path: root/js/public/Conversions.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* ECMAScript conversion operations. */

#ifndef js_Conversions_h
#define js_Conversions_h

#include "mozilla/Casting.h"
#include "mozilla/Compiler.h"
#include "mozilla/FloatingPoint.h"
#include "mozilla/MathAlgorithms.h"
#include "mozilla/WrappingOperations.h"

#include <cmath>
#include <stddef.h>  // size_t
#include <stdint.h>  // {u,}int{8,16,32,64}_t
#include <type_traits>

#include "jspubtd.h"
#include "jstypes.h"  // JS_PUBLIC_API

#include "js/RootingAPI.h"
#include "js/Value.h"

namespace js {

/* DO NOT CALL THIS. Use JS::ToBoolean. */
extern JS_PUBLIC_API bool ToBooleanSlow(JS::HandleValue v);

/* DO NOT CALL THIS.  Use JS::ToNumber. */
extern JS_PUBLIC_API bool ToNumberSlow(JSContext* cx, JS::HandleValue v,
                                       double* dp);

/* DO NOT CALL THIS. Use JS::ToInt8. */
extern JS_PUBLIC_API bool ToInt8Slow(JSContext* cx, JS::HandleValue v,
                                     int8_t* out);

/* DO NOT CALL THIS. Use JS::ToUint8. */
extern JS_PUBLIC_API bool ToUint8Slow(JSContext* cx, JS::HandleValue v,
                                      uint8_t* out);

/* DO NOT CALL THIS. Use JS::ToInt16. */
extern JS_PUBLIC_API bool ToInt16Slow(JSContext* cx, JS::HandleValue v,
                                      int16_t* out);

/* DO NOT CALL THIS. Use JS::ToInt32. */
extern JS_PUBLIC_API bool ToInt32Slow(JSContext* cx, JS::HandleValue v,
                                      int32_t* out);

/* DO NOT CALL THIS. Use JS::ToUint32. */
extern JS_PUBLIC_API bool ToUint32Slow(JSContext* cx, JS::HandleValue v,
                                       uint32_t* out);

/* DO NOT CALL THIS. Use JS::ToUint16. */
extern JS_PUBLIC_API bool ToUint16Slow(JSContext* cx, JS::HandleValue v,
                                       uint16_t* out);

/* DO NOT CALL THIS. Use JS::ToInt64. */
extern JS_PUBLIC_API bool ToInt64Slow(JSContext* cx, JS::HandleValue v,
                                      int64_t* out);

/* DO NOT CALL THIS. Use JS::ToUint64. */
extern JS_PUBLIC_API bool ToUint64Slow(JSContext* cx, JS::HandleValue v,
                                       uint64_t* out);

/* DO NOT CALL THIS. Use JS::ToString. */
extern JS_PUBLIC_API JSString* ToStringSlow(JSContext* cx, JS::HandleValue v);

/* DO NOT CALL THIS. Use JS::ToObject. */
extern JS_PUBLIC_API JSObject* ToObjectSlow(JSContext* cx, JS::HandleValue v,
                                            bool reportScanStack);

}  // namespace js

namespace JS {

namespace detail {

#ifdef JS_DEBUG
/**
 * Assert that we're not doing GC on cx, that we're in a request as
 * needed, and that the compartments for cx and v are correct.
 * Also check that GC would be safe at this point.
 */
extern JS_PUBLIC_API void AssertArgumentsAreSane(JSContext* cx, HandleValue v);
#else
inline void AssertArgumentsAreSane(JSContext* cx, HandleValue v) {}
#endif /* JS_DEBUG */

}  // namespace detail

/**
 * ES6 draft 20141224, 7.1.1, second algorithm.
 *
 * Most users shouldn't call this -- use JS::ToBoolean, ToNumber, or ToString
 * instead.  This will typically only be called from custom convert hooks that
 * wish to fall back to the ES6 default conversion behavior shared by most
 * objects in JS, codified as OrdinaryToPrimitive.
 */
extern JS_PUBLIC_API bool OrdinaryToPrimitive(JSContext* cx, HandleObject obj,
                                              JSType type,
                                              MutableHandleValue vp);

/* ES6 draft 20141224, 7.1.2. */
MOZ_ALWAYS_INLINE bool ToBoolean(HandleValue v) {
  if (v.isBoolean()) {
    return v.toBoolean();
  }
  if (v.isInt32()) {
    return v.toInt32() != 0;
  }
  if (v.isNullOrUndefined()) {
    return false;
  }
  if (v.isDouble()) {
    double d = v.toDouble();
    return !mozilla::IsNaN(d) && d != 0;
  }
  if (v.isSymbol()) {
    return true;
  }

