Write out global initializers and data rel directly to ELF file.

src/IceELFObjectWriter.h

 //===- subzero/src/IceELFObjectWriter.h - ELF object writer -----*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // Abstraction for a writer that is responsible for writing an ELF file.
@@ skipping 11 matching lines @@
 using namespace llvm::ELF;
 
 namespace Ice {
 
 // Higher level ELF object writer.  Manages section information and writes
 // the final ELF object.  The object writer will write to file the code
 // and data as it is being defined (rather than keep a copy).
 // After all definitions are written out, it will finalize the bookkeeping
 // sections and write them out.  Expected usage:
 //
-// (1) writeInitialELFHeader
-// (2) writeDataInitializer*
-// (3) writeFunctionCode*
-// (4) writeNonUserSections
+// (1) writeInitialELFHeader (invoke once)
+// (2) writeDataSection  (invoke once)
+// (3) writeFunctionCode (may invoke once per function)

[Jim Stichnoth, 2015/01/28 20:35:08]

Why "may"? Just based on the method signature, it seems like it *must* be
invoked once per function definition.

[jvoung (off chromium), 2015/01/28 23:37:53]

Done. Hmm yes =)

+// (4) writeConstantPool (may invoke once per type)

[Jim Stichnoth, 2015/01/28 20:35:08]

"(invoke at most once per primitive type)"

Would that be accurate?

[jvoung (off chromium), 2015/01/28 23:37:53]

Yes, or how about "must invoke once per pooled primitive type"?

Probably not worth fdata-section'ing these, and that might defeat the
constant-merging (not fully sure if that's based of the section name).

[Jim Stichnoth, 2015/01/29 00:38:14]

I agree - my main purpose of data-sections was to allow szbuild.py
frankenbinaries, and that doesn't apply to the internal constant pool entries.

+// (5) writeNonUserSections (invoke once)
+//
+// The requirement for writeDataSection to be invoked only once can
+// be relaxed if using -fdata-sections. The requirement to invoke only once
+// without -fdata-sections is so that variables that belong to each possible
+// SectionType are contiguous in the file. With -fdata-sections, each global
+// variable is in a separate section and therefore the sections will be
+// trivially contiguous.
+//
+// The motivation for requiring that writeFunctionCode happen after
+// writeDataSection: to keep the .text and .data sections contiguous in the
+// file. Having both -fdata-sections and -ffunction-sections does allow
+// relaxing this requirement.
 class ELFObjectWriter {
   ELFObjectWriter(const ELFObjectWriter &) = delete;
   ELFObjectWriter &operator=(const ELFObjectWriter &) = delete;
 
 public:
   ELFObjectWriter(GlobalContext &Ctx, ELFStreamer &Out);
 
   // Write the initial ELF header. This is just to reserve space in the ELF
   // file. Reserving space allows the other functions to write text
   // and data directly to the file and get the right file offsets.
   void writeInitialELFHeader();
 
+  // Copy initializer data for globals to file and note the offset and size
+  // of each global's definition in the symbol table.
+  // Use the given target's RelocationKind for any relocations.
+  void writeDataSection(const VariableDeclarationList &Vars,
+                        FixupKind RelocationKind);
+
   // Copy data of a function's text section to file and note the offset of the
   // symbol's definition in the symbol table.
   // Copy the text fixups for use after all functions are written.
+  // The text buffer and fixups are extracted from the Assembler object.
   void writeFunctionCode(const IceString &FuncName, bool IsInternal,
                          const Assembler *Asm);
 
-  // Copy initializer data for a global to file and note the offset and
-  // size of the global's definition in the symbol table.
-  // TODO(jvoung): This needs to know which section. This also needs the
-  // relocations to hook them up to the symbol table references. Also
-  // TODO is handling BSS (which just needs to note the size).
-  void writeDataInitializer(const IceString &VarName,
-                            const llvm::StringRef Data);
-
+  // Queries the GlobalContext for constant pools of the given type
+  // and writes out read-only data sections for those constants. This also
+  // fills the symbol table with labels for each constant pool entry.
   template <typename ConstType> void writeConstantPool(Type Ty);
 
