Optimize stringToCMap() usage

CMAP guarantees there is 1:1 mapping between the Unicode code points
array and the glyph indexes array, which means the QString length
always greater than or equal to a sufficient glyph indexes array length.

Simply add some asserts and improve the memory consumption
and the performance where possible.

Change-Id: I0bc8cdba5f86c1c0ba9e79c8d5f96cbe56ec463e
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

9 files changed, 244 insertions, 131 deletions

diff --git a/src/gui/text/qdistancefield.cpp b/src/gui/text/qdistancefield.cpp
index e584b66a25..f2b88c4692 100644
--- a/src/gui/text/qdistancefield.cpp
+++ b/src/gui/text/qdistancefield.cpp
@@ -739,13 +739,19 @@ bool qt_fontHasNarrowOutlines(QFontEngine *fontEngine)
     if (!fe)
         return false;
 
-    QGlyphLayout glyphs;
+    const QChar uc(QLatin1Char('O'));
+
     glyph_t glyph;
-    glyphs.glyphs = &glyph;
+
+    QGlyphLayout glyphs;
     glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
     int numGlyphs = 1;
-    QChar uc = QLatin1Char('O');
-    fe->stringToCMap(&uc, 1, &glyphs, &numGlyphs, QFontEngine::GlyphIndicesOnly);
+
+    if (!fe->stringToCMap(&uc, 1, &glyphs, &numGlyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(numGlyphs == 1);
+
     QImage im = fe->alphaMapForGlyph(glyph, QFixed(), QTransform());
 
     Q_ASSERT(fe->ref.load() == 0);
diff --git a/src/gui/text/qfont.cpp b/src/gui/text/qfont.cpp
index f2951611ff..12761d0d3e 100644
--- a/src/gui/text/qfont.cpp
+++ b/src/gui/text/qfont.cpp
@@ -2533,7 +2533,9 @@ bool QFontInfo::fixedPitch() const
         QChar ch[2] = { QLatin1Char('i'), QLatin1Char('m') };
         QGlyphLayoutArray<2> g;
         int l = 2;
-        engine->stringToCMap(ch, 2, &g, &l, 0);
+        if (!engine->stringToCMap(ch, 2, &g, &l, 0))
+            Q_UNREACHABLE();
+        Q_ASSERT(l == 2);
         engine->fontDef.fixedPitch = g.advances[0] == g.advances[1];
         engine->fontDef.fixedPitchComputed = true;
     }
diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index 32225750e4..d4e2090c8f 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/src/gui/text/qfontengine.cpp
@@ -384,23 +384,39 @@ glyph_metrics_t QFontEngine::boundingBox(glyph_t glyph, const QTransform &matrix
 
 QFixed QFontEngine::xHeight() const
 {
-    QGlyphLayoutArray<8> glyphs;
-    int nglyphs = 7;
     QChar x((ushort)'x');
-    stringToCMap(&x, 1, &glyphs, &nglyphs, GlyphIndicesOnly);
 
-    glyph_metrics_t bb = const_cast<QFontEngine *>(this)->boundingBox(glyphs.glyphs[0]);
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!stringToCMap(&x, 1, &glyphs, &nglyphs, GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
+    glyph_metrics_t bb = const_cast<QFontEngine *>(this)->boundingBox(glyph);
     return bb.height;
 }
 
 QFixed QFontEngine::averageCharWidth() const
 {
-    QGlyphLayoutArray<8> glyphs;
-    int nglyphs = 7;
     QChar x((ushort)'x');
-    stringToCMap(&x, 1, &glyphs, &nglyphs, GlyphIndicesOnly);
 
-    glyph_metrics_t bb = const_cast<QFontEngine *>(this)->boundingBox(glyphs.glyphs[0]);
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!stringToCMap(&x, 1, &glyphs, &nglyphs, GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
+    glyph_metrics_t bb = const_cast<QFontEngine *>(this)->boundingBox(glyph);
     return bb.xoff;
 }
 
