[LLD] [COFF] Don't error out on duplicate absolute symbols with the same value

Both MS link.exe and GNU ld.bfd handle it this way; one can have
multiple object files defining the same absolute symbols, as long
as it defines it to the same value. But if there are multiple absolute
symbols with differing values, it is treated as an error.

Differential Revision: https://reviews.llvm.org/D71981

Author: mstorsjo

lld/COFF/SymbolTable.cpp

@@ -591,7 +591,10 @@ Symbol *SymbolTable::addAbsolute(StringRef n, COFFSymbolRef sym) {
   s->isUsedInRegularObj = true;
   if (wasInserted || isa<Undefined>(s) || s->isLazy())
     replaceSymbol<DefinedAbsolute>(s, n, sym);
-  else if (!isa<DefinedCOFF>(s))
+  else if (auto *da = dyn_cast<DefinedAbsolute>(s)) {
+    if (!da->isEqual(sym))
+      reportDuplicate(s, nullptr);
+  } else if (!isa<DefinedCOFF>(s))
     reportDuplicate(s, nullptr);
   return s;
 }
@@ -603,7 +606,10 @@ Symbol *SymbolTable::addAbsolute(StringRef n, uint64_t va) {
   s->isUsedInRegularObj = true;
   if (wasInserted || isa<Undefined>(s) || s->isLazy())
     replaceSymbol<DefinedAbsolute>(s, n, va);
-  else if (!isa<DefinedCOFF>(s))
+  else if (auto *da = dyn_cast<DefinedAbsolute>(s)) {
+    if (!da->isEqual(va))
+      reportDuplicate(s, nullptr);
+  } else if (!isa<DefinedCOFF>(s))
     reportDuplicate(s, nullptr);
   return s;
 }

lld/COFF/Symbols.h

@@ -229,6 +229,14 @@ class DefinedAbsolute : public Defined {
   uint64_t getRVA() { return va - config->imageBase; }
   void setVA(uint64_t v) { va = v; }
 
+  bool isEqual(COFFSymbolRef s) const {
+    return va == s.getValue();
+  }
+
+  bool isEqual(uint64_t otherVa) const {
+    return va == otherVa;
+  }
+
   // Section index relocations against absolute symbols resolve to
   // this 16 bit number, and it is the largest valid section index
   // plus one. This variable keeps it.

lld/test/COFF/duplicate-absolute-same.s

@@ -0,0 +1,14 @@
+// REQUIRES: x86
+// RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t.obj %s
+// RUN: echo -e ".globl myabsolute\nmyabsolute = 0" > %t.dupl.s
+// RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t.dupl.obj %t.dupl.s
+// RUN: lld-link /out:%t.exe %t.obj %t.dupl.obj -subsystem:console -entry:entry 2>&1 | FileCheck --allow-empty %s
+
+// CHECK-NOT: error: duplicate symbol: myabsolute
+
+.globl myabsolute
+myabsolute = 0
+
+.globl entry
+entry:
+    ret

lld/test/COFF/duplicate-absolute.s

@@ -1,6 +1,6 @@
 // REQUIRES: x86
 // RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t.obj %s
-// RUN: echo -e ".globl myabsolute\nmyabsolute = 0" > %t.dupl.s
+// RUN: echo -e ".globl myabsolute\nmyabsolute = 1" > %t.dupl.s
 // RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t.dupl.obj %t.dupl.s
 // RUN: not lld-link /out:%t.exe %t.obj %t.dupl.obj 2>&1 | FileCheck %s