src/ir/bits.h

/*
 * Copyright 2017 WebAssembly Community Group participants
 *
 * 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.
 */

#ifndef wasm_ir_bits_h
#define wasm_ir_bits_h

#include "ir/literal-utils.h"
#include "support/bits.h"
#include "wasm-builder.h"

namespace wasm {

struct Bits {
  // get a mask to keep only the low # of bits
  static int32_t lowBitMask(int32_t bits) {
    uint32_t ret = -1;
    if (bits >= 32) {
      return ret;
    }
    return ret >> (32 - bits);
  }

  // checks if the input is a mask of lower bits, i.e., all 1s up to some high
  // bit, and all zeros from there. returns the number of masked bits, or 0 if
  // this is not such a mask
  static uint32_t getMaskedBits(uint32_t mask) {
    if (mask == uint32_t(-1)) {
      return 32; // all the bits
    }
    if (mask == 0) {
      return 0; // trivially not a mask
    }
    // otherwise, see if adding one turns this into a 1-bit thing, 00011111 + 1
    // => 00100000
    if (PopCount(mask + 1) != 1) {
      return 0;
    }
    // this is indeed a mask
    return 32 - CountLeadingZeroes(mask);
  }

  // gets the number of effective shifts a shift operation does. In
  // wasm, only 5 bits matter for 32-bit shifts, and 6 for 64.
  static Index getEffectiveShifts(Index amount, Type type) {
    if (type == Type::i32) {
      return amount & 31;
    } else if (type == Type::i64) {
      return amount & 63;
    }
    WASM_UNREACHABLE("unexpected type");
  }

  static Index getEffectiveShifts(Expression* expr) {
    auto* amount = expr->cast<Const>();
    if (amount->type == Type::i32) {
      return getEffectiveShifts(amount->value.geti32(), Type::i32);
    } else if (amount->type == Type::i64) {
      return getEffectiveShifts(amount->value.geti64(), Type::i64);
    }
    WASM_UNREACHABLE("unexpected type");
  }

  static Expression* makeSignExt(Expression* value, Index bytes, Module& wasm) {
    if (value->type == Type::i32) {
      if (bytes == 1 || bytes == 2) {
        auto shifts = bytes == 1 ? 24 : 16;
        Builder builder(wasm);
        return builder.makeBinary(
          ShrSInt32,
          builder.makeBinary(
            ShlInt32,
            value,
            LiteralUtils::makeFromInt32(shifts, Type::i32, wasm)),
          LiteralUtils::makeFromInt32(shifts, Type::i32, wasm));
      }
      assert(bytes == 4);
      return value; // nothing to do
    } else {
      assert(value->type == Type::i64);
      if (bytes == 1 || bytes == 2 || bytes == 4) {
        auto shifts = bytes == 1 ? 56 : (bytes == 2 ? 48 : 32);
        Builder builder(wasm);
        return builder.makeBinary(
          ShrSInt64,
          builder.makeBinary(
            ShlInt64,
            value,
            LiteralUtils::makeFromInt32(shifts, Type::i64, wasm)),
          LiteralUtils::makeFromInt32(shifts, Type::i64, wasm));
      }
      assert(bytes == 8);
      return value; // nothing to do
    }
  }
};

} // namespace wasm

#endif // wasm_ir_bits_h