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Reduce branches in padding adjustment in FromBase64_ComputeResultLength #115022

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Reduce branches in padding adjustment in FromBase64_ComputeResultLength #115022

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7c6832d
Reduce branches in padding adjustment in FromBase64_ComputeResultLength
rameel Apr 24, 2025
ed1ea95
Merge branch 'main' into optimize-base64-padding-map
rameel Apr 28, 2025
7ebb256
Use switch instead of array lookup
rameel Apr 28, 2025
2e29648
Add missing semicolon
rameel Apr 28, 2025
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14 changes: 6 additions & 8 deletions src/libraries/System.Private.CoreLib/src/System/Convert.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2901,15 +2901,13 @@ private static unsafe int FromBase64_ComputeResultLength(char* inputPtr, int inp

// Perf: reuse the variable that stored the number of '=' to store the number of bytes encoded by the
// last group that contains the '=':
if (padding != 0)
padding = padding switch
{
if (padding == 1)
padding = 2;
else if (padding == 2)
padding = 1;
else
throw new FormatException(SR.Format_BadBase64Char);
}
0 => 0,
1 => 2,
2 => 1,
_ => throw new FormatException(SR.Format_BadBase64Char)
};
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The title of the PR is "reduce branches", but this doesn't actually reduce branches today, does it?

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How about this: "Improve readability of Base64 padding adjustment logic"?

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However, the change seems too minor to have a meaningful impact. It might be more worthwhile to focus on removing "unsafe" in FromBase64_ComputeResultLength and related methods. Here's the commit for that: rameel@fe9f28b

Unfortunately due to the current JIT's inability to eliminate unnecessary bounds checks in cases like this (see #115091):

while (chars.Length != 0)
{
    if (chars[^1] is not (' ' or '\n' or '\r' or '\t'))
        break;

    chars = chars.Slice(0, chars.Length - 1); // Unnecessary bounds checks
}

and a performance regression compared to NET9 (#115090) I decided to postpone creating PR for this until later.

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Thanks, anyway


// Done:
return (usefulInputLength / 4) * 3 + padding;
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