Fontconfig font database: Short-circuit matching by filename

If the filename matches, no other matching is necessary. Fontconfig
doesn't have a fast path for that, so implement one here.
Fontconfig is unlikely to add that fast path, see here:
https://gitlab.freedesktop.org/fontconfig/fontconfig/issues/103

With -O1 builds of Qt and KDE stack, 358 fonts installed according
to KDE systemsetting, on a Ryzen 1800X, startup time of kwrite
decreases as following according to perf stat:
msec task-clock: ~480 ms to ~455 ms
cycles: ~1.73e9 to ~1.65e9

Change-Id: I630a80e4bed2647d5bbd95247005aab7d0cb0363
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 50 insertions, 22 deletions

diff --git a/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp b/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp
index 7abf295782..e28b40c240 100644
--- a/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp
+++ b/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp
@@ -927,38 +927,66 @@ void QFontconfigDatabase::setupFontEngine(QFontEngineFT *engine, const QFontDef
             antialias = antialiasingEnabled - 1;
     }
 
-    QFontEngine::GlyphFormat format;
-    // try and get the pattern
+    // try to find a match for fid
+    const QFontEngine::FaceId fid = engine->faceId();
     FcPattern *pattern = FcPatternCreate();
+    FcPattern *match = nullptr;
+
+    // try a trivial match by filename - FC_FILE is highest priority, so if it matches, FcFontMatch
+    // will just find the file (fine) and spend a millisecond or so doing unnecessary work (bad).
+    if (!fid.filename.isEmpty() && QFile::exists(QString::fromUtf8(fid.filename))) {
+        FcBlanks *blanks = FcConfigGetBlanks(nullptr);
+        int count = 0;
+        FcPattern *fileMatch = FcFreeTypeQuery((const FcChar8 *)fid.filename.data(), fid.index,
+                                               blanks, &count);
+        if (fileMatch) {
+            // Apply Fontconfig configuration - FcFreeTypeQuery only returns information stored in
+            // the font file, we also want to respect system and user settings.
+            FcConfigSubstitute(0, pattern, FcMatchPattern);
+            FcDefaultSubstitute(pattern);
+            match = FcFontRenderPrepare(0, pattern, fileMatch);
+            FcPatternDestroy(fileMatch);
+        }
+    }
 
-    FcValue value;
-    value.type = FcTypeString;
-    QByteArray cs = fontDef.family.toUtf8();
-    value.u.s = (const FcChar8 *)cs.data();
-    FcPatternAdd(pattern,FC_FAMILY,value,true);
+    if (!match) {
+        FcValue value;
 
-    QFontEngine::FaceId fid = engine->faceId();
+        // Fontconfig rules might process this information for arbitrary purposes, so add it,
+        // even though we already know that it doesn't match an existing file.
+        if (!fid.filename.isEmpty()) {
+            value.type = FcTypeString;
+            value.u.s = (const FcChar8 *)fid.filename.data();
+            FcPatternAdd(pattern, FC_FILE, value, true);
 
-    if (!fid.filename.isEmpty()) {
-        value.u.s = (const FcChar8 *)fid.filename.data();
-        FcPatternAdd(pattern,FC_FILE,value,true);
+            value.type = FcTypeInteger;
+            value.u.i = fid.index;
+            FcPatternAdd(pattern, FC_INDEX, value, true);
+        }
 
-        value.type = FcTypeInteger;
-        value.u.i = fid.index;
-        FcPatternAdd(pattern,FC_INDEX,value,true);
-    }
+        const QByteArray cs = fontDef.family.toUtf8();
+        value.type = FcTypeString;
+        value.u.s = (const FcChar8 *)cs.data();
+        FcPatternAdd(pattern, FC_FAMILY, value, true);
 
-    if (fontDef.pixelSize > 0.1)
-        FcPatternAddDouble(pattern, FC_PIXEL_SIZE, fontDef.pixelSize);
+        if (fontDef.pixelSize > 0.1) {
+            value.type = FcTypeDouble;
+            value.u.d = fontDef.pixelSize;
+            FcPatternAdd(pattern, FC_PIXEL_SIZE, value, true);
+        }
 
-    FcResult result;
+        FcResult result;
 
-    FcConfigSubstitute(0, pattern, FcMatchPattern);
-    FcDefaultSubstitute(pattern);
+        FcConfigSubstitute(0, pattern, FcMatchPattern);
+        FcDefaultSubstitute(pattern);
 
-    FcPattern *match = FcFontMatch(0, pattern, &result);
+        match = FcFontMatch(0, pattern, &result);
+    }
+
+    QFontEngine::GlyphFormat format;
     if (match) {
-        engine->setDefaultHintStyle(defaultHintStyleFromMatch((QFont::HintingPreference)fontDef.hintingPreference, match, useXftConf));
+        engine->setDefaultHintStyle(defaultHintStyleFromMatch(
+            (QFont::HintingPreference)fontDef.hintingPreference, match, useXftConf));
 
         FcBool fc_autohint;
         if (FcPatternGetBool(match, FC_AUTOHINT,0, &fc_autohint) == FcResultMatch)