Mark max length work buffers uninitialized

Otherwise the hardening with initializing all buffers causes serious
performance regressions

Pick-to: 6.8
Change-Id: I3f7a0b7f0e0d08644b1dbb520cf1f6d5e052b270
Reviewed-by: Thiago Macieira <[email protected]>

15 files changed, 117 insertions, 111 deletions

diff --git a/src/corelib/global/qcompilerdetection.h b/src/corelib/global/qcompilerdetection.h
index 8e9c64fc5b0..4b93bdefed4 100644
--- a/src/corelib/global/qcompilerdetection.h
+++ b/src/corelib/global/qcompilerdetection.h
@@ -1272,6 +1272,12 @@
 #  endif
 #endif
 
+#if defined(__has_attribute) && __has_attribute(uninitialized)
+#  define Q_DECL_UNINITIALIZED __attribute__((uninitialized))
+#else
+#  define Q_DECL_UNINITIALIZED
+#endif
+
 
 /*
     Sanitize compiler feature availability
diff --git a/src/corelib/text/qlocale_tools.cpp b/src/corelib/text/qlocale_tools.cpp
index 6bd7ef13b1b..126d5e65545 100644
--- a/src/corelib/text/qlocale_tools.cpp
+++ b/src/corelib/text/qlocale_tools.cpp
@@ -510,7 +510,7 @@ QString qulltoBasicLatin(qulonglong number, int base, bool negative)
     // We do not need a terminator.
     const unsigned maxlen = 65;
     static_assert(CHAR_BIT * sizeof(number) + 1 <= maxlen);
-    char16_t buff[maxlen];
+    Q_DECL_UNINITIALIZED char16_t buff[maxlen];
     char16_t *const end = buff + maxlen, *p = end;
 
     qulltoString_helper<char16_t>(number, base, p);
@@ -526,7 +526,7 @@ QString qulltoa(qulonglong number, int base, const QStringView zero)
     // per digit. We do not need a terminator.
     const unsigned maxlen = 128;
     static_assert(CHAR_BIT * sizeof(number) <= maxlen);
-    char16_t buff[maxlen];
+    Q_DECL_UNINITIALIZED char16_t buff[maxlen];
     char16_t *const end = buff + maxlen, *p = end;
 
     if (base != 10 || zero == u"0") {
diff --git a/src/gui/image/qimage.cpp b/src/gui/image/qimage.cpp
index 9cfe722bb5f..b18952fc12f 100644
--- a/src/gui/image/qimage.cpp
+++ b/src/gui/image/qimage.cpp
@@ -2913,7 +2913,7 @@ bool QImage::allGray() const
         break;
     }
 
-    uint buffer[BufferSize];
+    Q_DECL_UNINITIALIZED uint buffer[BufferSize];
     const QPixelLayout *layout = &qPixelLayouts[d->format];
     const auto fetch = layout->fetchToARGB32PM;
     for (int j = 0; j < d->height; ++j) {
diff --git a/src/gui/image/qimage_conversions.cpp b/src/gui/image/qimage_conversions.cpp
index ddc7ce59aa3..5f300afd5f5 100644
--- a/src/gui/image/qimage_conversions.cpp
+++ b/src/gui/image/qimage_conversions.cpp
@@ -185,7 +185,7 @@ void convert_generic(QImageData *dest, const QImageData *src, Qt::ImageConversio
     }
 
     auto convertSegment = [=](int yStart, int yEnd) {
-        uint buf[BufferSize];
+        Q_DECL_UNINITIALIZED uint buf[BufferSize];
         uint *buffer = buf;
         const uchar *srcData = src->data + src->bytes_per_line * yStart;
         uchar *destData = dest->data + dest->bytes_per_line * yStart;
@@ -247,7 +247,7 @@ void convert_generic_over_rgb64(QImageData *dest, const QImageData *src, Qt::Ima
     const ConvertAndStorePixelsFunc64 store = qStoreFromRGBA64PM[dest->format];
 
     auto convertSegment = [=](int yStart, int yEnd) {
-        QRgba64 buf[BufferSize];
+        Q_DECL_UNINITIALIZED QRgba64 buf[BufferSize];
         QRgba64 *buffer = buf;
         const uchar *srcData = src->data + yStart * src->bytes_per_line;
         uchar *destData = dest->data + yStart * dest->bytes_per_line;
@@ -301,7 +301,7 @@ void convert_generic_over_rgba32f(QImageData *dest, const QImageData *src, Qt::I
     const ConvertAndStorePixelsFuncFP store = qStoreFromRGBA32F[dest->format];
 
