[MC] Set addrsig symbol as weak/weakExternal

[zequanwu]

Summary

No all symbols in .addrsig table have definitions. So, setting them as weak to avoid undefined symbol error during linking.

[pcc]

Wouldn't this make all address significant symbols weak external?

The intent is that we don't emit additional symbols if the address significance table is enabled (see e.g. the call to GV.use_empty() before calling emitAddrsigSym in llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp). If you're still seeing undefined symbols with that, I think we need to make that condition more powerful instead.

llvm/lib/MC/ELFObjectWriter.cpp

public:
 
   void executePostLayoutBinding(MCAssembler &Asm,
                                 const MCAsmLayout &Layout) override;
 
   void markGnuAbi() override { SeenGnuAbi = true; }
   bool seenGnuAbi() const { return SeenGnuAbi; }
   void emitAddrsigSection() override { EmitAddrsigSection = true; }
   void addAddrsigSymbol(const MCSymbol *Sym) override {
+    const MCSymbolELF *S = cast<MCSymbolELF>(Sym);
+    S->setBinding(ELF::STB_WEAK);
     AddrsigSyms.push_back(Sym);
   }
 
   friend struct ELFWriter;
 };
 
 class ELFSingleObjectWriter : public ELFObjectWriter {
   raw_pwrite_stream &OS;

llvm/lib/MC/WinCOFFObjectWriter.cpp

public:
 
   void createFileSymbols(MCAssembler &Asm);
   void setWeakDefaultNames();
   void assignSectionNumbers();
   void assignFileOffsets(MCAssembler &Asm, const MCAsmLayout &Layout);
 
   void emitAddrsigSection() override { EmitAddrsigSection = true; }
   void addAddrsigSymbol(const MCSymbol *Sym) override {
-    AddrsigSyms.push_back(Sym);
+    const MCSymbolCOFF *S = cast<MCSymbolCOFF>(Sym);
+    S->setIsWeakExternal();
+    AddrsigSyms.push_back(S);
   }
 
   uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
 };
 
 } // end anonymous namespace
 
 //------------------------------------------------------------------------------

llvm/test/MC/COFF/addrsig.s

 // RUN: llvm-mc -filetype=obj -triple x86_64-pc-win32 %s -o - | llvm-readobj -S --symbols --sd --addrsig - | FileCheck %s
 
 // CHECK:      Name: .llvm_addrsig
 // CHECK-NEXT: VirtualSize: 0x0
 // CHECK-NEXT: VirtualAddress: 0x0
 // CHECK-NEXT: RawDataSize: 4
 // CHECK-NEXT: PointerToRawData:
 // CHECK-NEXT: PointerToRelocations: 0x0
 // CHECK-NEXT: PointerToLineNumbers: 0x0
 // CHECK-NEXT: RelocationCount: 0
 // CHECK-NEXT: LineNumberCount: 0
 // CHECK-NEXT: Characteristics [ (0x100800)
 // CHECK-NEXT:   IMAGE_SCN_ALIGN_1BYTES (0x100000)
 // CHECK-NEXT:   IMAGE_SCN_LNK_REMOVE (0x800)
 // CHECK-NEXT: ]
 // CHECK-NEXT: SectionData (
-// CHECK-NEXT:   0000: 080A0B02
+// CHECK-NEXT:   0000: 080C0F02
 // CHECK-NEXT: )
 
 // CHECK: Symbols [
 // CHECK: Name: .text
 // CHECK: AuxSectionDef
 // CHECK: Name: .data
 // CHECK: AuxSectionDef
 // CHECK: Name: .bss
 // CHECK: AuxSectionDef
 // CHECK: Name: .llvm_addrsig
 // CHECK: AuxSectionDef
 // CHECK: Name: g1
+// 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.g1.default (10)
+// CHECK-NEXT: Search: Alias (0x3)
 // CHECK: Name: g2
 // CHECK: Name: g3
+// 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.g3.default (14)
+// CHECK-NEXT: Search: Alias (0x3)
 // CHECK: Name: local
+// 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.local.default (17)
+// CHECK-NEXT: Search: Alias (0x3)
 
