[RISCV] Use init_array instead of ctors for RISCV target, by default

[mgrang]

Summary

LLVM defaults to the newer .init_array/.fini_array scheme for static
constructors rather than the less desirable .ctors/.dtors (the UseCtors
flag defaults to false). This wasn't being respected in the RISC-V
backend because it fails to call TargetLoweringObjectFileELF::InitializeELF with the the appropriate
flag for UseInitArray.
This patch fixes this by implementing RISCVELFTargetObjectFile and overriding its Initialize method to call
InitializeELF(TM.Options.UseInitArray).

[asb]

Hi Mandeep, thanks for the patch. This looks good to me, but there are just two changes I would suggest prior to committing:

• Basing the test on test/CodeGen/Hexagon/ctor.ll would be better - that demonstrates that the -use-ctors flag is respected, which seems worth doing
• The patch description explains what was changed, but not how or why. I might add something like "LLVM defaults to the newer .init_array/.fini_array scheme for static constructors rather than the less desirable .ctors/.dtors (the UseCtors flag defaults to false). This wasn't being respected in the RISC-V backend because it fails to call TargetLoweringObjectFileELF::InitializeELF with the the appropriate flag for UseInitArray. This patch fixes this by implementing RISCVELFTargetObjectFile and overriding its Initialize method to call InitializeELF(TM.Options.UseInitArray)."

[mgrang]

Updated test.

lib/Target/RISCV/CMakeLists.txt

add_llvm_target(RISCVCodeGen
   RISCVFrameLowering.cpp
   RISCVInstrInfo.cpp
   RISCVISelDAGToDAG.cpp
   RISCVISelLowering.cpp
   RISCVMCInstLower.cpp
   RISCVRegisterInfo.cpp
   RISCVSubtarget.cpp
   RISCVTargetMachine.cpp
+  RISCVTargetObjectFile.cpp
   )
 
 add_subdirectory(AsmParser)
 add_subdirectory(Disassembler)
 add_subdirectory(InstPrinter)
 add_subdirectory(MCTargetDesc)
 add_subdirectory(TargetInfo)

lib/Target/RISCV/RISCVTargetMachine.cpp

 //===-- RISCVTargetMachine.cpp - Define TargetMachine for RISCV -----------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // Implements the info about RISCV target spec.
 //
 //===----------------------------------------------------------------------===//
 
 #include "RISCV.h"
 #include "RISCVTargetMachine.h"
+#include "RISCVTargetObjectFile.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/CodeGen/TargetLoweringObjectFileImpl.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/IR/LegacyPassManager.h"
 #include "llvm/Support/FormattedStream.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Target/TargetOptions.h"
RISCVTargetMachine::RISCVTargetMachine(const Target &T, const Triple &TT,
                                        StringRef CPU, StringRef FS,
                                        const TargetOptions &Options,
                                        Optional<Reloc::Model> RM,
                                        Optional<CodeModel::Model> CM,
                                        CodeGenOpt::Level OL, bool JIT)
     : LLVMTargetMachine(T, computeDataLayout(TT), TT, CPU, FS, Options,
                         getEffectiveRelocModel(TT, RM),
                         getEffectiveCodeModel(CM), OL),
-      TLOF(make_unique<TargetLoweringObjectFileELF>()),
+      TLOF(make_unique<RISCVELFTargetObjectFile>()),
       Subtarget(TT, CPU, FS, *this) {
   initAsmInfo();
 }
 
 namespace {
 class RISCVPassConfig : public TargetPassConfig {
 public:
   RISCVPassConfig(RISCVTargetMachine &TM, PassManagerBase &PM)

lib/Target/RISCV/RISCVTargetObjectFile.h

+//===-- RISCVTargetObjectFile.h - RISCV Object Info -*- C++ ---------*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_TARGET_RISCV_RISCVTARGETOBJECTFILE_H
+#define LLVM_LIB_TARGET_RISCV_RISCVTARGETOBJECTFILE_H
+
+#include "llvm/CodeGen/TargetLoweringObjectFileImpl.h"
+
+namespace llvm {
+class RISCVTargetMachine;
+
+/// This implementation is used for RISCV ELF targets.
+class RISCVELFTargetObjectFile : public TargetLoweringObjectFileELF {
+  void Initialize(MCContext &Ctx, const TargetMachine &TM) override;
+};
+
+} // end namespace llvm
+
+#endif

lib/Target/RISCV/RISCVTargetObjectFile.cpp

+//===-- RISCVTargetObjectFile.cpp - RISCV Object Info -----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "RISCVTargetObjectFile.h"
+#include "RISCVTargetMachine.h"
+
+using namespace llvm;
+
+void RISCVELFTargetObjectFile::Initialize(MCContext &Ctx,
+                                          const TargetMachine &TM) {
+  TargetLoweringObjectFileELF::Initialize(Ctx, TM);
+  InitializeELF(TM.Options.UseInitArray);
+}

test/CodeGen/RISCV/init-array.ll

+; RUN: llc -mtriple=riscv32-unknown-linux-gnu -verify-machineinstrs -o - %s \
+; RUN: | FileCheck %s
+; RUN: llc -mtriple=riscv64-unknown-linux-gnu -verify-machineinstrs -o - %s \
+; RUN: | FileCheck %s
+; RUN: llc -mtriple=riscv32-unknown-elf -verify-machineinstrs -o - %s \
+; RUN: | FileCheck %s
+; RUN: llc -mtriple=riscv64-unknown-elf -verify-machineinstrs -o - %s \
+; RUN: | FileCheck %s
+
+define internal void @_GLOBAL__I_a() section ".text.startup" {
+  ret void
+}
+
[e-mail]_ctors = appending global [1 x { i32, void ()* }] [{ i32, void ()* } { i32 65535, void ()* @_GLOBAL__I_a }]
+
+; CHECK: .section .init_array