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Refactor DecompressAllTags to reduce duplicate code
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A goto branch is still required, but we've swapped one for another and
it's much cleaner. This should also reduce code size.
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@brantburnett
brantburnett committed Jan 6, 2024
1 parent 77834c8 commit 172fcd7
Showing 1 changed file with 27 additions and 68 deletions.
95 changes: 27 additions & 68 deletions Snappier/Internal/SnappyDecompressor.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -222,28 +222,31 @@ internal void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
op = ref Unsafe.Add(ref op, bytesWritten);
}

if (!Unsafe.IsAddressLessThan(ref input, ref inputLimitMinMaxTagLength))
while (true)
{
uint newScratchLength = RefillTag(ref input, ref inputEnd, ref scratch);
if (newScratchLength == uint.MaxValue)
if (!Unsafe.IsAddressLessThan(ref input, ref inputLimitMinMaxTagLength))
{
goto exit;
}
uint newScratchLength = RefillTag(ref input, ref inputEnd, ref scratch);
if (newScratchLength == uint.MaxValue)
{
break;
}

if (newScratchLength > 0)
{
// Data has been moved to the scratch buffer
input = ref scratch;
inputEnd = ref Unsafe.Add(ref input, newScratchLength);
inputLimitMinMaxTagLength = ref Unsafe.Subtract(ref inputEnd,
Math.Min(newScratchLength, Constants.MaximumTagLength - 1));
if (newScratchLength > 0)
{
// Data has been moved to the scratch buffer
input = ref scratch;
inputEnd = ref Unsafe.Add(ref input, newScratchLength);
inputLimitMinMaxTagLength = ref Unsafe.Subtract(ref inputEnd,
Math.Min(newScratchLength, Constants.MaximumTagLength - 1));
}
}
}

uint preload = Helpers.UnsafeReadUInt32(ref input);
uint preload = Helpers.UnsafeReadUInt32(ref input);

// Some branches refill preload in a more optimal manner, they jump here to avoid the code above
skip_preload:

while (true)
{
byte c = (byte) preload;
input = ref Unsafe.Add(ref input, 1);

Expand All @@ -260,7 +263,7 @@ internal void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
// will not return true unless there's already at least five spare
// bytes in addition to the literal.
preload = Helpers.UnsafeReadUInt32(ref input);
continue;
goto skip_preload;
}

if (literalLength >= 61)
Expand All @@ -281,35 +284,12 @@ internal void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
Append(ref op, ref bufferEnd, in input, inputRemaining);
op = ref Unsafe.Add(ref op, inputRemaining);
_remainingLiteral = (int) (literalLength - inputRemaining);
goto exit;
break;
}
else
{
Append(ref op, ref bufferEnd, in input, literalLength);
op = ref Unsafe.Add(ref op, literalLength);
input = ref Unsafe.Add(ref input, literalLength);

if (!Unsafe.IsAddressLessThan(ref input, ref inputLimitMinMaxTagLength))
{
uint newScratchLength = RefillTag(ref input, ref inputEnd, ref scratch);
if (newScratchLength == uint.MaxValue)
{
goto exit;
}

if (newScratchLength > 0)
{
// Data has been moved to the scratch buffer
input = ref scratch;
inputEnd = ref Unsafe.Add(ref input, newScratchLength);
inputLimitMinMaxTagLength = ref Unsafe.Subtract(ref inputEnd,
Math.Min(newScratchLength, Constants.MaximumTagLength - 1));

}
}

preload = Helpers.UnsafeReadUInt32(ref input);
}
Append(ref op, ref bufferEnd, in input, literalLength);
op = ref Unsafe.Add(ref op, literalLength);
input = ref Unsafe.Add(ref input, literalLength);
}
else
{
Expand All @@ -325,10 +305,6 @@ internal void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
else
{
ushort entry = Unsafe.Add(ref charTable, c);

// We don't use BitConverter to read because we might be reading past the end of the span
// But we know that's safe because we'll be doing it in _scratch with extra data on the end.
// This reduces this step by several operations
preload = Helpers.UnsafeReadUInt32(ref input);

uint trailer = Helpers.ExtractLowBytes(preload, c & 3);
Expand All @@ -347,32 +323,15 @@ internal void DecompressAllTags(ReadOnlySpan<byte> inputSpan)
// By using the result of the previous load we reduce the critical
// dependency chain of ip to 4 cycles.
preload >>= (c & 3) * 8;
if (Unsafe.IsAddressLessThan(ref input, ref inputLimitMinMaxTagLength)) continue;
}

if (!Unsafe.IsAddressLessThan(ref input, ref inputLimitMinMaxTagLength))
{
uint newScratchLength = RefillTag(ref input, ref inputEnd, ref scratch);
if (newScratchLength == uint.MaxValue)
if (Unsafe.IsAddressLessThan(ref input, ref inputLimitMinMaxTagLength))
{
goto exit;
}

if (newScratchLength > 0)
{
// Data has been moved to the scratch buffer
input = ref scratch;
inputEnd = ref Unsafe.Add(ref input, newScratchLength);
inputLimitMinMaxTagLength = ref Unsafe.Subtract(ref inputEnd,
Math.Min(newScratchLength, Constants.MaximumTagLength - 1));
goto skip_preload;
}
}

preload = Helpers.UnsafeReadUInt32(ref input);
}
}

exit: ; // All input data is processed
// All input data is processed
_lookbackPosition = (int)Unsafe.ByteOffset(ref buffer, ref op);
}
}
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