  /* The slow path handles strings, BigInts and objects. */
  return js::ToBooleanSlow(v);
}

/* ES6 draft 20141224, 7.1.3. */
MOZ_ALWAYS_INLINE bool ToNumber(JSContext* cx, HandleValue v, double* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isNumber()) {
    *out = v.toNumber();
    return true;
  }
  return js::ToNumberSlow(cx, v, out);
}

// ES2020 draft rev 6b05bc56ba4e3c7a2b9922c4282d9eb844426d9b
// 7.1.5 ToInteger ( argument )
//
// Specialized for double values.
inline double ToInteger(double d) {
  if (d == 0) {
    return 0;
  }

  if (!mozilla::IsFinite(d)) {
    if (mozilla::IsNaN(d)) {
      return 0;
    }
    return d;
  }

  return std::trunc(d) + (+0.0);  // Add zero to convert -0 to +0.
}

/* ES6 draft 20141224, 7.1.5. */
MOZ_ALWAYS_INLINE bool ToInt32(JSContext* cx, JS::HandleValue v, int32_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = v.toInt32();
    return true;
  }
  return js::ToInt32Slow(cx, v, out);
}

/* ES6 draft 20141224, 7.1.6. */
MOZ_ALWAYS_INLINE bool ToUint32(JSContext* cx, HandleValue v, uint32_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = uint32_t(v.toInt32());
    return true;
  }
  return js::ToUint32Slow(cx, v, out);
}

/* ES6 draft 20141224, 7.1.7. */
MOZ_ALWAYS_INLINE bool ToInt16(JSContext* cx, JS::HandleValue v, int16_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = int16_t(v.toInt32());
    return true;
  }
  return js::ToInt16Slow(cx, v, out);
}

/* ES6 draft 20141224, 7.1.8. */
MOZ_ALWAYS_INLINE bool ToUint16(JSContext* cx, HandleValue v, uint16_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = uint16_t(v.toInt32());
    return true;
  }
  return js::ToUint16Slow(cx, v, out);
}

/* ES6 draft 20141224, 7.1.9 */
MOZ_ALWAYS_INLINE bool ToInt8(JSContext* cx, JS::HandleValue v, int8_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = int8_t(v.toInt32());
    return true;
  }
  return js::ToInt8Slow(cx, v, out);
}

/* ES6 ECMA-262, 7.1.10 */
MOZ_ALWAYS_INLINE bool ToUint8(JSContext* cx, JS::HandleValue v, uint8_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = uint8_t(v.toInt32());
    return true;
  }
  return js::ToUint8Slow(cx, v, out);
}

/*
 * Non-standard, with behavior similar to that of ToInt32, except in its
 * producing an int64_t.
 */
MOZ_ALWAYS_INLINE bool ToInt64(JSContext* cx, HandleValue v, int64_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = int64_t(v.toInt32());
    return true;
  }
  return js::ToInt64Slow(cx, v, out);
}

/*
 * Non-standard, with behavior similar to that of ToUint32, except in its
 * producing a uint64_t.
 */
MOZ_ALWAYS_INLINE bool ToUint64(JSContext* cx, HandleValue v, uint64_t* out) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isInt32()) {
    *out = uint64_t(v.toInt32());
    return true;
  }
  return js::ToUint64Slow(cx, v, out);
}

/* ES6 draft 20141224, 7.1.12. */
MOZ_ALWAYS_INLINE JSString* ToString(JSContext* cx, HandleValue v) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isString()) {
    return v.toString();
  }
  return js::ToStringSlow(cx, v);
}

/* ES6 draft 20141224, 7.1.13. */
inline JSObject* ToObject(JSContext* cx, HandleValue v) {
  detail::AssertArgumentsAreSane(cx, v);

  if (v.isObject()) {
    return &v.toObject();
  }
  return js::ToObjectSlow(cx, v, false);
}

/**
 * Convert a double value to UnsignedInteger (an unsigned integral type) using
 * ECMAScript-style semantics (that is, in like manner to how ECMAScript's
 * ToInt32 converts to int32_t).
 *
 *   If d is infinite or NaN, return 0.
 *   Otherwise compute d2 = sign(d) * floor(abs(d)), and return the
 *   UnsignedInteger value congruent to d2 % 2**(bit width of UnsignedInteger).
 *
 * The algorithm below is inspired by that found in
 * <https://trac.webkit.org/changeset/67825/webkit/trunk/JavaScriptCore/runtime/JSValue.cpp>
 * but has been generalized to all integer widths.
 */
template <typename UnsignedInteger>
inline UnsignedInteger ToUnsignedInteger(double d) {
  static_assert(std::is_unsigned_v<UnsignedInteger>,
                "UnsignedInteger must be an unsigned type");

  uint64_t bits = mozilla::BitwiseCast<uint64_t>(d);
  unsigned DoubleExponentShift = mozilla::FloatingPoint<double>::kExponentShift;

