[LTO] Add SelectionKind to IRSymtab and use it in ld.lld/LLVMgold

In PGO, a C++ external linkage function `foo` has a private counter
`__profc_foo` and a private `__profd_foo` in a `comdat nodeduplicate`.

A `__attribute__((weak))` function `foo` has a weak hidden counter `__profc_foo`
and a private `__profd_foo` in a `comdat nodeduplicate`.

In `ld.lld a.o b.o`, say a.o defines an external linkage `foo` and b.o
defines a weak `foo`. Currently we treat `comdat nodeduplicate` as `comdat any`,
ld.lld will incorrectly consider `b.o:__profc_foo` non-prevailing.  In the worst
case when `b.o:__profd_foo` is retained and `b.o:__profc_foo` isn't, there will
be dangling reference causing an `undefined hidden symbol` error.

Add SelectionKind to `Comdat` in IRSymtab and let linkers ignore nodeduplicate comdat.

Differential Revision: https://reviews.llvm.org/D106228

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/tools/gold/gold-plugin.cpp b/llvm/tools/gold/gold-plugin.cpp
index 9870f3a..6bcb1e4 100644
--- a/llvm/tools/gold/gold-plugin.cpp
+++ b/llvm/tools/gold/gold-plugin.cpp
@@ -623,8 +623,10 @@ static ld_plugin_status claim_file_hook(const ld_plugin_input_file *file,
     sym.comdat_key = nullptr;
     int CI = Sym.getComdatIndex();
     if (CI != -1) {
-      StringRef C = Obj->getComdatTable()[CI];
-      sym.comdat_key = strdup(C.str().c_str());
+      // Not setting comdat_key for nodeduplicate ensuress we don't deduplicate.
+      std::pair<StringRef, Comdat::SelectionKind> C = Obj->getComdatTable()[CI];
+      if (C.second != Comdat::NoDeduplicate)
+        sym.comdat_key = strdup(C.first.str().c_str());
     }
 
     sym.resolution = LDPR_UNKNOWN;