[MC] Allow overriding whether to output Elf_Rel or Elf_Rela relocations

[arichardson]

Summary

This makes it possible to test that tools correctly deal with REL vs RELA
differences in input files. In D42790 I am trying to fix LLD not writing
addends if the default output format uses Elf_Rel but an input contains
relocations in the Elf_Rela format. In order to test this I need to create
object files that use a different relocation format from the output.

lib/MC/MCELFObjectTargetWriter.cpp

 //===-- MCELFObjectTargetWriter.cpp - ELF Target Writer Subclass ----------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/MC/MCELFObjectWriter.h"
+#include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
 
+enum class ELFRelocationFormat { Default, Rel, Rela };
+static cl::opt<ELFRelocationFormat> RelocationFormat(
+    "elf-relocation-format",
+    cl::desc("Select whether to use REL or "
+             "RELA for relocations in the output object"),
+    cl::values(clEnumValN(ELFRelocationFormat::Default, "default",
+                          "Use the default kind for the current target"),
+               clEnumValN(ELFRelocationFormat::Rel, "rel",
+                          "Always emit REL relocations"),
+               clEnumValN(ELFRelocationFormat::Rela, "rela",
+                          "Always emit RELA relocations")),
+    cl::init(ELFRelocationFormat::Default), cl::Hidden);
+
+static bool shouldUseRelocationAddend(bool Default) {
+  switch (RelocationFormat) {
+  case ELFRelocationFormat::Rel:
+    return false;
+  case ELFRelocationFormat::Rela:
+    return true;
+  default:
+    return Default;
+  }
+}
+
 MCELFObjectTargetWriter::MCELFObjectTargetWriter(bool Is64Bit_, uint8_t OSABI_,
                                                  uint16_t EMachine_,
                                                  bool HasRelocationAddend_)
     : OSABI(OSABI_), EMachine(EMachine_),
-      HasRelocationAddend(HasRelocationAddend_), Is64Bit(Is64Bit_) {}
+      HasRelocationAddend(shouldUseRelocationAddend(HasRelocationAddend_)),
+      Is64Bit(Is64Bit_) {}
 
 bool MCELFObjectTargetWriter::needsRelocateWithSymbol(const MCSymbol &Sym,
                                                       unsigned Type) const {
   return false;
 }
 
 void
 MCELFObjectTargetWriter::sortRelocs(const MCAssembler &Asm,
                                     std::vector<ELFRelocationEntry> &Relocs) {
 }

test/MC/AArch64/elf-override-rela-and-rel.s

+// Check that we can override whether to ouput Elf_Rel vs Elf_Rela relocations
+
+// By default we use Elf_Rela relocations:
+// RUN: llvm-mc -filetype=obj -triple aarch64-none-linux-gnu %s -o - | \
+// RUN: llvm-readobj -r -s -section-data -  | FileCheck %s -check-prefix RELA
+
+// RELA:  Section {
+// RELA:    Name: .data
+// RELA:    SectionData (
+// RELA-NEXT:      0000: 00000000 00000000                    |........|
+// RELA-NEXT:    )
+// RELA:      Relocations [
+// RELA-NEXT:   Section (4) .rela.data {
+// RELA-NEXT:     0x0 R_AARCH64_ABS64 foo 0x123456
+// RELA-NEXT:   }
+// RELA-NEXT: ]
+
+// RUN: llvm-mc -filetype=obj -triple aarch64-none-linux-gnu %s -o - -elf-relocation-format=rel | \
+// RUN: llvm-readobj -r -s -section-data -  | FileCheck %s -check-prefix REL
+// REL:  Section {
+// REL:    Name: .data
+// REL:    SectionData (
+// REL-NEXT:      0000: 56341200 00000000                    |V4......|
+//                         ^---- Addend 0x123456 for the relocation below
+// REL-NEXT:    )
+// REL:      Relocations [
+// REL-NEXT:   Section (4) .rel.data {
+// The real addend is not printed for REL as it is in .data
+// REL-NEXT:     0x0 R_AARCH64_ABS64 foo 0x0
+// REL-NEXT:   }
+// REL-NEXT: ]
+
+// RUN: llvm-mc -filetype=obj -triple aarch64-none-linux-gnu %s -o - -elf-relocation-format=rela | \
+// RUN: llvm-readobj -r -s -section-data -  | FileCheck %s -check-prefix RELA
+// RUN: llvm-mc -filetype=obj -triple aarch64-none-linux-gnu %s -o - -elf-relocation-format=default | \
+// RUN: llvm-readobj -r -s -section-data -  | FileCheck %s -check-prefix RELA
+
+// Check that we only accept rela, rel, and default for the -elf-relocation-format flag
+// RUN: not llvm-mc -filetype=obj -triple aarch64-none-linux-gnu %s -o - \
+// RUN: -elf-relocation-format=none 2>&1 | FileCheck %s -check-prefix ERR
+// ERR: llvm-mc: for the -elf-relocation-format option: Cannot find option named 'none'
+.extern foo
+
+.data
+.quad foo + 0x123456
 No newline at end of file