  // Extract the exponent component.  (Be careful here!  It's not technically
  // the exponent in NaN, infinities, and subnormals.)
  int_fast16_t exp =
      int_fast16_t((bits & mozilla::FloatingPoint<double>::kExponentBits) >>
                   DoubleExponentShift) -
      int_fast16_t(mozilla::FloatingPoint<double>::kExponentBias);

  // If the exponent's less than zero, abs(d) < 1, so the result is 0.  (This
  // also handles subnormals.)
  if (exp < 0) {
    return 0;
  }

  uint_fast16_t exponent = mozilla::AssertedCast<uint_fast16_t>(exp);

  // If the exponent is greater than or equal to the bits of precision of a
  // double plus UnsignedInteger's width, the number is either infinite, NaN,
  // or too large to have lower-order bits in the congruent value.  (Example:
  // 2**84 is exactly representable as a double.  The next exact double is
  // 2**84 + 2**32.  Thus if UnsignedInteger is uint32_t, an exponent >= 84
  // implies floor(abs(d)) == 0 mod 2**32.)  Return 0 in all these cases.
  constexpr size_t ResultWidth = CHAR_BIT * sizeof(UnsignedInteger);
  if (exponent >= DoubleExponentShift + ResultWidth) {
    return 0;
  }

  // The significand contains the bits that will determine the final result.
  // Shift those bits left or right, according to the exponent, to their
  // locations in the unsigned binary representation of floor(abs(d)).
  static_assert(sizeof(UnsignedInteger) <= sizeof(uint64_t),
                "left-shifting below would lose upper bits");
  UnsignedInteger result =
      (exponent > DoubleExponentShift)
          ? UnsignedInteger(bits << (exponent - DoubleExponentShift))
          : UnsignedInteger(bits >> (DoubleExponentShift - exponent));

  // Two further complications remain.  First, |result| may contain bogus
  // sign/exponent bits.  Second, IEEE-754 numbers' significands (excluding
  // subnormals, but we already handled those) have an implicit leading 1
  // which may affect the final result.
  //
  // It may appear that there's complexity here depending on how ResultWidth
  // and DoubleExponentShift relate, but it turns out there's not.
  //
  // Assume ResultWidth < DoubleExponentShift:
  //   Only right-shifts leave bogus bits in |result|.  For this to happen,
  //   we must right-shift by > |DoubleExponentShift - ResultWidth|, implying
  //   |exponent < ResultWidth|.
  //   The implicit leading bit only matters if it appears in the final
  //   result -- if |2**exponent mod 2**ResultWidth != 0|.  This implies
  //   |exponent < ResultWidth|.
  // Otherwise assume ResultWidth >= DoubleExponentShift:
  //   Any left-shift less than |ResultWidth - DoubleExponentShift| leaves
  //   bogus bits in |result|.  This implies |exponent < ResultWidth|.  Any
  //   right-shift less than |ResultWidth| does too, which implies
  //   |DoubleExponentShift - ResultWidth < exponent|.  By assumption, then,
  //   |exponent| is negative, but we excluded that above.  So bogus bits
  //   need only |exponent < ResultWidth|.
  //   The implicit leading bit matters identically to the other case, so
  //   again, |exponent < ResultWidth|.
  if (exponent < ResultWidth) {
    const auto implicitOne =
        static_cast<UnsignedInteger>(UnsignedInteger{1} << exponent);
    result &= implicitOne - 1;  // remove bogus bits
    result += implicitOne;      // add the implicit bit
  }

  // Compute the congruent value in the signed range.
  return (bits & mozilla::FloatingPoint<double>::kSignBit) ? ~result + 1
                                                           : result;
}

template <typename SignedInteger>
inline SignedInteger ToSignedInteger(double d) {
  static_assert(std::is_signed_v<SignedInteger>,
                "SignedInteger must be a signed type");

  using UnsignedInteger = std::make_unsigned_t<SignedInteger>;
  UnsignedInteger u = ToUnsignedInteger<UnsignedInteger>(d);

  return mozilla::WrapToSigned(u);
}

// clang crashes compiling this when targeting arm:
// https://llvm.org/bugs/show_bug.cgi?id=22974
#if defined(__arm__) && MOZ_IS_GCC

template <>
inline int32_t ToSignedInteger<int32_t>(double d) {
  int32_t i;
  uint32_t tmp0;
  uint32_t tmp1;
  uint32_t tmp2;
  asm(
      // We use a pure integer solution here. In the 'softfp' ABI, the argument
      // will start in r0 and r1, and VFP can't do all of the necessary ECMA
      // conversions by itself so some integer code will be required anyway. A
      // hybrid solution is faster on A9, but this pure integer solution is
      // notably faster for A8.

