Optimize GlyphTypeface: Avoid create the cmap Dictionary when get Count only

[lindexi]

Description

As the code shows:

wpf/src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/FontCache/FontFaceLayoutInfo.cs

Lines 585 to 609 in f7ebd69

	private Dictionary<int, ushort> CMap
	{
	get
	{
	if (_cmap == null)
	{
	lock (this)
	{
	if (_cmap == null)
	{
	_cmap = new Dictionary<int, ushort>();
	ushort glyphIndex;
	for (int codePoint = 0; codePoint <= FontFamilyMap.LastUnicodeScalar; ++codePoint)
	{
	if (TryGetValue(codePoint, out glyphIndex))
	{
	_cmap.Add(codePoint, glyphIndex);
	}
	}
	}
	}
	}
	return _cmap;
	}
	}

We can know that the cost of creating CMap is expensive. When we get the Count only, we do not need to create the CMap Dictionary.

We can get the count from FontFace directly:

wpf/src/Microsoft.DotNet.Wpf/src/DirectWriteForwarder/CPP/DWriteWrapper/FontFace.h

Line 126 in f7ebd69

property UINT16 GlyphCount

Customer Impact

Performance

Regression

Testing

CI only.

Risk

Low. I do not change the behavior. The old behavior is get the count after the CMap Dictionary created, and the new behavior is get the count from FontFace directly.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR optimizes the performance of getting the Count property in the FontFaceLayoutInfo class by avoiding the expensive creation of the CMap Dictionary when only the count is needed.

• Introduces conditional logic to check if the CMap is already created before accessing its Count
• Directly retrieves glyph count from FontFace when CMap is not yet initialized
• Maintains existing behavior while improving performance for count-only operations

---

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[miloush]

To be fair if you just want a count you can use GlyphTypeface.GlyphCount. How did you come with this, is it in some hot path?

[h3xds1nz]

I might be misremembering, but doesn't CMap get created the first time you do a single text run / line anyways? Not mentioning that the generation method is slow af and should be rewritten but as miloush says, you have the prop on GlyphTypeface directly for count on public interface and FontFaceLayoutInfo does have it as well so now you're code duping baiscally.

So I'm indeed curious about the scenario where you don't end up creating it and can get away with this.

[lindexi]

@miloush

To be fair if you just want a count you can use GlyphTypeface.GlyphCount. How did you come with this, is it in some hot path?

We just use the CharacterToGlyphMap as the Dictionary and pass it to other business code. The hot path will count the CharacterToGlyphMap Dictionary for all fonts.

[lindexi]

@h3xds1nz As you can see, ont of the IDictionary hot path is TryGetValue method, and which do not create the CMap:

wpf/src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/FontCache/FontFaceLayoutInfo.cs

Lines 634 to 657 in f7ebd69

	public bool TryGetValue(int key, out ushort value)
	{
	ushort localValue;
	unsafe
	{
	uint uKey = checked((uint)key);
	uint *pKey = &uKey;
	MS.Internal.Text.TextInterface.FontFace fontFace = _font.GetFontFace();
	try
	{
	fontFace.GetArrayOfGlyphIndices(pKey, 1, &localValue);
	}
	finally
	{
	fontFace.Release();
	}
	value = localValue;
	}
	// if a glyph is not present, index 0 is returned
	return (value != 0);
	}

So that, it is meaningful to do this.