Fix bug where WinCOFFObjectWriter would assume starting from an empty output.

[klimek]

Summary

Starting on an input stream that is not at offset 0 would trigger the
assert in WinCOFFObjectWriter.cpp:1065:

assert(getStream().tell() <= (*i)->Header.PointerToRawData &&
               "Section::PointerToRawData is insane!");

[bkramer]

Looks good. Nice test case :)

[klimek]

Thx. Submitted as r253464.

include/llvm/MC/MCObjectWriter.h

 
 protected:
   unsigned IsLittleEndian : 1;
 
 protected: // Can only create subclasses.
   MCObjectWriter(raw_pwrite_stream &OS, bool IsLittleEndian)
       : OS(&OS), IsLittleEndian(IsLittleEndian) {}
 
+  unsigned getInitialOffset() {
+    return OS->tell();
+  }
+
 public:
   virtual ~MCObjectWriter();
 
   /// lifetime management
   virtual void reset() {}
 
   bool isLittleEndian() const { return IsLittleEndian; }
 

lib/MC/WinCOFFObjectWriter.cpp

for (auto &Section : Sections) {
     if (Assoc->Number == -1)
       continue;
 
     Section->Symbol->Aux[0].Aux.SectionDefinition.Number = Assoc->Number;
   }
 
   // Assign file offsets to COFF object file structures.
 
-  unsigned offset = 0;
+  unsigned offset = getInitialOffset();
 
   if (UseBigObj)
     offset += COFF::Header32Size;
   else
     offset += COFF::Header16Size;
   offset += COFF::SectionSize * Header.NumberOfSections;
 
   for (const auto &Section : Asm) {

test/MC/COFF/stdin.s

+// REQUIRES: shell
+// RUN: ( echo "test"; llvm-mc -filetype=obj -triple i686-pc-win32 %s ) > %t
+