      // %0 is the result register, and may alias either of the %[QR]1
      //    registers.
      // %Q4 holds the lower part of the mantissa.
      // %R4 holds the sign, exponent, and the upper part of the mantissa.
      // %1, %2 and %3 are used as temporary values.

      // Extract the exponent.
      "   mov     %1, %R4, LSR #20\n"
      "   bic     %1, %1, #(1 << 11)\n"  // Clear the sign.

      // Set the implicit top bit of the mantissa. This clobbers a bit of the
      // exponent, but we have already extracted that.
      "   orr     %R4, %R4, #(1 << 20)\n"

      // Special Cases
      //   We should return zero in the following special cases:
      //    - Exponent is 0x000 - 1023: +/-0 or subnormal.
      //    - Exponent is 0x7ff - 1023: +/-INFINITY or NaN
      //      - This case is implicitly handled by the standard code path
      //        anyway, as shifting the mantissa up by the exponent will
      //        result in '0'.
      //
      // The result is composed of the mantissa, prepended with '1' and
      // bit-shifted left by the (decoded) exponent. Note that because the
      // r1[20] is the bit with value '1', r1 is effectively already shifted
      // (left) by 20 bits, and r0 is already shifted by 52 bits.

      // Adjust the exponent to remove the encoding offset. If the decoded
      // exponent is negative, quickly bail out with '0' as such values round to
      // zero anyway. This also catches +/-0 and subnormals.
      "   sub     %1, %1, #0xff\n"
      "   subs    %1, %1, #0x300\n"
      "   bmi     8f\n"

      //  %1 = (decoded) exponent >= 0
      //  %R4 = upper mantissa and sign

      // ---- Lower Mantissa ----
      "   subs    %3, %1, #52\n"  // Calculate exp-52
      "   bmi     1f\n"

      // Shift r0 left by exp-52.
      // Ensure that we don't overflow ARM's 8-bit shift operand range.
      // We need to handle anything up to an 11-bit value here as we know that
      // 52 <= exp <= 1024 (0x400). Any shift beyond 31 bits results in zero
      // anyway, so as long as we don't touch the bottom 5 bits, we can use
      // a logical OR to push long shifts into the 32 <= (exp&0xff) <= 255
      // range.
      "   bic     %2, %3, #0xff\n"
      "   orr     %3, %3, %2, LSR #3\n"
      // We can now perform a straight shift, avoiding the need for any
      // conditional instructions or extra branches.
      "   mov     %Q4, %Q4, LSL %3\n"
      "   b       2f\n"
      "1:\n"  // Shift r0 right by 52-exp.
              // We know that 0 <= exp < 52, and we can shift up to 255 bits so
              // 52-exp will always be a valid shift and we can sk%3 the range
              // check for this case.
      "   rsb     %3, %1, #52\n"
      "   mov     %Q4, %Q4, LSR %3\n"

      //  %1 = (decoded) exponent
      //  %R4 = upper mantissa and sign
      //  %Q4 = partially-converted integer

      "2:\n"
      // ---- Upper Mantissa ----
      // This is much the same as the lower mantissa, with a few different
      // boundary checks and some masking to hide the exponent & sign bit in the
      // upper word.
      // Note that the upper mantissa is pre-shifted by 20 in %R4, but we shift
      // it left more to remove the sign and exponent so it is effectively
      // pre-shifted by 31 bits.
      "   subs    %3, %1, #31\n"       // Calculate exp-31
      "   mov     %1, %R4, LSL #11\n"  // Re-use %1 as a temporary register.
      "   bmi     3f\n"

      // Shift %R4 left by exp-31.
      // Avoid overflowing the 8-bit shift range, as before.
      "   bic     %2, %3, #0xff\n"
      "   orr     %3, %3, %2, LSR #3\n"
      // Perform the shift.
      "   mov     %2, %1, LSL %3\n"
      "   b       4f\n"
      "3:\n"  // Shift r1 right by 31-exp.
              // We know that 0 <= exp < 31, and we can shift up to 255 bits so
              // 31-exp will always be a valid shift and we can skip the range
              // check for this case.
      "   rsb     %3, %3, #0\n"      // Calculate 31-exp from -(exp-31)
      "   mov     %2, %1, LSR %3\n"  // Thumb-2 can't do "LSR %3" in "orr".

