[MC] [COFF] Make sure that weak external symbols are undefined symbols

[mstorsjo]

Summary

For comdats (e.g. caused by -ffunction-sections), Section is already set here; make sure it's null, for the weak external symbol to be undefined.

This fixes PR46779.

This setup did reveal another issue wrt mingw style associative comdats in combination with weak symbols, when the actual leader symbol isn't named func but .weak.func.default in these cases. I'm looking at ways of fixing that as well (as a general headsup re potentially getting the fix for that into the release as well).

llvm/lib/MC/WinCOFFObjectWriter.cpp

if (Base && Base->getFragment()) {
     Sec = SectionMap[Base->getFragment()->getParent()];
     if (Sym->Section && Sym->Section != Sec)
       report_fatal_error("conflicting sections for symbol");
   }
 
   COFFSymbol *Local = nullptr;
   if (cast<MCSymbolCOFF>(MCSym).isWeakExternal()) {
     Sym->Data.StorageClass = COFF::IMAGE_SYM_CLASS_WEAK_EXTERNAL;
+    Sym->Section = nullptr;
 
     COFFSymbol *WeakDefault = getLinkedSymbol(MCSym);
     if (!WeakDefault) {
       std::string WeakName = (".weak." + MCSym.getName() + ".default").str();
       WeakDefault = createSymbol(WeakName);
       if (!Sec)
         WeakDefault->Data.SectionNumber = COFF::IMAGE_SYM_ABSOLUTE;
       else

llvm/test/MC/COFF/weak-comdat.s

+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-win32 %s -o %t.o
+// RUN: llvm-readobj --symbols %t.o | FileCheck %s
+
+// Test that the weak symbol is properly undefined, while originally being
+// the leader symbol for a comdat. (This can easily happen if building with
+// -ffunction-sections).
+
+        .section .text$func,"xr",one_only,func
+        .weak   func
+func:
+        ret
+
+// CHECK:       Symbol {
+// CHECK:         Name: func
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: IMAGE_SYM_UNDEFINED (0)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: WeakExternal (0x69)
+// CHECK-NEXT:    AuxSymbolCount: 1
+// CHECK-NEXT:    AuxWeakExternal {
+// CHECK-NEXT:      Linked: .weak.func.default (10)
+// CHECK-NEXT:      Search: Alias (0x3)
+// CHECK-NEXT:    }
+// CHECK-NEXT:  }
+// CHECK-NEXT:  Symbol {
+// CHECK-NEXT:    Name: .weak.func.default
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: .text$func (4)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: External (0x2)
+// CHECK-NEXT:    AuxSymbolCount: 0
+// CHECK-NEXT:  }