src/compiler/codegen/local_optimizations.cc

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "../compiler_internals.h"

namespace art {

#define DEBUG_OPT(X)

/* Check RAW, WAR, and WAR dependency on the register operands */
#define CHECK_REG_DEP(use, def, check) ((def & check->useMask) || \
                                        ((use | def) & check->defMask))

/* Scheduler heuristics */
#define MAX_HOIST_DISTANCE 20
#define LDLD_DISTANCE 4
#define LD_LATENCY 2

static bool IsDalvikRegisterClobbered(LIR* lir1, LIR* lir2)
{
  int reg1Lo = DECODE_ALIAS_INFO_REG(lir1->aliasInfo);
  int reg1Hi = reg1Lo + DECODE_ALIAS_INFO_WIDE(lir1->aliasInfo);
  int reg2Lo = DECODE_ALIAS_INFO_REG(lir2->aliasInfo);
  int reg2Hi = reg2Lo + DECODE_ALIAS_INFO_WIDE(lir2->aliasInfo);

  return (reg1Lo == reg2Lo) || (reg1Lo == reg2Hi) || (reg1Hi == reg2Lo);
}

/* Convert a more expensive instruction (ie load) into a move */
static void ConvertMemOpIntoMove(CompilationUnit* cUnit, LIR* origLIR, int dest, int src)
{
  /* Insert a move to replace the load */
  LIR* moveLIR;
  moveLIR = OpRegCopyNoInsert( cUnit, dest, src);
  /*
   * Insert the converted instruction after the original since the
   * optimization is scannng in the top-down order and the new instruction
   * will need to be re-checked (eg the new dest clobbers the src used in
   * thisLIR).
   */
  InsertLIRAfter(origLIR, moveLIR);
}

/*
 * Perform a pass of top-down walk, from the second-last instruction in the
 * superblock, to eliminate redundant loads and stores.
 *
 * An earlier load can eliminate a later load iff
 *   1) They are must-aliases
 *   2) The native register is not clobbered in between
 *   3) The memory location is not written to in between
 *
 * An earlier store can eliminate a later load iff
 *   1) They are must-aliases
 *   2) The native register is not clobbered in between
 *   3) The memory location is not written to in between
 *
 * A later store can be eliminated by an earlier store iff
 *   1) They are must-aliases
 *   2) The memory location is not written to in between
 */
static void ApplyLoadStoreElimination(CompilationUnit* cUnit, LIR* headLIR, LIR* tailLIR)
{
  LIR* thisLIR;

  if (headLIR == tailLIR) return;

  for (thisLIR = PREV_LIR(tailLIR);
      thisLIR != headLIR;
      thisLIR = PREV_LIR(thisLIR)) {
    int sinkDistance = 0;

    /* Skip non-interesting instructions */
    if ((thisLIR->flags.isNop == true) ||
        isPseudoOpcode(thisLIR->opcode) ||
        (GetTargetInstFlags(thisLIR->opcode) & IS_BRANCH) ||
        !(GetTargetInstFlags(thisLIR->opcode) & (IS_LOAD | IS_STORE))) {
      continue;
    }

    int nativeRegId;
    if (cUnit->instructionSet == kX86) {
      // If x86, location differs depending on whether memory/reg operation.
      nativeRegId = (GetTargetInstFlags(thisLIR->opcode) & IS_STORE) ? thisLIR->operands[2]
          : thisLIR->operands[0];
    } else {
      nativeRegId = thisLIR->operands[0];
    }
    bool isThisLIRLoad = GetTargetInstFlags(thisLIR->opcode) & IS_LOAD;
    LIR* checkLIR;
    /* Use the mem mask to determine the rough memory location */
    uint64_t thisMemMask = (thisLIR->useMask | thisLIR->defMask) & ENCODE_MEM;

    /*
     * Currently only eliminate redundant ld/st for constant and Dalvik
     * register accesses.
     */
    if (!(thisMemMask & (ENCODE_LITERAL | ENCODE_DALVIK_REG))) continue;

    uint64_t stopDefRegMask = thisLIR->defMask & ~ENCODE_MEM;
    uint64_t stopUseRegMask;
    if (cUnit->instructionSet == kX86) {
      stopUseRegMask = (IS_BRANCH | thisLIR->useMask) & ~ENCODE_MEM;
    } else {
      /*
       * Add pc to the resource mask to prevent this instruction
       * from sinking past branch instructions. Also take out the memory
       * region bits since stopMask is used to check data/control
       * dependencies.
       */
        stopUseRegMask = (GetPCUseDefEncoding() | thisLIR->useMask) & ~ENCODE_MEM;
    }

    for (checkLIR = NEXT_LIR(thisLIR);
        checkLIR != tailLIR;
        checkLIR = NEXT_LIR(checkLIR)) {