@@ -459,11 +475,22 @@ void QFontEngine::getGlyphPositions(const QGlyphLayout &glyphs, const QTransform
             ++current;
             if (glyphs.justifications[i].nKashidas) {
                 QChar ch(0x640); // Kashida character
-                QGlyphLayoutArray<8> g;
-                int nglyphs = 7;
-                stringToCMap(&ch, 1, &g, &nglyphs, 0);
+
+                glyph_t kashidaGlyph;
+                QFixed kashidaWidth;
+
+                QGlyphLayout g;
+                g.numGlyphs = 1;
+                g.glyphs = &kashidaGlyph;
+                g.advances = &kashidaWidth;
+
+                int nglyphs = 1;
+                if (!stringToCMap(&ch, 1, &g, &nglyphs, 0))
+                    Q_UNREACHABLE();
+                Q_ASSERT(nglyphs == 1);
+
                 for (uint k = 0; k < glyphs.justifications[i].nKashidas; ++k) {
-                    xpos -= g.advances[0];
+                    xpos -= kashidaWidth;
 
                     QFixed gpos_x = xpos + glyphs.offsets[i].x;
                     QFixed gpos_y = ypos + glyphs.offsets[i].y;
@@ -475,7 +502,7 @@ void QFontEngine::getGlyphPositions(const QGlyphLayout &glyphs, const QTransform
                     }
                     positions[current].x = gpos_x;
                     positions[current].y = gpos_y;
-                    glyphs_out[current] = g.glyphs[0];
+                    glyphs_out[current] = kashidaGlyph;
                     ++current;
                 }
             } else {
@@ -1536,9 +1563,10 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
                 if (!(flags & GlyphIndicesOnly))
                     glyphs->advances[glyph_pos] = QFixed();
                 int num = 2;
-                QGlyphLayout offs = glyphs->mid(glyph_pos, num);
-                engine->stringToCMap(str + i, surrogate ? 2 : 1, &offs, &num, flags);
-                Q_ASSERT(num == 1); // surrogates only give 1 glyph
+                QGlyphLayout g = glyphs->mid(glyph_pos, num);
+                if (!engine->stringToCMap(str + i, surrogate ? 2 : 1, &g, &num, flags))
+                    Q_UNREACHABLE();
+                Q_ASSERT(num == 1);
                 if (glyphs->glyphs[glyph_pos]) {
                     // set the high byte to indicate which engine the glyph came from
                     glyphs->glyphs[glyph_pos] |= (x << 24);
@@ -1849,16 +1877,11 @@ bool QFontEngineMulti::canRender(const QChar *string, int len)
     QGlyphLayout g;
     g.numGlyphs = nglyphs;
     g.glyphs = glyphs.data();
-    if (!stringToCMap(string, len, &g, &nglyphs, GlyphIndicesOnly)) {
-        glyphs.resize(nglyphs);
-        g.numGlyphs = nglyphs;
-        g.glyphs = glyphs.data();
-        if (!stringToCMap(string, len, &g, &nglyphs, GlyphIndicesOnly))
-            Q_ASSERT_X(false, Q_FUNC_INFO, "stringToCMap shouldn't fail twice");
-    }
+    if (!stringToCMap(string, len, &g, &nglyphs, GlyphIndicesOnly))
+        Q_UNREACHABLE();
 
     for (int i = 0; i < nglyphs; i++) {
-        if (g.glyphs[i] == 0)
+        if (glyphs[i] == 0)
             return false;
     }
 
