[XCOFF][AIX] Support basic relocation type on AIX

Summary:

This patch intends to support three most common relocation type
on AIX: R_POS, R_TOC, R_RBR.
These three relocation type will be needed for object file generation
on AIX for small code model.
We will have follow up patches to bring relocation support for
large code model on AIX.

Reviewers: hubert.reinterpretcast, daltenty, DiggerLin

Differential Revision: https://reviews.llvm.org/D72027

Author: jasonliudev

llvm/include/llvm/BinaryFormat/XCOFF.h

@@ -20,7 +20,12 @@ namespace llvm {
 namespace XCOFF {
 
 // Constants used in the XCOFF definition.
-enum { FileNamePadSize = 6, NameSize = 8, SymbolTableEntrySize = 18 };
+enum {
+  FileNamePadSize = 6,
+  NameSize = 8,
+  SymbolTableEntrySize = 18,
+  RelocationSerializationSize32 = 10
+};
 
 enum ReservedSectionNum { N_DEBUG = -2, N_ABS = -1, N_UNDEF = 0 };
 

llvm/include/llvm/MC/MCXCOFFObjectWriter.h

@@ -28,6 +28,13 @@ class MCXCOFFObjectTargetWriter : public MCObjectTargetWriter {
   }
   bool is64Bit() const { return Is64Bit; }
 
+  // Returns relocation info such as type, sign and size.
+  // First element of the pair contains type,
+  // second element contains sign and size.
+  virtual std::pair<uint8_t, uint8_t>
+  getRelocTypeAndSignSize(const MCValue &Target, const MCFixup &Fixup,
+                          bool IsPCRel) const = 0;
+
 private:
   bool Is64Bit;
 };

llvm/lib/MC/MCXCOFFStreamer.cpp

@@ -69,9 +69,15 @@ void MCXCOFFStreamer::EmitInstToData(const MCInst &Inst,
   raw_svector_ostream VecOS(Code);
   Assembler.getEmitter().encodeInstruction(Inst, VecOS, Fixups, STI);
 
-  // TODO: Handle Fixups later
-
+  // Add the fixups and data.
   MCDataFragment *DF = getOrCreateDataFragment(&STI);
+  const size_t ContentsSize = DF->getContents().size();
+  auto &DataFragmentFixups = DF->getFixups();
+  for (auto &Fixup : Fixups) {
+    Fixup.setOffset(Fixup.getOffset() + ContentsSize);
+    DataFragmentFixups.push_back(Fixup);
+  }
+
   DF->setHasInstructions(STI);
   DF->getContents().append(Code.begin(), Code.end());
 }