ErlangGCPrinter.cpp

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//===-- ErlangGCPrinter.cpp - Erlang/OTP frametable emitter -----*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the compiler plugin that is used in order to emit
// garbage collection information in a convenient layout for parsing and
// loading in the Erlang/OTP runtime.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/CodeGen/GCMetadataPrinter.h"
#include "llvm/CodeGen/GCs.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Metadata.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/Target/TargetLoweringObjectFile.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetSubtargetInfo.h"

using namespace llvm;

namespace {

  class ErlangGCPrinter : public GCMetadataPrinter {
  public:
    void beginAssembly(AsmPrinter &AP) override;
    void finishAssembly(AsmPrinter &AP) override;
  };

}

static GCMetadataPrinterRegistry::Add<ErlangGCPrinter>
X("erlang", "erlang-compatible garbage collector");

void llvm::linkErlangGCPrinter() { }

void ErlangGCPrinter::beginAssembly(AsmPrinter &AP) { }

void ErlangGCPrinter::finishAssembly(AsmPrinter &AP) {
  MCStreamer &OS = AP.OutStreamer;
  unsigned IntPtrSize =
      AP.TM.getSubtargetImpl()->getDataLayout()->getPointerSize();

  // Put this in a custom .note section.
  AP.OutStreamer.SwitchSection(AP.getObjFileLowering().getContext()
    .getELFSection(".note.gc", ELF::SHT_PROGBITS, 0,
                   SectionKind::getDataRel()));

  // For each function...
  for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
    GCFunctionInfo &MD = **FI;

    /** A compact GC layout. Emit this data structure:
     *
     * struct {
     *   int16_t PointCount;
     *   void *SafePointAddress[PointCount];
     *   int16_t StackFrameSize; (in words)
     *   int16_t StackArity;
     *   int16_t LiveCount;
     *   int16_t LiveOffsets[LiveCount];
     * } __gcmap_<FUNCTIONNAME>;
     **/

    // Align to address width.
    AP.EmitAlignment(IntPtrSize == 4 ? 2 : 3);

    // Emit PointCount.
    OS.AddComment("safe point count");
    AP.EmitInt16(MD.size());

    // And each safe point...
    for (GCFunctionInfo::iterator PI = MD.begin(), PE = MD.end(); PI != PE;
         ++PI) {
      // Emit the address of the safe point.
      OS.AddComment("safe point address");
      MCSymbol *Label = PI->Label;
      AP.EmitLabelPlusOffset(Label/*Hi*/, 0/*Offset*/, 4/*Size*/);
    }

    // Stack information never change in safe points! Only print info from the
    // first call-site.
    GCFunctionInfo::iterator PI = MD.begin();

    // Emit the stack frame size.
    OS.AddComment("stack frame size (in words)");
    AP.EmitInt16(MD.getFrameSize() / IntPtrSize);

    // Emit stack arity, i.e. the number of stacked arguments.
    unsigned RegisteredArgs = IntPtrSize == 4 ? 5 : 6;
    unsigned StackArity = MD.getFunction().arg_size() > RegisteredArgs ?
                          MD.getFunction().arg_size() - RegisteredArgs : 0;
    OS.AddComment("stack arity");
    AP.EmitInt16(StackArity);

    // Emit the number of live roots in the function.
    OS.AddComment("live root count");
    AP.EmitInt16(MD.live_size(PI));

    // And for each live root...
    for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
                                       LE = MD.live_end(PI);
                                       LI != LE; ++LI) {
      // Emit live root's offset within the stack frame.
      OS.AddComment("stack index (offset / wordsize)");
      AP.EmitInt16(LI->StackOffset / IntPtrSize);
    }
  }
}