diff --git a/src/gui/text/qfontengine_ft.cpp b/src/gui/text/qfontengine_ft.cpp
index 438c7b2141..a9195216ed 100644
--- a/src/gui/text/qfontengine_ft.cpp
+++ b/src/gui/text/qfontengine_ft.cpp
@@ -1274,13 +1274,22 @@ qreal QFontEngineFT::minRightBearing() const
 {
     if (rbearing == SHRT_MIN) {
         lbearing = rbearing = 0;
-        const QChar *ch = (const QChar *)(const void*)char_table;
-        QGlyphLayoutArray<char_table_entries> glyphs;
+
+        const QChar *ch = reinterpret_cast<const QChar *>(char_table);
+
+        glyph_t glyphs[char_table_entries];
+
+        QGlyphLayout g;
+        g.glyphs = glyphs;
+        g.numGlyphs = char_table_entries;
         int ng = char_table_entries;
-        stringToCMap(ch, char_table_entries, &glyphs, &ng, GlyphIndicesOnly);
+        if (!stringToCMap(ch, char_table_entries, &g, &ng, GlyphIndicesOnly))
+            Q_UNREACHABLE();
+        Q_ASSERT(ng == char_table_entries);
+
         while (--ng) {
-            if (glyphs.glyphs[ng]) {
-                glyph_metrics_t gi = const_cast<QFontEngineFT *>(this)->boundingBox(glyphs.glyphs[ng]);
+            if (glyphs[ng]) {
+                glyph_metrics_t gi = const_cast<QFontEngineFT *>(this)->boundingBox(glyphs[ng]);
                 lbearing = qMin(lbearing, gi.x);
                 rbearing = qMin(rbearing, (gi.xoff - gi.x - gi.width));
             }
diff --git a/src/gui/text/qfontmetrics.cpp b/src/gui/text/qfontmetrics.cpp
index 08105834ea..2f4709afe4 100644
--- a/src/gui/text/qfontmetrics.cpp
+++ b/src/gui/text/qfontmetrics.cpp
@@ -458,12 +458,19 @@ int QFontMetrics::leftBearing(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<10> glyphs;
-    int nglyphs = 9;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly);
-    // ### can nglyphs != 1 happen at all? Not currently I think
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
     qreal lb;
-    engine->getGlyphBearings(glyphs.glyphs[0], &lb);
+    engine->getGlyphBearings(glyph, &lb);
     return qRound(lb);
 }
 
@@ -493,12 +500,19 @@ int QFontMetrics::rightBearing(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<10> glyphs;
-    int nglyphs = 9;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly);
-    // ### can nglyphs != 1 happen at all? Not currently I think
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
     qreal rb;
-    engine->getGlyphBearings(glyphs.glyphs[0], 0, &rb);
+    engine->getGlyphBearings(glyph, 0, &rb);
     return qRound(rb);
 }
 
@@ -538,11 +552,8 @@ int QFontMetrics::width(const QString &text, int len, int flags) const
         int numGlyphs = len;
         QVarLengthGlyphLayoutArray glyphs(numGlyphs);
         QFontEngine *engine = d->engineForScript(QChar::Script_Common);
-        if (!engine->stringToCMap(text.data(), len, &glyphs, &numGlyphs, 0)) {
-            glyphs.resize(numGlyphs);
-            if (!engine->stringToCMap(text.data(), len, &glyphs, &numGlyphs, 0))
-                Q_ASSERT_X(false, Q_FUNC_INFO, "stringToCMap shouldn't fail twice");
-        }
+        if (!engine->stringToCMap(text.data(), len, &glyphs, &numGlyphs, 0))
+            Q_UNREACHABLE();
 
         QFixed width;
         for (int i = 0; i < numGlyphs; ++i)
@@ -594,10 +605,20 @@ int QFontMetrics::width(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<8> glyphs;
-    int nglyphs = 7;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0);
-    return qRound(glyphs.advances[0]);
+    QFixed advance;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyph_t glyph;
+    glyphs.glyphs = &glyph;
+    glyphs.advances = &advance;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
+    return qRound(advance);
 }
 
 /*! \obsolete
@@ -639,10 +660,20 @@ int QFontMetrics::charWidth(const QString &text, int pos) const
 
         d->alterCharForCapitalization(ch);
 
-        QGlyphLayoutArray<8> glyphs;
-        int nglyphs = 7;
-        engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0);
-        width = qRound(glyphs.advances[0]);
+        QFixed advance;
+
+        QGlyphLayout glyphs;
+        glyphs.numGlyphs = 1;
+        glyph_t glyph;
+        glyphs.glyphs = &glyph;
+        glyphs.advances = &advance;
+
+        int nglyphs = 1;
+        if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0))
+            Q_UNREACHABLE();
+        Q_ASSERT(nglyphs == 1);
+
+        width = qRound(advance);
     }
     return width;
 }
@@ -708,10 +739,18 @@ QRect QFontMetrics::boundingRect(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<10> glyphs;
-    int nglyphs = 9;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly);
-    glyph_metrics_t gm = engine->boundingBox(glyphs.glyphs[0]);
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
+    glyph_metrics_t gm = engine->boundingBox(glyph);
     return QRect(qRound(gm.x), qRound(gm.y), qRound(gm.width), qRound(gm.height));
 }
 