 // CHECK:      Addrsig [
 // CHECK-NEXT:   Sym: g1 (8)
-// CHECK-NEXT:   Sym: g3 (10)
-// CHECK-NEXT:   Sym: local (11)
+// CHECK-NEXT:   Sym: g3 (12)
+// CHECK-NEXT:   Sym: local (15)
 // CHECK-NEXT:   Sym: .data (2)
 // CHECK-NEXT: ]
 
 .addrsig
 .addrsig_sym g1
 .globl g2
 .addrsig_sym g3
 .addrsig_sym local
 .addrsig_sym .Llocal
 
 local:
 
 .data
 .Llocal:

llvm/test/MC/ELF/addrsig.s

 // CHECK-NEXT:   Address:
 // CHECK-NEXT:   Offset:
 // CHECK-NEXT:   Size: 4
 // CHECK-NEXT:   Link: 4
 // CHECK-NEXT:   Info: 0
 // CHECK-NEXT:   AddressAlignment: 1
 // CHECK-NEXT:   EntrySize: 0
 // CHECK-NEXT:   SectionData (
-// CHECK-NEXT:     0000: 03050201
+// CHECK-NEXT:     0000: 02040501
 // CHECK-NEXT:   )
 // CHECK-NEXT: }
 // CHECK-NEXT: Section {
 // CHECK-NEXT:   Index: 4
 // CHECK-NEXT:   Name: .symtab
 
 // CHECK:      Symbol {
 // CHECK:      Symbol {
 // CHECK-NEXT:   Name:
 // CHECK-NEXT:   Value:
 // CHECK-NEXT:   Size:
 // CHECK-NEXT:   Binding:
 // CHECK-NEXT:   Type:
 // CHECK-NEXT:   Other:
 // CHECK-NEXT:   Section: [[SEC:.*]]
 // CHECK-NEXT: }
 // CHECK-NEXT: Symbol {
-// CHECK-NEXT:   Name: local
+// CHECK-NEXT:   Name: g1
 // CHECK-NEXT:   Value:
 // CHECK-NEXT:   Size:
-// CHECK-NEXT:   Binding:
+// CHECK-NEXT:   Binding: Weak
 // CHECK-NEXT:   Type:
 // CHECK-NEXT:   Other:
-// CHECK-NEXT:   Section: [[SEC]]
+// CHECK-NEXT:   Section:
 // CHECK-NEXT: }
 // CHECK-NEXT: Symbol {
-// CHECK-NEXT:   Name: g1
+// CHECK-NEXT:   Name: g2
 // CHECK-NEXT:   Value:
 // CHECK-NEXT:   Size:
 // CHECK-NEXT:   Binding:
 // CHECK-NEXT:   Type:
 // CHECK-NEXT:   Other:
 // CHECK-NEXT:   Section:
 // CHECK-NEXT: }
 // CHECK-NEXT: Symbol {
-// CHECK-NEXT:   Name: g2
+// CHECK-NEXT:   Name: g3
 // CHECK-NEXT:   Value:
 // CHECK-NEXT:   Size:
-// CHECK-NEXT:   Binding:
+// CHECK-NEXT:   Binding: Weak
 // CHECK-NEXT:   Type:
 // CHECK-NEXT:   Other:
 // CHECK-NEXT:   Section:
 // CHECK-NEXT: }
 // CHECK-NEXT: Symbol {
-// CHECK-NEXT:   Name: g3
+// CHECK-NEXT:   Name: local
+// CHECK-NEXT:   Value:
+// CHECK-NEXT:   Size:
+// CHECK-NEXT:   Binding: Weak
+// CHECK-NEXT:   Type:
+// CHECK-NEXT:   Other:
+// CHECK-NEXT:   Section: [[SEC]]
+// CHECK-NEXT: }
 
 // CHECK:      Addrsig [
-// CHECK-NEXT:   Sym: g1 (3)
-// CHECK-NEXT:   Sym: g3 (5)
-// CHECK-NEXT:   Sym: local (2)
+// CHECK-NEXT:   Sym: g1 (2)
+// CHECK-NEXT:   Sym: g3 (4)
+// CHECK-NEXT:   Sym: local (5)
 // CHECK-NEXT:   Sym:  (1)
 // CHECK-NEXT: ]
 
 // ASM:      .addrsig
 // ASM-NEXT: .addrsig_sym g1
 .addrsig
 .addrsig_sym g1
 .globl g2