     auto convertSegment = [=](int yStart, int yEnd) {
-        QRgbaFloat32 buf[BufferSize];
+        Q_DECL_UNINITIALIZED QRgbaFloat32 buf[BufferSize];
         QRgbaFloat32 *buffer = buf;
         const uchar *srcData = src->data + yStart * src->bytes_per_line;
         uchar *destData = dest->data + yStart * dest->bytes_per_line;
@@ -408,7 +408,7 @@ bool convert_generic_inplace(QImageData *data, QImage::Format dst_format, Qt::Im
     }
 
     auto convertSegment = [=](int yStart, int yEnd) {
-        uint buf[BufferSize];
+        Q_DECL_UNINITIALIZED uint buf[BufferSize];
         uint *buffer = buf;
         uchar *srcData = data->data + data->bytes_per_line * yStart;
         uchar *destData = srcData; // This can be temporarily wrong if we doing a shrinking conversion
@@ -507,7 +507,7 @@ bool convert_generic_inplace_over_rgb64(QImageData *data, QImage::Format dst_for
     }
 
     auto convertSegment = [=](int yStart, int yEnd) {
-        QRgba64 buf[BufferSize];
+        Q_DECL_UNINITIALIZED QRgba64 buf[BufferSize];
         QRgba64 *buffer = buf;
         uchar *srcData = data->data + yStart * data->bytes_per_line;
         uchar *destData = srcData;
@@ -601,7 +601,7 @@ bool convert_generic_inplace_over_rgba32f(QImageData *data, QImage::Format dst_f
     }
 
     auto convertSegment = [=](int yStart, int yEnd) {
-        QRgbaFloat32 buf[BufferSize];
+        Q_DECL_UNINITIALIZED QRgbaFloat32 buf[BufferSize];
         QRgbaFloat32 *buffer = buf;
         uchar *srcData = data->data + yStart * data->bytes_per_line;
         uchar *destData = srcData;
@@ -1468,7 +1468,7 @@ static void convert_ARGB_to_gray16(QImageData *dest, const QImageData *src, Qt::
             ? QColorTransformPrivate::InputPremultiplied
             : QColorTransformPrivate::Unpremultiplied;
 
-    QRgba64 tmp_line[BufferSize];
+    Q_DECL_UNINITIALIZED QRgba64 tmp_line[BufferSize];
     for (int i = 0; i < src->height; ++i) {
         const QRgb *src_line = reinterpret_cast<const QRgb *>(src_data);
         quint16 *dest_line = reinterpret_cast<quint16 *>(dest_data);
@@ -1507,7 +1507,7 @@ static void convert_RGBA64_to_gray8(QImageData *dest, const QImageData *src, Qt:
             ? QColorTransformPrivate::InputPremultiplied
             : QColorTransformPrivate::Unpremultiplied;
 
-    quint16 gray_line[BufferSize];
+    Q_DECL_UNINITIALIZED quint16 gray_line[BufferSize];
     for (int i = 0; i < src->height; ++i) {
         const QRgba64 *src_line = reinterpret_cast<const QRgba64 *>(src_data);
         uchar *dest_line = dest_data;
diff --git a/src/gui/painting/qcolortransform.cpp b/src/gui/painting/qcolortransform.cpp
index 90665e84ff6..43e2dd6f3d1 100644
--- a/src/gui/painting/qcolortransform.cpp
+++ b/src/gui/painting/qcolortransform.cpp
@@ -1975,7 +1975,7 @@ void QColorTransformPrivate::apply(D *dst, const S *src, qsizetype count, Transf
     if (colorSpaceOut->isThreeComponentMatrix())
         updateLutsOut();
 
