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/* -*- 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
#ifndef gc_BufferAllocatorInternals_h
#define gc_BufferAllocatorInternals_h
namespace js::gc {
// Encode a size with a single byte by using separate value and shift parts.
//
// This is essentially a floating point representation with the value being the
// mantissa and the shift being the exponent.
template <size_t GranularityShift = 4>
struct EncodedSize {
static constexpr size_t ShiftBits = 4;
static constexpr size_t ValueBits = 4;
static_assert(ShiftBits + ValueBits == 8);
static constexpr size_t Granularity = Bit(GranularityShift);
static constexpr size_t MaxSize =
BitMask(ValueBits + 1) << (BitMask(ShiftBits) - 1 + GranularityShift);
uint8_t bits = 0;
EncodedSize() = default;
explicit EncodedSize(size_t bytes) { set(bytes); }
void set(size_t bytes) {
MOZ_ASSERT(bytes < MaxSize);
// Round up to granularity and shift.
size_t i = (bytes + BitMask(GranularityShift)) >> GranularityShift;
// Sizes that fit in ValueBits are represented directly.
if (i < Bit(ValueBits)) {
bits = i;
return;
}
// Larger sizes store only ValueBits of the value plus a shift. The MSB is
// implied and is not stored as part of value.
MOZ_ASSERT(i <= UINT32_MAX);
size_t topBit = 31 - mozilla::CountLeadingZeroes32(i);
MOZ_ASSERT(i & Bit(topBit));
size_t shift = topBit - ValueBits;
size_t value = (i >> shift) & BitMask(ValueBits);
bits = ((shift + 1) << ValueBits) | value;
if (i & BitMask(shift)) {
bits++; // Value overflows into shift.
}
}
size_t get() const {
size_t shift = shiftPart();
size_t value = valuePart();
size_t i;
if (shift == 0) {
i = value;
} else {
i = (Bit(ValueBits) | value) << (shift - 1);
}
return i << GranularityShift;
}
size_t valuePart() const { return bits & BitMask(ValueBits); }
size_t shiftPart() const { return bits >> ValueBits; }
};
} // namespace js::gc
#endif // gc_BufferAllocatorInternals_h