      /*
       * Skip already dead instructions (whose dataflow information is
       * outdated and misleading).
       */
      if (checkLIR->flags.isNop) continue;

      uint64_t checkMemMask = (checkLIR->useMask | checkLIR->defMask) & ENCODE_MEM;
      uint64_t aliasCondition = thisMemMask & checkMemMask;
      bool stopHere = false;

      /*
       * Potential aliases seen - check the alias relations
       */
      if (checkMemMask != ENCODE_MEM && aliasCondition != 0) {
        bool isCheckLIRLoad = GetTargetInstFlags(checkLIR->opcode) & IS_LOAD;
        if  (aliasCondition == ENCODE_LITERAL) {
          /*
           * Should only see literal loads in the instruction
           * stream.
           */
          DCHECK(!(GetTargetInstFlags(checkLIR->opcode) & IS_STORE));
          /* Same value && same register type */
          if (checkLIR->aliasInfo == thisLIR->aliasInfo &&
              SameRegType(checkLIR->operands[0], nativeRegId)) {
            /*
             * Different destination register - insert
             * a move
             */
            if (checkLIR->operands[0] != nativeRegId) {
              ConvertMemOpIntoMove(cUnit, checkLIR, checkLIR->operands[0],
                                   nativeRegId);
            }
            checkLIR->flags.isNop = true;
          }
        } else if (aliasCondition == ENCODE_DALVIK_REG) {
          /* Must alias */
          if (checkLIR->aliasInfo == thisLIR->aliasInfo) {
            /* Only optimize compatible registers */
            bool regCompatible = SameRegType(checkLIR->operands[0], nativeRegId);
            if ((isThisLIRLoad && isCheckLIRLoad) ||
                (!isThisLIRLoad && isCheckLIRLoad)) {
              /* RAR or RAW */
              if (regCompatible) {
                /*
                 * Different destination register -
                 * insert a move
                 */
                if (checkLIR->operands[0] !=
                  nativeRegId) {
                  ConvertMemOpIntoMove(cUnit, checkLIR, checkLIR->operands[0],
                                       nativeRegId);
                }
                checkLIR->flags.isNop = true;
              } else {
                /*
                 * Destinaions are of different types -
                 * something complicated going on so
                 * stop looking now.
                 */
                stopHere = true;
              }
            } else if (isThisLIRLoad && !isCheckLIRLoad) {
              /* WAR - register value is killed */
              stopHere = true;
            } else if (!isThisLIRLoad && !isCheckLIRLoad) {
              /* WAW - nuke the earlier store */
              thisLIR->flags.isNop = true;
              stopHere = true;
            }
          /* Partial overlap */
          } else if (IsDalvikRegisterClobbered(thisLIR, checkLIR)) {
            /*
             * It is actually ok to continue if checkLIR
             * is a read. But it is hard to make a test
             * case for this so we just stop here to be
             * conservative.
             */
            stopHere = true;
          }
        }
        /* Memory content may be updated. Stop looking now. */
        if (stopHere) {
          break;
        /* The checkLIR has been transformed - check the next one */
        } else if (checkLIR->flags.isNop) {
          continue;
        }
      }


      /*
       * this and check LIRs have no memory dependency. Now check if
       * their register operands have any RAW, WAR, and WAW
       * dependencies. If so, stop looking.
       */
      if (stopHere == false) {
        stopHere = CHECK_REG_DEP(stopUseRegMask, stopDefRegMask, checkLIR);
      }

      if (stopHere == true) {
        if (cUnit->instructionSet == kX86) {
          // Prevent stores from being sunk between ops that generate ccodes and
          // ops that use them.
          uint64_t flags = GetTargetInstFlags(checkLIR->opcode);
          if (sinkDistance > 0 && (flags & IS_BRANCH) && (flags & USES_CCODES)) {
            checkLIR = PREV_LIR(checkLIR);
            sinkDistance--;
          }
        }
        DEBUG_OPT(dumpDependentInsnPair(thisLIR, checkLIR, "REG CLOBBERED"));
        /* Only sink store instructions */
        if (sinkDistance && !isThisLIRLoad) {
          LIR* newStoreLIR = static_cast<LIR*>(NewMem(cUnit, sizeof(LIR), true, kAllocLIR));
          *newStoreLIR = *thisLIR;
          /*
           * Stop point found - insert *before* the checkLIR
           * since the instruction list is scanned in the
           * top-down order.
           */
          InsertLIRBefore(checkLIR, newStoreLIR);
          thisLIR->flags.isNop = true;
        }
        break;
      } else if (!checkLIR->flags.isNop) {
        sinkDistance++;
      }
    }
  }
}