@@ -1326,12 +1365,19 @@ qreal QFontMetricsF::leftBearing(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<10> glyphs;
-    int nglyphs = 9;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly);
-    // ### can nglyphs != 1 happen at all? Not currently I think
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
     qreal lb;
-    engine->getGlyphBearings(glyphs.glyphs[0], &lb);
+    engine->getGlyphBearings(glyph, &lb);
     return lb;
 }
 
@@ -1361,12 +1407,19 @@ qreal QFontMetricsF::rightBearing(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<10> glyphs;
-    int nglyphs = 9;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly);
-    // ### can nglyphs != 1 happen at all? Not currently I think
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
     qreal rb;
-    engine->getGlyphBearings(glyphs.glyphs[0], 0, &rb);
+    engine->getGlyphBearings(glyph, 0, &rb);
     return rb;
 
 }
@@ -1431,10 +1484,20 @@ qreal QFontMetricsF::width(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<8> glyphs;
-    int nglyphs = 7;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0);
-    return glyphs.advances[0].toReal();
+    QFixed advance;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyph_t glyph;
+    glyphs.glyphs = &glyph;
+    glyphs.advances = &advance;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
+    return advance.toReal();
 }
 
 /*!
@@ -1496,10 +1559,18 @@ QRectF QFontMetricsF::boundingRect(QChar ch) const
 
     d->alterCharForCapitalization(ch);
 
-    QGlyphLayoutArray<10> glyphs;
-    int nglyphs = 9;
-    engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly);
-    glyph_metrics_t gm = engine->boundingBox(glyphs.glyphs[0]);
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.numGlyphs = 1;
+    glyphs.glyphs = &glyph;
+
+    int nglyphs = 1;
+    if (!engine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(nglyphs == 1);
+
+    glyph_metrics_t gm = engine->boundingBox(glyph);
     return QRectF(gm.x.toReal(), gm.y.toReal(), gm.width.toReal(), gm.height.toReal());
 }
 
diff --git a/src/gui/text/qfontsubset.cpp b/src/gui/text/qfontsubset.cpp
index 152e15a54d..2109b16bb5 100644
--- a/src/gui/text/qfontsubset.cpp
+++ b/src/gui/text/qfontsubset.cpp
@@ -201,12 +201,22 @@ static void checkRanges(QPdf::ByteStream &ts, QByteArray &ranges, int &nranges)
 QVector<int> QFontSubset::getReverseMap() const
 {
     QVector<int> reverseMap(0x10000, 0);
-    QGlyphLayoutArray<10> glyphs;
+
+    glyph_t glyph;
+
+    QGlyphLayout glyphs;
+    glyphs.glyphs = &glyph;
+    glyphs.numGlyphs = 1;
+
     for (uint uc = 0; uc < 0x10000; ++uc) {
         QChar ch(uc);
-        int nglyphs = 10;
-        fontEngine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly);
-        int idx = glyph_indices.indexOf(glyphs.glyphs[0]);
+
+        int nglyphs = 1;
+        if (!fontEngine->stringToCMap(&ch, 1, &glyphs, &nglyphs, QFontEngine::GlyphIndicesOnly))
+            Q_UNREACHABLE();
+        Q_ASSERT(nglyphs == 1);
+
+        int idx = glyph_indices.indexOf(glyph);
         if (idx >= 0 && !reverseMap.at(idx))
             reverseMap[idx] = uc;
     }
diff --git a/src/gui/text/qharfbuzzng.cpp b/src/gui/text/qharfbuzzng.cpp
index bf34c74b49..7819cd4465 100644
--- a/src/gui/text/qharfbuzzng.cpp
+++ b/src/gui/text/qharfbuzzng.cpp
@@ -397,13 +397,7 @@ _hb_qt_font_get_glyph(hb_font_t * /*font*/, void *font_data,
     QFontEngine *fe = (QFontEngine *)font_data;
     Q_ASSERT(fe);
 
-    glyph_t glyphs[2] = { 0, 0 };
-
-    QGlyphLayout g;
-    g.numGlyphs = 2;
-    g.glyphs = glyphs;
-
-    QChar chars[4];
+    QChar chars[2];
     int numChars = 0;
     if (Q_UNLIKELY(QChar::requiresSurrogates(unicode))) {
         chars[numChars++] = QChar(QChar::highSurrogate(unicode));
@@ -422,11 +416,14 @@ _hb_qt_font_get_glyph(hb_font_t * /*font*/, void *font_data,
     }
 #endif
 