-    QUninitialized<QColorVector, WorkBlockSize> buffer;
+    Q_DECL_UNINITIALIZED QUninitialized<QColorVector, WorkBlockSize> buffer;
     qsizetype i = 0;
     while (i < count) {
         const qsizetype len = qMin(count - i, WorkBlockSize);
diff --git a/src/gui/painting/qdrawhelper.cpp b/src/gui/painting/qdrawhelper.cpp
index 496d38cc45b..8beb299fab4 100644
--- a/src/gui/painting/qdrawhelper.cpp
+++ b/src/gui/painting/qdrawhelper.cpp
@@ -667,7 +667,7 @@ static void QT_FASTCALL destStoreGray16(QRasterBuffer *rasterBuffer, int x, int
         QColorTransform tf = QColorSpacePrivate::get(fromCS)->transformationToXYZ();
         QColorTransformPrivate *tfd = QColorTransformPrivate::get(tf);
 
-        QRgba64 tmp_line[BufferSize];
+        Q_DECL_UNINITIALIZED QRgba64 tmp_line[BufferSize];
         for (int k = 0; k < length; ++k)
             tmp_line[k] = QRgba64::fromArgb32(buffer[k]);
         tfd->apply(data, tmp_line, length, QColorTransformPrivate::InputPremultiplied);
@@ -750,7 +750,7 @@ static void QT_FASTCALL destStore64Gray8(QRasterBuffer *rasterBuffer, int x, int
         QColorTransform tf = QColorSpacePrivate::get(fromCS)->transformationToXYZ();
         QColorTransformPrivate *tfd = QColorTransformPrivate::get(tf);
 
-        quint16 gray_line[BufferSize];
+        Q_DECL_UNINITIALIZED quint16 gray_line[BufferSize];
         tfd->apply(gray_line, buffer, length, QColorTransformPrivate::InputPremultiplied);
         for (int k = 0; k < length; ++k)
             data[k] = qt_div_257(gray_line[k]);
@@ -1117,7 +1117,7 @@ static const QRgba64 *QT_FASTCALL fetchTransformed64(QRgba64 *buffer, const Oper
 {
     const QPixelLayout *layout = &qPixelLayouts[data->texture.format];
     if (layout->bpp < QPixelLayout::BPP64) {
-        uint buffer32[BufferSize];
+        Q_DECL_UNINITIALIZED uint buffer32[BufferSize];
         Q_ASSERT(length <= BufferSize);
         if (layout->bpp == QPixelLayout::BPP32)
             fetchTransformed_fetcher<blendType, QPixelLayout::BPP32, uint>(buffer32, data, y, x, length);
@@ -1140,7 +1140,7 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedFP(QRgbaFloat32 *buffer,
 {
     const QPixelLayout *layout = &qPixelLayouts[data->texture.format];
     if (layout->bpp < QPixelLayout::BPP64) {
-        uint buffer32[BufferSize];
+        Q_DECL_UNINITIALIZED uint buffer32[BufferSize];
         Q_ASSERT(length <= BufferSize);
         if (layout->bpp == QPixelLayout::BPP32)
             fetchTransformed_fetcher<blendType, QPixelLayout::BPP32, uint>(buffer32, data, y, x, length);
@@ -1148,7 +1148,7 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedFP(QRgbaFloat32 *buffer,
             fetchTransformed_fetcher<blendType, QPixelLayout::BPPNone, uint>(buffer32, data, y, x, length);
         qConvertToRGBA32F[data->texture.format](buffer, buffer32, length, data->texture.colorTable, nullptr);
     } else if (layout->bpp < QPixelLayout::BPP32FPx4) {
-        quint64 buffer64[BufferSize];
+        Q_DECL_UNINITIALIZED quint64 buffer64[BufferSize];
         fetchTransformed_fetcher<blendType, QPixelLayout::BPP64, quint64>(buffer64, data, y, x, length);
         convert64ToRGBA32F[data->texture.format](buffer, buffer64, length);
     } else {
@@ -1376,7 +1376,7 @@ static void QT_FASTCALL fetchTransformedBilinearARGB32PM_simple_scale_helper(uin
     const int offset = (fx + adjust) >> 16;
     int x = offset;
 
-    IntermediateBuffer intermediate;
+    Q_DECL_UNINITIALIZED IntermediateBuffer intermediate;
     // count is the size used in the intermediate.buffer.
     int count = (qint64(length) * qAbs(fdx) + fixed_scale - 1) / fixed_scale + 2;
     // length is supposed to be <= BufferSize either because data->m11 < 1 or
@@ -2191,7 +2191,7 @@ static void QT_FASTCALL fetchTransformedBilinear_simple_scale_helper(uint *b, ui
     const int offset = (fx + adjust) >> 16;
     int x = offset;
 