/*
 * Perform a pass of bottom-up walk, from the second instruction in the
 * superblock, to try to hoist loads to earlier slots.
 */
void ApplyLoadHoisting(CompilationUnit* cUnit, LIR* headLIR, LIR* tailLIR)
{
  LIR* thisLIR, *checkLIR;
  /*
   * Store the list of independent instructions that can be hoisted past.
   * Will decide the best place to insert later.
   */
  LIR* prevInstList[MAX_HOIST_DISTANCE];

  /* Empty block */
  if (headLIR == tailLIR) return;

  /* Start from the second instruction */
  for (thisLIR = NEXT_LIR(headLIR);
     thisLIR != tailLIR;
     thisLIR = NEXT_LIR(thisLIR)) {

    /* Skip non-interesting instructions */
    if ((thisLIR->flags.isNop == true) ||
        isPseudoOpcode(thisLIR->opcode) ||
        !(GetTargetInstFlags(thisLIR->opcode) & IS_LOAD)) {
      continue;
    }

    uint64_t stopUseAllMask = thisLIR->useMask;

    if (cUnit->instructionSet != kX86) {
      /*
       * Branches for null/range checks are marked with the true resource
       * bits, and loads to Dalvik registers, constant pools, and non-alias
       * locations are safe to be hoisted. So only mark the heap references
       * conservatively here.
       */
      if (stopUseAllMask & ENCODE_HEAP_REF) {
        stopUseAllMask |= GetPCUseDefEncoding();
      }
    }

    /* Similar as above, but just check for pure register dependency */
    uint64_t stopUseRegMask = stopUseAllMask & ~ENCODE_MEM;
    uint64_t stopDefRegMask = thisLIR->defMask & ~ENCODE_MEM;

    int nextSlot = 0;
    bool stopHere = false;

    /* Try to hoist the load to a good spot */
    for (checkLIR = PREV_LIR(thisLIR);
        checkLIR != headLIR;
        checkLIR = PREV_LIR(checkLIR)) {

      /*
       * Skip already dead instructions (whose dataflow information is
       * outdated and misleading).
       */
      if (checkLIR->flags.isNop) continue;

      uint64_t checkMemMask = checkLIR->defMask & ENCODE_MEM;
      uint64_t aliasCondition = stopUseAllMask & checkMemMask;
      stopHere = false;

      /* Potential WAR alias seen - check the exact relation */
      if (checkMemMask != ENCODE_MEM && aliasCondition != 0) {
        /* We can fully disambiguate Dalvik references */
        if (aliasCondition == ENCODE_DALVIK_REG) {
          /* Must alias or partually overlap */
          if ((checkLIR->aliasInfo == thisLIR->aliasInfo) ||
            IsDalvikRegisterClobbered(thisLIR, checkLIR)) {
            stopHere = true;
          }
        /* Conservatively treat all heap refs as may-alias */
        } else {
          DCHECK_EQ(aliasCondition, ENCODE_HEAP_REF);
          stopHere = true;
        }
        /* Memory content may be updated. Stop looking now. */
        if (stopHere) {
          prevInstList[nextSlot++] = checkLIR;
          break;
        }
      }

      if (stopHere == false) {
        stopHere = CHECK_REG_DEP(stopUseRegMask, stopDefRegMask,
                     checkLIR);
      }

      /*
       * Store the dependent or non-pseudo/indepedent instruction to the
       * list.
       */
      if (stopHere || !isPseudoOpcode(checkLIR->opcode)) {
        prevInstList[nextSlot++] = checkLIR;
        if (nextSlot == MAX_HOIST_DISTANCE) break;
      }

      /* Found a new place to put the load - move it here */
      if (stopHere == true) {
        DEBUG_OPT(dumpDependentInsnPair(checkLIR, thisLIR "HOIST STOP"));
        break;
      }
    }

