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AK/LEB128: Speed up reading unsigned LEB128 values

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Unroll the first byte as a fast path, and remove a branch. This speeds
up the instantiation of spidermonkey by 10ms.
This commit is contained in:
Diego Frias 2024-07-26 14:53:21 -07:00 committed by Andreas Kling
parent fc57f5111d
commit a6ebd100ec
Notes: github-actions[bot] 2024-07-27 06:20:41 +00:00 
Author: https://github.com/dzfrias
Commit: https://github.com/LadybirdBrowser/ladybird/commit/a6ebd100ecd
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/850
1 changed files with 15 additions and 19 deletions
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32
AK/LEB128.h
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@ -27,31 +27,27 @@ public:
static ErrorOr<LEB128<ValueType>> read_from_stream(Stream& stream)
requires(Unsigned<ValueType>)
{
ValueType result {};
size_t num_bytes = 0;
while (true) {
if (stream.is_eof())
return Error::from_string_literal("Stream reached end-of-file while reading LEB128 value");
// First byte is unrolled for speed
auto byte = TRY(stream.read_value<u8>());
if ((byte & 0x80) == 0)
return LEB128<ValueType> { byte };
ValueType masked_byte = byte & ~(1 << 7);
bool const shift_too_large_for_result = num_bytes * 7 > sizeof(ValueType) * 8;
if (shift_too_large_for_result)
ValueType result = byte & 0x7F;
size_t num_bytes = 1;
while (true) {
auto byte = TRY(stream.read_value<u8>());
ValueType masked = byte & 0x7F;
result |= masked << (num_bytes * 7);
if (num_bytes * 7 >= sizeof(ValueType) * 8 - 7 && (byte >> (sizeof(ValueType) * 8 - (num_bytes * 7))) != 0) {
if ((byte & 0x80) != 0)
return Error::from_string_literal("Read value contains more bits than fit the chosen ValueType");
bool const shift_too_large_for_byte = ((masked_byte << (num_bytes * 7)) >> (num_bytes * 7)) != masked_byte;
if (shift_too_large_for_byte)
return Error::from_string_literal("Read byte is too large to fit the chosen ValueType");
result = (result) | (masked_byte << (num_bytes * 7));
if (!(byte & (1 << 7)))
break;
++num_bytes;
}
++num_bytes;
if ((byte & 0x80) == 0)
return LEB128<ValueType> { result };
}
}
static ErrorOr<LEB128<ValueType>> read_from_stream(Stream& stream)
requires(Signed<ValueType>)