iniital checkin of ELFWriter implementation

For now, the elf writer is only capable of emitting an empty elf file, with
a section table and a section table string table.  This will be enhanced
in the future :)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22291 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ELFWriter.cpp

@@ -0,0 +1,230 @@
+//===-- ELFWriter.cpp - Target-independent ELF Writer code ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by the LLVM research group and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the target-independent ELF writer.  This file writes out
+// the ELF file in the following order:
+//
+//  #1. ELF Header
+//  #2. '.data' section
+//  #3. '.bss' section
+//  ...
+//  #X. '.shstrtab' section
+//  #Y. Section Table
+//
+// The entries in the section table are laid out as:
+//  #0. Null entry [required]
+//  #1. ".data" entry - global variables with initializers.     [ if needed ]
+//  #2. ".bss" entry  - global variables without initializers.  [ if needed ]
+//  #3. ".text" entry - the program code
+//  ...
+//  #N. ".shstrtab" entry - String table for the section names.
+
+//
+// NOTE: This code should eventually be extended to support 64-bit ELF (this
+// won't be hard), but we haven't done so yet!
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/ELFWriter.h"
+#include "llvm/Module.h"
+#include "llvm/Target/TargetMachine.h"
+using namespace llvm;
+
+ELFWriter::ELFWriter(std::ostream &o, TargetMachine &tm) : O(o), TM(tm) {
+  e_machine = 0;  // e_machine defaults to 'No Machine'
+  e_flags = 0;    // e_flags defaults to 0, no flags.
+
+  is64Bit = TM.getTargetData().getPointerSizeInBits() == 64;  
+  isLittleEndian = TM.getTargetData().isLittleEndian();
+}
+
+// doInitialization - Emit the file header and all of the global variables for
+// the module to the ELF file.
+bool ELFWriter::doInitialization(Module &M) {
+  outbyte(0x7F);                     // EI_MAG0
+  outbyte('E');                      // EI_MAG1
+  outbyte('L');                      // EI_MAG2
+  outbyte('F');                      // EI_MAG3
+  outbyte(is64Bit ? 2 : 1);          // EI_CLASS
+  outbyte(isLittleEndian ? 1 : 2);   // EI_DATA
+  outbyte(1);                        // EI_VERSION
+  for (unsigned i = OutputBuffer.size(); i != 16; ++i)
+    outbyte(0);                      // EI_PAD up to 16 bytes.
+  
+  // This should change for shared objects.
+  outhalf(1);                        // e_type = ET_REL
+  outhalf(e_machine);                // e_machine = whatever the target wants
+  outword(1);                        // e_version = 1
+  outaddr(0);                        // e_entry = 0 -> no entry point in .o file
+  outaddr(0);                        // e_phoff = 0 -> no program header for .o
+
+  ELFHeader_e_shoff_Offset = OutputBuffer.size();
+  outaddr(0);                        // e_shoff
+  outword(e_flags);                  // e_flags = whatever the target wants
+
+  assert(!is64Bit && "These sizes need to be adjusted for 64-bit!");
+  outhalf(52);                       // e_ehsize = ELF header size
+  outhalf(0);                        // e_phentsize = prog header entry size
+  outhalf(0);                        // e_phnum     = # prog header entries = 0
+  outhalf(40);                       // e_shentsize = sect header entry size
+
+  
+  ELFHeader_e_shnum_Offset = OutputBuffer.size();
+  outhalf(0);                        // e_shnum     = # of section header ents
+  ELFHeader_e_shstrndx_Offset = OutputBuffer.size();
+  outhalf(0);                        // e_shstrndx  = Section # of '.shstrtab'
+
+  // Add the null section.
+  SectionList.push_back(ELFSection());
+
+  // Okay, the ELF header has been completed, emit the .data section next.
+  ELFSection DataSection(".data", OutputBuffer.size());
+  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
+       I != E; ++I)
+    EmitDATASectionGlobal(I);
+
+  // If the .data section is nonempty, add it to our list.
+  if ((DataSection.Size = OutputBuffer.size()-DataSection.Offset)) {
+    DataSection.Align = 4;   // FIXME: Compute!
+    SectionList.push_back(DataSection);
+  }
+
+  // Okay, emit the .bss section next.
+  ELFSection BSSSection(".bss", OutputBuffer.size());
+  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
+       I != E; ++I)
+    EmitBSSSectionGlobal(I);
+
+  // If the .bss section is nonempty, add it to our list.
+  if ((BSSSection.Size = OutputBuffer.size()-BSSSection.Offset)) {
+    BSSSection.Align = 4;  // FIXME: Compute!
+    SectionList.push_back(BSSSection);
+  }
+
+  return false;
+}
+
+// isCOMM - A global variable should be emitted to the common area if it is zero
+// initialized and has linkage that permits it to be merged with other globals.