      //  %Q4 = partially-converted integer (lower)
      //  %R4 = upper mantissa and sign
      //  %2 = partially-converted integer (upper)

      "4:\n"
      // Combine the converted parts.
      "   orr     %Q4, %Q4, %2\n"
      // Negate the result if we have to, and move it to %0 in the process. To
      // avoid conditionals, we can do this by inverting on %R4[31], then adding
      // %R4[31]>>31.
      "   eor     %Q4, %Q4, %R4, ASR #31\n"
      "   add     %0, %Q4, %R4, LSR #31\n"
      "   b       9f\n"
      "8:\n"
      // +/-INFINITY, +/-0, subnormals, NaNs, and anything else out-of-range
      // that will result in a conversion of '0'.
      "   mov     %0, #0\n"
      "9:\n"
      : "=r"(i), "=&r"(tmp0), "=&r"(tmp1), "=&r"(tmp2), "=&r"(d)
      : "4"(d)
      : "cc");
  return i;
}

#endif  // defined (__arm__) && MOZ_IS_GCC

namespace detail {

template <typename IntegerType,
          bool IsUnsigned = std::is_unsigned_v<IntegerType>>
struct ToSignedOrUnsignedInteger;

template <typename IntegerType>
struct ToSignedOrUnsignedInteger<IntegerType, true> {
  static IntegerType compute(double d) {
    return ToUnsignedInteger<IntegerType>(d);
  }
};

template <typename IntegerType>
struct ToSignedOrUnsignedInteger<IntegerType, false> {
  static IntegerType compute(double d) {
    return ToSignedInteger<IntegerType>(d);
  }
};

}  // namespace detail

template <typename IntegerType>
inline IntegerType ToSignedOrUnsignedInteger(double d) {
  return detail::ToSignedOrUnsignedInteger<IntegerType>::compute(d);
}

/* WEBIDL 4.2.4 */
inline int8_t ToInt8(double d) { return ToSignedInteger<int8_t>(d); }

/* ECMA-262 7.1.10 ToUInt8() specialized for doubles. */
inline int8_t ToUint8(double d) { return ToUnsignedInteger<uint8_t>(d); }

/* WEBIDL 4.2.6 */
inline int16_t ToInt16(double d) { return ToSignedInteger<int16_t>(d); }

inline uint16_t ToUint16(double d) { return ToUnsignedInteger<uint16_t>(d); }

/* ES5 9.5 ToInt32 (specialized for doubles). */
inline int32_t ToInt32(double d) { return ToSignedInteger<int32_t>(d); }

/* ES5 9.6 (specialized for doubles). */
inline uint32_t ToUint32(double d) { return ToUnsignedInteger<uint32_t>(d); }

/* WEBIDL 4.2.10 */
inline int64_t ToInt64(double d) { return ToSignedInteger<int64_t>(d); }

/* WEBIDL 4.2.11 */
inline uint64_t ToUint64(double d) { return ToUnsignedInteger<uint64_t>(d); }

/**
 * An amount of space large enough to store the null-terminated result of
 * |ToString| on any Number.
 *
 * The <https://tc39.es/ecma262/#sec-tostring-applied-to-the-number-type>
 * |NumberToString| algorithm is specified in terms of results, not an
 * algorithm.  It is extremely unclear from the algorithm's definition what its
 * longest output can be.  |-(2**-19 - 2**-72)| requires 25 + 1 characters and
 * is believed to be at least *very close* to the upper bound, so we round that
 * *very generously* upward to a 64-bit pointer-size boundary (to be extra
 * cautious) and assume that's adequate.
 *
 * If you can supply better reasoning for a tighter bound, file a bug to improve
 * this!
 */
static constexpr size_t MaximumNumberToStringLength = 31 + 1;

/**
 * Store in |out| the null-terminated, base-10 result of |ToString| applied to
 * |d| per <https://tc39.es/ecma262/#sec-tostring-applied-to-the-number-type>.
 * (This will produce "NaN", "-Infinity", or "Infinity" for non-finite |d|.)
 */
extern JS_PUBLIC_API void NumberToString(
    double d, char (&out)[MaximumNumberToStringLength]);

}  // namespace JS

#endif /* js_Conversions_h */