    /*
     * Reached the top - use headLIR as the dependent marker as all labels
     * are barriers.
     */
    if (stopHere == false && nextSlot < MAX_HOIST_DISTANCE) {
      prevInstList[nextSlot++] = headLIR;
    }

    /*
     * At least one independent instruction is found. Scan in the reversed
     * direction to find a beneficial slot.
     */
    if (nextSlot >= 2) {
      int firstSlot = nextSlot - 2;
      int slot;
      LIR* depLIR = prevInstList[nextSlot-1];
      /* If there is ld-ld dependency, wait LDLD_DISTANCE cycles */
      if (!isPseudoOpcode(depLIR->opcode) &&
        (GetTargetInstFlags(depLIR->opcode) & IS_LOAD)) {
        firstSlot -= LDLD_DISTANCE;
      }
      /*
       * Make sure we check slot >= 0 since firstSlot may be negative
       * when the loop is first entered.
       */
      for (slot = firstSlot; slot >= 0; slot--) {
        LIR* curLIR = prevInstList[slot];
        LIR* prevLIR = prevInstList[slot+1];

        /* Check the highest instruction */
        if (prevLIR->defMask == ENCODE_ALL) {
          /*
           * If the first instruction is a load, don't hoist anything
           * above it since it is unlikely to be beneficial.
           */
          if (GetTargetInstFlags(curLIR->opcode) & IS_LOAD) continue;
          /*
           * If the remaining number of slots is less than LD_LATENCY,
           * insert the hoisted load here.
           */
          if (slot < LD_LATENCY) break;
        }

        // Don't look across a barrier label
        if ((prevLIR->opcode == kPseudoTargetLabel) ||
            (prevLIR->opcode == kPseudoSafepointPC) ||
            (prevLIR->opcode == kPseudoBarrier)) {
          break;
        }

        /*
         * Try to find two instructions with load/use dependency until
         * the remaining instructions are less than LD_LATENCY.
         */
        bool prevIsLoad = isPseudoOpcode(prevLIR->opcode) ? false :
            (GetTargetInstFlags(prevLIR->opcode) & IS_LOAD);
        if (((curLIR->useMask & prevLIR->defMask) && prevIsLoad) || (slot < LD_LATENCY)) {
          break;
        }
      }

      /* Found a slot to hoist to */
      if (slot >= 0) {
        LIR* curLIR = prevInstList[slot];
        LIR* newLoadLIR = static_cast<LIR*>(NewMem(cUnit, sizeof(LIR), true, kAllocLIR));
        *newLoadLIR = *thisLIR;
        /*
         * Insertion is guaranteed to succeed since checkLIR
         * is never the first LIR on the list
         */
        InsertLIRBefore(curLIR, newLoadLIR);
        thisLIR->flags.isNop = true;
      }
    }
  }
}

void ApplyLocalOptimizations(CompilationUnit* cUnit, LIR* headLIR,
                    LIR* tailLIR)
{
  if (!(cUnit->disableOpt & (1 << kLoadStoreElimination))) {
    ApplyLoadStoreElimination(cUnit, headLIR, tailLIR);
  }
  if (!(cUnit->disableOpt & (1 << kLoadHoisting))) {
    ApplyLoadHoisting(cUnit, headLIR, tailLIR);
  }
}

/*
 * Nop any unconditional branches that go to the next instruction.
 * Note: new redundant branches may be inserted later, and we'll
 * use a check in final instruction assembly to nop those out.
 */
void RemoveRedundantBranches(CompilationUnit* cUnit)
{
  LIR* thisLIR;

  for (thisLIR = cUnit->firstLIRInsn; thisLIR != cUnit->lastLIRInsn; thisLIR = NEXT_LIR(thisLIR)) {

    /* Branch to the next instruction */
    if (BranchUnconditional(thisLIR)) {
      LIR* nextLIR = thisLIR;

      while (true) {
        nextLIR = NEXT_LIR(nextLIR);

        /*
         * Is the branch target the next instruction?
         */
        if (nextLIR == thisLIR->target) {
          thisLIR->flags.isNop = true;
          break;
        }

        /*
         * Found real useful stuff between the branch and the target.
         * Need to explicitly check the lastLIRInsn here because it
         * might be the last real instruction.
         */
        if (!isPseudoOpcode(nextLIR->opcode) ||
          (nextLIR == cUnit->lastLIRInsn))
          break;
      }
    }
  }
}

}  // namespace art