The Tor Browser: js/src/vm/NumericConversions.h@129ffea94266 (annotated)

js/src/vm/NumericConversions.h@129ffea94266 (annotated)

js/src/vm/NumericConversions.h

Sat, 03 Jan 2015 20:18:00 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Sat, 03 Jan 2015 20:18:00 +0100
branch: TOR_BUG_3246
changeset 7: 129ffea94266
permissions: -rw-r--r--

Conditionally enable double key logic according to:
private browsing mode or privacy.thirdparty.isolate preference and
implement in GetCookieStringCommon and FindCookie where it counts...
With some reservations of how to convince FindCookie users to test
condition and pass a nullptr when disabling double key logic.

 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
  * vim: set ts=8 sts=4 et sw=4 tw=99:
  * 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/. */
 #ifndef vm_NumericConversions_h
 #define vm_NumericConversions_h
 #include "mozilla/Assertions.h"
 #include "mozilla/Casting.h"
 #include "mozilla/FloatingPoint.h"
 #include "mozilla/TypeTraits.h"
 #include <math.h>
 namespace js {
 namespace detail {
 /*
  * Convert a double value to ResultType (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 ResultType
  *   value congruent to d2 mod 2**(bit width of ResultType).
  *
  * The algorithm below is inspired by that found in
  * <http://trac.webkit.org/changeset/67825/trunk/JavaScriptCore/runtime/JSValue.cpp>
  * but has been generalized to all integer widths.
  */
 template<typename ResultType>
 inline ResultType
 ToUintWidth(double d)
 {
     static_assert(mozilla::IsUnsigned<ResultType>::value,
                   "ResultType must be an unsigned type");
     uint64_t bits = mozilla::BitwiseCast<uint64_t>(d);
     unsigned DoubleExponentShift = mozilla::FloatingPoint<double>::ExponentShift;
     // 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>::ExponentBits) >> DoubleExponentShift) -
         int_fast16_t(mozilla::FloatingPoint<double>::ExponentBias);
     // 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::SafeCast<uint_fast16_t>(exp);
     // If the exponent is greater than or equal to the bits of precision of a
     // double plus ResultType'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 ResultType is int32_t, an exponent >= 84 implies
     // floor(abs(d)) == 0 mod 2**32.)  Return 0 in all these cases.
     const size_t ResultWidth = CHAR_BIT * sizeof(ResultType);
     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(ResultType) <= sizeof(uint64_t),
                   "Left-shifting below would lose upper bits");
     ResultType result = (exponent > DoubleExponentShift)
                         ? ResultType(bits << (exponent - DoubleExponentShift))
                         : ResultType(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) {
         ResultType implicitOne = ResultType(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>::SignBit) ? ~result + 1 : result;
 }
 template<typename ResultType>
 inline ResultType
 ToIntWidth(double d)
 {
     static_assert(mozilla::IsSigned<ResultType>::value,
                   "ResultType must be a signed type");
     const ResultType MaxValue = (1ULL << (CHAR_BIT * sizeof(ResultType) - 1)) - 1;
     const ResultType MinValue = -MaxValue - 1;
     typedef typename mozilla::MakeUnsigned<ResultType>::Type UnsignedResult;
     UnsignedResult u = ToUintWidth<UnsignedResult>(d);
     if (u <= UnsignedResult(MaxValue))
         return static_cast<ResultType>(u);
     return (MinValue + static_cast<ResultType>(u - MaxValue)) - 1;
 }
 } /* namespace detail */
 /* ES5 9.5 ToInt32 (specialized for doubles). */
 inline int32_t
 ToInt32(double d)
 {
 #if defined (__arm__) && defined (__GNUC__)
     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;
 #else
     return detail::ToIntWidth<int32_t>(d);
 #endif
 }
 /* ES5 9.6 (specialized for doubles). */
 inline uint32_t
 ToUint32(double d)
 {
     return detail::ToUintWidth<uint32_t>(d);
 }
 /* WEBIDL 4.2.10 */
 inline int64_t
 ToInt64(double d)
 {
     return detail::ToIntWidth<int64_t>(d);
 }
 /* WEBIDL 4.2.11 */
 inline uint64_t
 ToUint64(double d)
 {
     return detail::ToUintWidth<uint64_t>(d);
 }
 /* ES5 9.4 ToInteger (specialized for doubles). */
 inline double
 ToInteger(double d)
 {
     if (d == 0)
         return d;
     if (!mozilla::IsFinite(d)) {
         if (mozilla::IsNaN(d))
             return 0;
         return d;
     }
     return d < 0 ? ceil(d) : floor(d);
 }
 } /* namespace js */
 #endif /* vm_NumericConversions_h */

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js/src/vm/NumericConversions.h@129ffea94266 (annotated)

js/src/vm/NumericConversions.h