+static bool isCOMM(GlobalVariable *GV) {
+  return GV->getInitializer()->isNullValue() &&
+    (GV->hasLinkOnceLinkage() || GV->hasInternalLinkage() ||
+     GV->hasWeakLinkage());
+}
+
+// EmitDATASectionGlobal - Emit a global variable to the .data section if it
+// belongs there.
+void ELFWriter::EmitDATASectionGlobal(GlobalVariable *GV) {
+  if (!GV->hasInitializer()) return;
+
+  // Do not emit a symbol here if it should be emitted to the common area.
+  if (isCOMM(GV)) return;
+
+  EmitGlobal(GV);
+}
+
+void ELFWriter::EmitBSSSectionGlobal(GlobalVariable *GV) {
+  if (!GV->hasInitializer()) return;
+
+  // FIXME: We don't support BSS yet!
+  return;
+
+  EmitGlobal(GV);
+}
+
+void ELFWriter::EmitGlobal(GlobalVariable *GV) {
+}
+
+
+bool ELFWriter::runOnMachineFunction(MachineFunction &MF) {
+  return false;
+}
+
+/// doFinalization - Now that the module has been completely processed, emit
+/// the ELF file to 'O'.
+bool ELFWriter::doFinalization(Module &M) {
+  // Emit the string table for the sections in the ELF file we have.
+  EmitSectionTableStringTable();
+
+  // Emit the .o file section table.
+  EmitSectionTable();
+
+  // Emit the .o file to the specified stream.
+  O.write((char*)&OutputBuffer[0], OutputBuffer.size());
+
+  // Free the output buffer.
+  std::vector<unsigned char>().swap(OutputBuffer);
+  return false;
+}
+
+/// EmitSectionTableStringTable - This method adds and emits a section for the
+/// ELF Section Table string table: the string table that holds all of the
+/// section names.
+void ELFWriter::EmitSectionTableStringTable() {
+  // First step: add the section for the string table to the list of sections:
+  SectionList.push_back(ELFSection(".shstrtab", OutputBuffer.size()));
+  SectionList.back().Type = 3;     // SHT_STRTAB
+
+  // Now that we know which section number is the .shstrtab section, update the
+  // e_shstrndx entry in the ELF header.
+  fixhalf(SectionList.size()-1, ELFHeader_e_shstrndx_Offset);
+
+  // Set the NameIdx of each section in the string table and emit the bytes for
+  // the string table.
+  unsigned Index = 0;
+
+  for (unsigned i = 0, e = SectionList.size(); i != e; ++i) {
+    // Set the index into the table.  Note if we have lots of entries with
+    // common suffixes, we could memoize them here if we cared.
+    SectionList[i].NameIdx = Index;
+
+    // Add the name to the output buffer, including the null terminator.
+    OutputBuffer.insert(OutputBuffer.end(), SectionList[i].Name.begin(),
+                        SectionList[i].Name.end());
+    // Add a null terminator.
+    OutputBuffer.push_back(0);
+
+    // Keep track of the number of bytes emitted to this section.
+    Index += SectionList[i].Name.size()+1;
+  }
+
+  // Set the size of .shstrtab now that we know what it is.
+  SectionList.back().Size = Index;
+}
+
+/// EmitSectionTable - Now that we have emitted the entire contents of the file
+/// (all of the sections), emit the section table which informs the reader where
+/// the boundaries are.
+void ELFWriter::EmitSectionTable() {
+  // Now that all of the sections have been emitted, set the e_shnum entry in
+  // the ELF header.
+  fixhalf(SectionList.size(), ELFHeader_e_shnum_Offset);
+  
+  // Now that we know the offset in the file of the section table (which we emit
+  // next), update the e_shoff address in the ELF header.
+  fixaddr(OutputBuffer.size(), ELFHeader_e_shoff_Offset);
+  
+  // Emit all of the section table entries.
+  for (unsigned i = 0, e = SectionList.size(); i != e; ++i) {
+    const ELFSection &S = SectionList[i];
+    outword(S.NameIdx);  // sh_name - Symbol table name idx
+    outword(S.Type);     // sh_type - Section contents & semantics
+    outword(S.Flags);    // sh_flags - Section flags.
+    outaddr(S.Addr);     // sh_addr - The mem address this section appears in.
+    outaddr(S.Offset);   // sh_offset - The offset from the start of the file.
+    outword(S.Size);     // sh_size - The section size.
+    outword(S.Link);     // sh_link - Section header table index link.
+    outword(S.Info);     // sh_info - Auxillary information.
+    outword(S.Align);    // sh_addralign - Alignment of section.
+    outword(S.EntSize);  // sh_entsize - Size of each entry in the section.
+  }
+
+  // Release the memory allocated for the section list.
+  std::vector<ELFSection>().swap(SectionList);
+}