-    int numGlyphs = g.numGlyphs;
-    bool ok = fe->stringToCMap(chars, numChars, &g, &numGlyphs, QFontEngine::GlyphIndicesOnly);
-    Q_ASSERT(ok); Q_UNUSED(ok)
+    QGlyphLayout g;
+    g.numGlyphs = numChars;
+    g.glyphs = glyph;
 
-    *glyph = g.glyphs[0];
+    int numGlyphs = numChars;
+    if (!fe->stringToCMap(chars, numChars, &g, &numGlyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
+    Q_ASSERT(numGlyphs == 1);
 
     return true;
 }
@@ -448,7 +445,7 @@ _hb_qt_font_get_glyph_h_advance(hb_font_t *font, void *font_data,
 
     fe->recalcAdvances(&g, QFontEngine::ShaperFlags(hb_qt_font_get_use_design_metrics(font)));
 
-    return g.advances[0].value();
+    return advance.value();
 }
 
 static hb_position_t
@@ -497,7 +494,7 @@ _hb_qt_font_get_glyph_h_kerning(hb_font_t *font, void *font_data,
 
     fe->doKerning(&g, QFontEngine::ShaperFlags(hb_qt_font_get_use_design_metrics(font)));
 
-    return g.advances[0].value();
+    return advance.value();
 }
 
 static hb_position_t
diff --git a/src/gui/text/qrawfont.cpp b/src/gui/text/qrawfont.cpp
index e26f7cdc93..449278df06 100644
--- a/src/gui/text/qrawfont.cpp
+++ b/src/gui/text/qrawfont.cpp
@@ -479,16 +479,8 @@ QVector<quint32> QRawFont::glyphIndexesForString(const QString &text) const
     QGlyphLayout glyphs;
     glyphs.numGlyphs = numGlyphs;
     glyphs.glyphs = glyphIndexes.data();
-    if (!d->fontEngine->stringToCMap(text.data(), text.size(), &glyphs, &numGlyphs, QFontEngine::GlyphIndicesOnly)) {
-        glyphIndexes.resize(numGlyphs);
-
-        glyphs.numGlyphs = numGlyphs;
-        glyphs.glyphs = glyphIndexes.data();
-        if (!d->fontEngine->stringToCMap(text.data(), text.size(), &glyphs, &numGlyphs, QFontEngine::GlyphIndicesOnly)) {
-            Q_ASSERT_X(false, Q_FUNC_INFO, "stringToCMap shouldn't fail twice");
-            return QVector<quint32>();
-        }
-    }
+    if (!d->fontEngine->stringToCMap(text.data(), text.size(), &glyphs, &numGlyphs, QFontEngine::GlyphIndicesOnly))
+        Q_UNREACHABLE();
 
     glyphIndexes.resize(numGlyphs);
     return glyphIndexes;
@@ -565,10 +557,11 @@ bool QRawFont::advancesForGlyphIndexes(const quint32 *glyphIndexes, QPointF *adv
     if (!d->isValid() || numGlyphs <= 0)
         return false;
 
+    QVarLengthArray<QFixed> tmpAdvances(numGlyphs);
+
     QGlyphLayout glyphs;
     glyphs.glyphs = const_cast<glyph_t *>(glyphIndexes);
     glyphs.numGlyphs = numGlyphs;
-    QVarLengthArray<QFixed> tmpAdvances(numGlyphs);
     glyphs.advances = tmpAdvances.data();
 
     bool design = layoutFlags & UseDesignMetrics;
@@ -578,7 +571,7 @@ bool QRawFont::advancesForGlyphIndexes(const quint32 *glyphIndexes, QPointF *adv
         d->fontEngine->doKerning(&glyphs, design ? QFontEngine::DesignMetrics : QFontEngine::ShaperFlag(0));
 
     for (int i=0; i<numGlyphs; ++i)
-        advances[i] = QPointF(glyphs.advances[i].toReal(), 0.0);
+        advances[i] = QPointF(tmpAdvances[i].toReal(), 0.0);
 
