Fix PR21189 -- Emit symbol subsection required to debug LLVM-built binaries with VS2012+

[timurrrr]

Summary

David, Eric,

This is a preliminary version.
I haven't updated some of the test/DebugInfo/COFF lit tests since changing the code just a little [which might happend during the review] requires significant amount of work to update the tests.
Please comment on the code and an example test modification and I'll update the rest of the tests if everything's OK with you.

––
Tim

[timurrrr]

ping?

[timurrrr]

Rebase now that r220526 has landed.

[timurrrr]

Using getDISubprogram(F).getDisplayName() to get demangled function name now

[timurrrr]

Oops, I've uploaded a few wrong hunks. Lemme fix that in a minute!

[timurrrr]

Fixed issues with the previous patch

[majnemer]

LGTM

[timurrrr]

Updated the rest of the tests.
I'm going to try a few more things locally and commit if everything's OK.

[timurrrr]

r220544!

lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp

static void EmitLabelDiff(MCStreamer &Streamer,
   MCContext &Context = Streamer.getContext();
   const MCExpr *FromRef = MCSymbolRefExpr::Create(From, Variant, Context),
                *ToRef   = MCSymbolRefExpr::Create(To, Variant, Context);
   const MCExpr *AddrDelta =
       MCBinaryExpr::Create(MCBinaryExpr::Sub, ToRef, FromRef, Context);
   Streamer.EmitValue(AddrDelta, Size);
 }
 
-const DISubprogram getDISubprogram(const Function &F, const LLVMContext &Ctx) {
-  for (auto &BI : F) {
-    const BasicBlock *BB = &BI;
-    for (auto &Inst : BB->getInstList()) {
-      DebugLoc DLoc = Inst.getDebugLoc();
-      if (DLoc.isUnknown())
-        continue;
-      const MDNode *Scope = DLoc.getScopeNode(Ctx);
-      DISubprogram Subprogram = getDISubprogram(Scope);
-      return Subprogram.describes(&F) ? Subprogram : DISubprogram();
-    }   
-  }
-
-  return DISubprogram();
-}
-
 void WinCodeViewLineTables::emitDebugInfoForFunction(const Function *GV) {
   // For each function there is a separate subsection
   // which holds the PC to file:line table.
   const MCSymbol *Fn = Asm->getSymbol(GV);
   assert(Fn);
 
   const FunctionInfo &FI = FnDebugInfo[GV];
   if (FI.Instrs.empty())
     return;
   assert(FI.End && "Don't know where the function ends?");
 
-  // FIXME: Find a way to get the demangled function name.
-  const StringRef FuncName = getDISubprogram(*GV, GV->getParent()->getContext()).getDisplayName();
-  printf("ZZZ ---%s---\n", FuncName.str().c_str());
-  fflush(0);
+  const StringRef FuncName = getDISubprogram(GV).getDisplayName();
   // Emit a symbol subsection, required by VS2012+ to find function boundaries.
   MCSymbol *SymbolsBegin = Asm->MMI->getContext().CreateTempSymbol(),
            *SymbolsEnd = Asm->MMI->getContext().CreateTempSymbol();
   Asm->OutStreamer.AddComment("Symbol subsection for " + Twine(FuncName));
   Asm->EmitInt32(COFF::DEBUG_SYMBOL_SUBSECTION);
   EmitLabelDiff(Asm->OutStreamer, SymbolsBegin, SymbolsEnd);
   Asm->OutStreamer.EmitLabel(SymbolsBegin);
   {

test/DebugInfo/COFF/simple.ll

 ; X86-NEXT: [[PROC_SEGMENT_START]]:
 ; X86-NEXT: .short  4423
 ; X86-NEXT: .zero   12
 ; X86-NEXT: .long [[END_OF_F]]-_f
 ; X86-NEXT: .zero   12
 ; X86-NEXT: .secrel32 _f
 ; X86-NEXT: .secidx _f
 ; X86-NEXT: .byte   0
-; X86-NEXT: .ascii  "_f"
+; X86-NEXT: .byte   102
 ; X86-NEXT: .byte   0
 ; X86-NEXT: [[PROC_SEGMENT_END]]:
 ; X86-NEXT: .short  2
 ; X86-NEXT: .short  4431
 ; X86-NEXT: [[F1_END]]:
 ; Padding
-; X86-NEXT: .zero   2
+; X86-NEXT: .zero   3
 ; Line table
 ; X86-NEXT: .long   242
 ; X86-NEXT: .long [[F2_END:.*]]-[[F2_START:.*]]
 ; X86-NEXT: [[F2_START]]:
 ; X86-NEXT: .secrel32 _f
 ; X86-NEXT: .secidx _f
 ; X86-NEXT: .short  0
 ; X86-NEXT: .long [[END_OF_F]]-_f

 ; OBJ32-NEXT:   0x30 IMAGE_REL_I386_SECTION _f
 ; OBJ32-NEXT:   0x44 IMAGE_REL_I386_SECREL _f
 ; OBJ32-NEXT:   0x48 IMAGE_REL_I386_SECTION _f
 ; OBJ32-NEXT: ]
 ; OBJ32:      Subsection [
 ; OBJ32-NEXT:   Type: 0xF1
 ; OBJ32-NOT:    ]
 ; OBJ32:        ProcStart {
-; OBJ32-NEXT:     FunctionName: _f
+; OBJ32-NEXT:     FunctionName: f
 ; OBJ32-NEXT:     Section: _f
 ; OBJ32-NEXT:     CodeSize: 0x6
 ; OBJ32-NEXT:   }
 ; OBJ32-NEXT:   ProcEnd
 ; OBJ32-NEXT: ]
 ; OBJ32:      FunctionLineTable [
 ; OBJ32-NEXT:   Name: _f
 ; OBJ32-NEXT:   CodeSize: 0x6