-  // Do final layout and write out the rest of the object file, then
-  // patch up the initial ELF header with the final info.
+  // Do final layout and write out the rest of the object file.
+  // Finally, patch up the initial ELF header with the final info.
   void writeNonUserSections();
 
+  // Which type of ELF section a global variable initializer belongs to.
+  enum SectionType {
+    BaseSectionType = 0,
+    ROData = BaseSectionType,
+    Data,
+    BSS,
+    NumSectionTypes
+  };
+
 private:
   GlobalContext &Ctx;
   ELFStreamer &Str;
   bool SectionNumbersAssigned;
 
   // All created sections, separated into different pools.
   typedef std::vector<ELFSection *> SectionList;
   typedef std::vector<ELFTextSection *> TextSectionList;
   typedef std::vector<ELFDataSection *> DataSectionList;
   typedef std::vector<ELFRelocationSection *> RelSectionList;
   TextSectionList TextSections;
   RelSectionList RelTextSections;
   DataSectionList DataSections;
   RelSectionList RelDataSections;
-  DataSectionList RoDataSections;
-  RelSectionList RelRoDataSections;
+  DataSectionList RODataSections;
+  RelSectionList RelRODataSections;
+  DataSectionList BSSSections;
 
   // Handles to special sections that need incremental bookkeeping.
   ELFSection *NullSection;
   ELFStringTableSection *ShStrTab;
   ELFSymbolTableSection *SymTab;
   ELFStringTableSection *StrTab;
 
   template <typename T>
   T *createSection(const IceString &Name, Elf64_Word ShType,
                    Elf64_Xword ShFlags, Elf64_Xword ShAddralign,
                    Elf64_Xword ShEntsize);
 
+  // Create a relocation section, given the related section
+  // (e.g., .text, .data., .rodata).
+  ELFRelocationSection *
+  createRelocationSection(bool IsELF64, const ELFSection *RelatedSection);
+
   // Align the file position before writing out a section's data,
   // and return the position of the file.
   Elf64_Off alignFileOffset(Elf64_Xword Align);
 
   // Assign an ordering / section numbers to each section.
   // Fill in other information that is only known near the end
   // (such as the size, if it wasn't already incrementally updated).
   // This then collects all sections in the decided order, into one vector,
   // for conveniently writing out all of the section headers.
   void assignSectionNumbersInfo(SectionList &AllSections);
 
   // This function assigns .foo and .rel.foo consecutive section numbers.
   // It also sets the relocation section's sh_info field to the related
   // section's number.
   template <typename UserSectionList>
   void assignRelSectionNumInPairs(SizeT &CurSectionNumber,
                                   UserSectionList &UserSections,
                                   RelSectionList &RelSections,
                                   SectionList &AllSections);
 
   // Link the relocation sections to the symbol table.
   void assignRelLinkNum(SizeT SymTabNumber, RelSectionList &RelSections);
 
+  // Helper function for writeDataSection. Writes a data section of type
+  // SectionType, given the global variables Vars belonging to that SectionType.
+  void writeDataOfType(SectionType SectionType,
+                       const VariableDeclarationList &Vars,
+                       FixupKind RelocationKind, bool IsELF64);
+
   // Write the final relocation sections given the final symbol table.
   // May also be able to seek around the file and resolve function calls
   // that are for functions within the same section.
   void writeAllRelocationSections(bool IsELF64);
   void writeRelocationSections(bool IsELF64, RelSectionList &RelSections);
 
   // Write the ELF file header with the given information about sections.
   template <bool IsELF64>
   void writeELFHeaderInternal(Elf64_Off SectionHeaderOffset,
                               SizeT SectHeaderStrIndex, SizeT NumSections);
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEELFOBJECTWRITER_H