     return true;
 }
diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index 6db6945659..03c99ee085 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -932,18 +932,8 @@ void QTextEngine::shapeText(int item) const
 
         int nGlyphs = initialGlyphs.numGlyphs;
         QFontEngine::ShaperFlags shaperFlags(QFontEngine::GlyphIndicesOnly);
-        if (!fontEngine->stringToCMap(reinterpret_cast<const QChar *>(string), itemLength, &initialGlyphs, &nGlyphs, shaperFlags)) {
-            nGlyphs = qMax(nGlyphs, itemLength); // ### needed for QFontEngine::stringToCMap() to not fail twice
-            if (!ensureSpace(nGlyphs)) {
-                Q_UNREACHABLE(); // ### report OOM error somehow
-                return;
-            }
-            initialGlyphs = availableGlyphs(&si);
-            if (!fontEngine->stringToCMap(reinterpret_cast<const QChar *>(string), itemLength, &initialGlyphs, &nGlyphs, shaperFlags)) {
-                Q_UNREACHABLE(); // ### if this happens there is a bug in the fontengine
-                return;
-            }
-        }
+        if (!fontEngine->stringToCMap(reinterpret_cast<const QChar *>(string), itemLength, &initialGlyphs, &nGlyphs, shaperFlags))
+            Q_UNREACHABLE();
 
         uint lastEngine = 0;
         for (int i = 0, glyph_pos = 0; i < itemLength; ++i, ++glyph_pos) {
@@ -2003,11 +1993,22 @@ static void set(QJustificationPoint *point, int type, const QGlyphLayout &glyph,
 
     if (type >= QGlyphAttributes::Arabic_Normal) {
         QChar ch(0x640); // Kashida character
-        QGlyphLayoutArray<8> glyphs;
-        int nglyphs = 7;
-        fe->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0);
-        if (glyphs.glyphs[0] != 0 && glyphs.advances[0].value() != 0) {
-            point->kashidaWidth = glyphs.advances[0];
+
+        glyph_t kashidaGlyph;
+        QFixed kashidaWidth;
+
+        QGlyphLayout glyphs;
+        glyphs.numGlyphs = 1;
+        glyphs.glyphs = &kashidaGlyph;
+        glyphs.advances = &kashidaWidth;
+
+        int nglyphs = 1;
+        if (!fe->stringToCMap(&ch, 1, &glyphs, &nglyphs, 0))
+            Q_UNREACHABLE();
+        Q_ASSERT(nglyphs == 1);
+
+        if (kashidaGlyph != 0 && kashidaWidth != 0) {
+            point->kashidaWidth = kashidaWidth;
         } else {
             point->type = QGlyphAttributes::NoJustification;
             point->kashidaWidth = 0;
@@ -2639,10 +2640,10 @@ QString QTextEngine::elidedText(Qt::TextElideMode mode, const QFixed &width, int
             if (feForEllipsis->type() == QFontEngine::Mac)
                 feForEllipsis = fe;
 
-            if (feForEllipsis->canRender(&ellipsisChar, 1)) {
-                int nGlyphs = 1;
-                feForEllipsis->stringToCMap(&ellipsisChar, 1, &ellipsisGlyph, &nGlyphs, 0);
-            }
+            int nGlyphs = 1;
+            if (!feForEllipsis->stringToCMap(&ellipsisChar, 1, &ellipsisGlyph, &nGlyphs, 0))
+                Q_UNREACHABLE();
+            Q_ASSERT(nGlyphs == 1);
         }
 
         if (ellipsisGlyph.glyphs[0]) {
@@ -2654,8 +2655,9 @@ QString QTextEngine::elidedText(Qt::TextElideMode mode, const QFixed &width, int
             QGlyphLayoutArray<3> glyphs;
             int nGlyphs = 3;
             if (!fe->stringToCMap(dotDotDot.constData(), 3, &glyphs, &nGlyphs, 0))
-                // should never happen...
-                return layoutData->string;
+                Q_UNREACHABLE();
+            Q_ASSERT(nGlyphs == 3);
+
             for (int i = 0; i < nGlyphs; ++i)
                 ellipsisWidth += glyphs.advances[i];
             ellipsisText = dotDotDot;