZGVN1BIiE1aE:
+	.quad	0
+
+#--- unique.s
+## GCC -fgnu-unique. Note the different group signature for the second group.
+	.weak	_ZN1BIiE1aE
+	.section	.bss._ZN1BIiE1aE,"awG",@nobits,_ZN1BIiE1aE,comdat
+	.type	_ZN1BIiE1aE, @gnu_unique_object
+_ZN1BIiE1aE:
+	.zero	4
+
+	.weak	_ZGVN1BIiE1aE
+	.section	.bss._ZGVN1BIiE1aE,"awG",@nobits,_ZGVN1BIiE1aE,comdat
+	.type	_ZGVN1BIiE1aE, @gnu_unique_object
+_ZGVN1BIiE1aE:
+	.zero	8