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Optimize Ascii.Equals when widening #87141

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Merged
merged 4 commits into from
Jul 6, 2023
Merged

Optimize Ascii.Equals when widening #87141

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1467895
Optimize Ascii.Equals when widening
BrennanConroy Jun 5, 2023
6b1cb00
fb
BrennanConroy Jun 11, 2023
60e0c2f
add BoundedMemory tests to ensure that boundaries are respected + som…
adamsitnik Jul 6, 2023
dd02aeb
Merge remote-tracking branch 'upstream/main' into brecon/ascii
adamsitnik Jul 6, 2023
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@BrennanConroy
BrennanConroy committed Jun 11, 2023
commit 6b1cb008033dec1bcdc63065127b596514a5b5c9
16 changes: 10 additions & 6 deletions src/libraries/System.Private.CoreLib/src/System/Text/Ascii.Equality.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,10 @@ private static bool Equals<TLeft, TRight, TLoader>(ref TLeft left, ref TRight ri
{
ref TLeft currentLeftSearchSpace = ref left;
ref TRight currentRightSearchSpace = ref right;
// Add Vector256<TLeft>.Count because TLeft == TRight
// Or we are in the Widen case where we iterate 2 * TRight.Count which is the same as TLeft.Count
Debug.Assert(Vector256<TLeft>.Count == Vector256<TRight>.Count
|| (typeof(TLoader) == typeof(WideningLoader) && Vector256<TLeft>.Count == Vector256<TRight>.Count * 2));
ref TRight oneVectorAwayFromRightEnd = ref Unsafe.Add(ref currentRightSearchSpace, length - (uint)Vector256<TLeft>.Count);
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I'm not understanding why this is valid. We're subtracting from the "right" search space the number of "left" elements in a vector?

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It works because TLeft and TRight are either the same type, or we are in the widen case where Vector<TLeft> is twice the size of Vector<TRight> and the widen code will advance twice Vector<TRight>.Count which is equal to 1 Vector<TLeft>.Count.

But it is written in a confusing way. The whole TLoader abstraction helps with code sharing but makes this part kind of yucky. Maybe if the compare method advanced the pointers it would be better?

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Thanks. At a minimum a comment explaining would be helpful.


// Loop until either we've finished all elements or there's less than a vector's-worth remaining.
Expand Down Expand Up @@ -410,22 +414,22 @@ public static Vector256<ushort> Load256(ref byte ptr)

[MethodImpl(MethodImplOptions.AggressiveInlining)]
[CompExactlyDependsOn(typeof(Avx))]
public static bool Compare256(ref byte left, ref ushort right)
public static bool Compare256(ref byte utf8, ref ushort utf16)
{
// We widen the utf8 param so we can compare it to utf16, this doubles how much of the utf16 vector we search
Debug.Assert(Vector256<byte>.Count == Vector256<ushort>.Count * 2);

Vector256<byte> leftNotWidened = Vector256.LoadUnsafe(ref left);
Vector256<byte> leftNotWidened = Vector256.LoadUnsafe(ref utf8);
if (!AllCharsInVectorAreAscii(leftNotWidened))
{
return false;
}

(Vector256<ushort> lower, Vector256<ushort> upper) = Vector256.Widen(leftNotWidened);
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This is me adding value:

Suggested change
(Vector256<ushort> lower, Vector256<ushort> upper) = Vector256.Widen(leftNotWidened);
var (lower, upper) = Vector256.Widen(leftNotWidened);

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This is me adding value:

Please stop adding value. :-P

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The BCL has a rule that var can only be used when the type is apparent.

It's a battle that I lost before ever joining the team 😅

Vector256<ushort> rightValues0 = Vector256.LoadUnsafe(ref right);
Vector256<ushort> rightValues1 = Vector256.LoadUnsafe(ref Unsafe.Add(ref right, (uint)Vector256<ushort>.Count));
Vector256<ushort> rightValues0 = Vector256.LoadUnsafe(ref utf16);
Vector256<ushort> rightValues1 = Vector256.LoadUnsafe(ref utf16, (uint)Vector256<ushort>.Count);

if (!Vector256<ushort>.AllBitsSet.Equals(
Vector256.BitwiseAnd(Vector256.Equals(lower, rightValues0), Vector256.Equals(upper, rightValues1))))
if (((lower ^ rightValues0) | (upper ^ rightValues1)) != Vector256<ushort>.Zero)
{
return false;
}
Expand Down