From 1d7926a98123d40b5e694b5a656497d77f1c3c19 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin@martin.st>
Date: Tue, 21 Jul 2020 23:39:37 +0300
Subject: [PATCH] [MC] [COFF] Make sure that weak external symbols are
 undefined symbols

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.

Differential Revision: https://reviews.llvm.org/D84507
---
 lib/MC/WinCOFFObjectWriter.cpp |  1 +
 test/MC/COFF/weak-comdat.s     | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)
 create mode 100644 test/MC/COFF/weak-comdat.s

diff --git a/lib/MC/WinCOFFObjectWriter.cpp b/lib/MC/WinCOFFObjectWriter.cpp
index 4796ef53105..8e7bf1eb016 100644
--- a/lib/MC/WinCOFFObjectWriter.cpp
+++ b/lib/MC/WinCOFFObjectWriter.cpp
@@ -375,6 +375,7 @@ void WinCOFFObjectWriter::DefineSymbol(const MCSymbol &MCSym,
   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) {
diff --git a/test/MC/COFF/weak-comdat.s b/test/MC/COFF/weak-comdat.s
new file mode 100644
index 00000000000..8605da6b521
--- /dev/null
+++ b/test/MC/COFF/weak-comdat.s
@@ -0,0 +1,34 @@
+// 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:  }