path: root/lib/CodeGen/SelectionDAG/FastISel.cpp

//===-- FastISel.cpp - Implementation of the FastISel class ---------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the implementation of the FastISel class.
//
// "Fast" instruction selection is designed to emit very poor code quickly.
// Also, it is not designed to be able to do much lowering, so most illegal
// types (e.g. i64 on 32-bit targets) and operations are not supported.  It is
// also not intended to be able to do much optimization, except in a few cases
// where doing optimizations reduces overall compile time.  For example, folding
// constants into immediate fields is often done, because it's cheap and it
// reduces the number of instructions later phases have to examine.
//
// "Fast" instruction selection is able to fail gracefully and transfer
// control to the SelectionDAG selector for operations that it doesn't
// support.  In many cases, this allows us to avoid duplicating a lot of
// the complicated lowering logic that SelectionDAG currently has.
//
// The intended use for "fast" instruction selection is "-O0" mode
// compilation, where the quality of the generated code is irrelevant when
// weighed against the speed at which the code can be generated.  Also,
// at -O0, the LLVM optimizers are not running, and this makes the
// compile time of codegen a much higher portion of the overall compile
// time.  Despite its limitations, "fast" instruction selection is able to
// handle enough code on its own to provide noticeable overall speedups
// in -O0 compiles.
//
// Basic operations are supported in a target-independent way, by reading
// the same instruction descriptions that the SelectionDAG selector reads,
// and identifying simple arithmetic operations that can be directly selected
// from simple operators.  More complicated operations currently require
// target-specific code.
//
//===----------------------------------------------------------------------===//

#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/CodeGen/FastISel.h"
#include "llvm/CodeGen/FunctionLoweringInfo.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Analysis/DebugInfo.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/ErrorHandling.h"
using namespace llvm;

bool FastISel::hasTrivialKill(const Value *V) const {
  // Don't consider constants or arguments to have trivial kills.
  const Instruction *I = dyn_cast<Instruction>(V);
  if (!I)
    return false;

  // No-op casts are trivially coalesced by fast-isel.
  if (const CastInst *Cast = dyn_cast<CastInst>(I))
    if (Cast->isNoopCast(TD.getIntPtrType(Cast->getContext())) &&
        !hasTrivialKill(Cast->getOperand(0)))
      return false;

  // Only instructions with a single use in the same basic block are considered
  // to have trivial kills.
  return I->hasOneUse() &&
         !(I->getOpcode() == Instruction::BitCast ||
           I->getOpcode() == Instruction::PtrToInt ||
           I->getOpcode() == Instruction::IntToPtr) &&
         cast<Instruction>(I->use_begin())->getParent() == I->getParent();
}

unsigned FastISel::getRegForValue(const Value *V) {
  EVT RealVT = TLI.getValueType(V->getType(), /*AllowUnknown=*/true);
  // Don't handle non-simple values in FastISel.
  if (!RealVT.isSimple())
    return 0;

  // Ignore illegal types. We must do this before looking up the value
  // in ValueMap because Arguments are given virtual registers regardless
  // of whether FastISel can handle them.
  MVT VT = RealVT.getSimpleVT();
  if (!TLI.isTypeLegal(VT)) {
    // Promote MVT::i1 to a legal type though, because it's common and easy.
    if (VT == MVT::i1)
      VT = TLI.getTypeToTransformTo(V->getContext(), VT).getSimpleVT();
    else