[COFF] Improve relocation against discarded section error

[rnk]

Summary

Reuse the "referenced by" note diagnostic code that we already use for
undefined symbols. In my case, it turned this:

lld-link: error: relocation against symbol in discarded section: .text
lld-link: error: relocation against symbol in discarded section: .text
...

Into this:

lld-link: error: relocation against symbol in discarded section: .text
>>> referenced by libANGLE.lib(CompilerGL.obj):(.SCOVP$M)
>>> referenced by libANGLE.lib(CompilerGL.obj):(.SCOVP$M)
...

lld-link: error: relocation against symbol in discarded section: .text
>>> referenced by obj/third_party/angle/libGLESv2/entry_points_egl_ext.obj:(.SCOVP$M)
>>> referenced by obj/third_party/angle/libGLESv2/entry_points_egl_ext.obj:(.SCOVP$M)
...

I think the new output is more useful.

[ruiu]

LGTM

COFF/Chunks.cpp

 //===- Chunks.cpp ---------------------------------------------------------===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "Chunks.h"
 #include "InputFiles.h"
 #include "Symbols.h"
 #include "Writer.h"
+#include "SymbolTable.h"
 #include "lld/Common/ErrorHandler.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/BinaryFormat/COFF.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
if (!Sym) {
           check(File->getCOFFObj()->getSymbol(Rel.SymbolTableIndex));
       StringRef Name;
       File->getCOFFObj()->getSymbolName(Sym, Name);
 
       // MinGW mode object files (built by GCC) can have leftover sections
       // with relocations against discarded comdat sections. Such sections
       // are left as is, with relocations untouched.
       if (!Config->MinGW)
-        error("relocation against symbol in discarded section: " + Name);
+        error("relocation against symbol in discarded section: " + Name +
+              getSymbolLocations(File, Rel.SymbolTableIndex));
       continue;
     }
     // Get the output section of the symbol for this relocation.  The output
     // section is needed to compute SECREL and SECTION relocations used in debug
     // info.
     Chunk *C = Sym->getChunk();
     OutputSection *OS = C ? C->getOutputSection() : nullptr;
 
     // Only absolute and __ImageBase symbols lack an output section. For any
     // other symbol, this indicates that the chunk was discarded.  Normally
     // relocations against discarded sections are an error.  However, debug info
     // sections are not GC roots and can end up with these kinds of relocations.
     // Skip these relocations.
     if (!OS && !isa<DefinedAbsolute>(Sym) && !isa<DefinedSynthetic>(Sym)) {
       if (isCodeView() || isDWARF())
         continue;
       error("relocation against symbol in discarded section: " +
-            Sym->getName());
+            Sym->getName() + getSymbolLocations(File, Rel.SymbolTableIndex));
       continue;
     }
     uint64_t S = Sym->getRVA();
 
     // Compute the RVA of the relocation for relative relocations.
     uint64_t P = RVA + Rel.VirtualAddress;
     switch (Config->Machine) {
     case AMD64:

COFF/SymbolTable.h

private:
   StringRef findByPrefix(StringRef Prefix);
 
   llvm::DenseMap<llvm::CachedHashStringRef, Symbol *> SymMap;
   std::unique_ptr<BitcodeCompiler> LTO;
 };
 
 extern SymbolTable *Symtab;
 
+std::string getSymbolLocations(ObjFile *File, uint32_t SymIndex);
+
 } // namespace coff
 } // namespace lld
 
 #endif

COFF/SymbolTable.cpp

if (!D || D->getChunk() != SC || D->getValue() > Addr ||
       continue;
 
     Candidate = D;
   }
 
   return Candidate;
 }
 
-static std::string getSymbolLocations(ObjFile *File, uint32_t SymIndex) {
+std::string lld::coff::getSymbolLocations(ObjFile *File, uint32_t SymIndex) {
   struct Location {
     Symbol *Sym;
     std::pair<StringRef, uint32_t> FileLine;
   };
   std::vector<Location> Locations;
 
   for (Chunk *C : File->getChunks()) {
     auto *SC = dyn_cast<SectionChunk>(C);

test/COFF/reloc-discarded.s

 # REQUIRES: x86
 # RUN: echo -e '.section .bss,"bw",discard,main_global\n.global main_global\n main_global:\n .long 0' | \
 # RUN:     llvm-mc - -filetype=obj -o %t1.obj -triple x86_64-windows-msvc
 # RUN: llvm-mc %s -filetype=obj -o %t2.obj -triple x86_64-windows-msvc
 
 # LLD should report an error and not assert regardless of whether we are doing
 # GC.
 
 # RUN: not lld-link -entry:main -nodefaultlib %t1.obj %t2.obj -out:%t.exe -opt:ref   2>&1 | FileCheck %s
 # RUN: not lld-link -entry:main -nodefaultlib %t1.obj %t2.obj -out:%t.exe -opt:noref 2>&1 | FileCheck %s
 
 # CHECK: error: relocation against symbol in discarded section: assoc_global
+# CHECK: >>> referenced by {{.*}}reloc-discarded{{.*}}.obj:(main)
 
   .section  .bss,"bw",discard,main_global
   .globl  main_global
   .p2align  2
 main_global:
   .long 0
 
   .section  .CRT$XCU,"dr",associative,main_global