-    IntermediateBuffer intermediate;
+    Q_DECL_UNINITIALIZED IntermediateBuffer intermediate;
     uint *buf1 = intermediate.buffer_rb;
     uint *buf2 = intermediate.buffer_ag;
     const uint *ptr1;
@@ -2508,8 +2508,8 @@ static const uint *QT_FASTCALL fetchTransformedBilinear(uint *buffer, const Oper
             } else {
                 const auto fetcher = fetchTransformedBilinear_fetcher<blendType,bpp,uint>;
 
-                uint buf1[BufferSize];
-                uint buf2[BufferSize];
+                Q_DECL_UNINITIALIZED uint buf1[BufferSize];
+                Q_DECL_UNINITIALIZED uint buf2[BufferSize];
                 uint *b = buffer;
                 while (length) {
                     int len = qMin(length, BufferSize / 2);
@@ -2543,8 +2543,8 @@ static const uint *QT_FASTCALL fetchTransformedBilinear(uint *buffer, const Oper
         } else { // rotation or shear
             const auto fetcher = fetchTransformedBilinear_fetcher<blendType,bpp,uint>;
 
-            uint buf1[BufferSize];
-            uint buf2[BufferSize];
+            Q_DECL_UNINITIALIZED uint buf1[BufferSize];
+            Q_DECL_UNINITIALIZED uint buf2[BufferSize];
             uint *b = buffer;
             while (length) {
                 int len = qMin(length, BufferSize / 2);
@@ -2594,12 +2594,12 @@ static const uint *QT_FASTCALL fetchTransformedBilinear(uint *buffer, const Oper
         qreal fy = data->m22 * cy + data->m12 * cx + data->dy;
         qreal fw = data->m23 * cy + data->m13 * cx + data->m33;
 
-        uint buf1[BufferSize];
-        uint buf2[BufferSize];
+        Q_DECL_UNINITIALIZED uint buf1[BufferSize];
+        Q_DECL_UNINITIALIZED uint buf2[BufferSize];
         uint *b = buffer;
 
-        ushort distxs[BufferSize / 2];
-        ushort distys[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distxs[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distys[BufferSize / 2];
 
         while (length) {
             const int len = qMin(length, BufferSize / 2);
@@ -2634,10 +2634,10 @@ static const QRgba64 *QT_FASTCALL fetchTransformedBilinear64_uint32(QRgba64 *buf
     const qreal cx = x + qreal(0.5);
     const qreal cy = y + qreal(0.5);
 
-    uint sbuf1[BufferSize];
-    uint sbuf2[BufferSize];
-    alignas(8) QRgba64 buf1[BufferSize];
-    alignas(8) QRgba64 buf2[BufferSize];
+    Q_DECL_UNINITIALIZED uint sbuf1[BufferSize];
+    Q_DECL_UNINITIALIZED uint sbuf2[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buf1[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buf2[BufferSize];
     QRgba64 *b = buffer;
 
     if (canUseFastMatrixPath(cx, cy, length, data)) {
@@ -2730,8 +2730,8 @@ static const QRgba64 *QT_FASTCALL fetchTransformedBilinear64_uint32(QRgba64 *buf
         qreal fy = data->m22 * cy + data->m12 * cx + data->dy;
         qreal fw = data->m23 * cy + data->m13 * cx + data->m33;
 
-        ushort distxs[BufferSize / 2];
-        ushort distys[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distxs[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distys[BufferSize / 2];
 
         while (length) {
             const int len = qMin(length, BufferSize / 2);
@@ -2762,8 +2762,8 @@ static const QRgba64 *QT_FASTCALL fetchTransformedBilinear64_uint64(QRgba64 *buf
     const qreal cx = x + qreal(0.5);
     const qreal cy = y + qreal(0.5);
 
-    alignas(8) QRgba64 buf1[BufferSize];
-    alignas(8) QRgba64 buf2[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buf1[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buf2[BufferSize];
     QRgba64 *end = buffer + length;
     QRgba64 *b = buffer;
 
@@ -2850,8 +2850,8 @@ static const QRgba64 *QT_FASTCALL fetchTransformedBilinear64_uint64(QRgba64 *buf
         qreal fy = data->m22 * cy + data->m12 * cx + data->dy;
         qreal fw = data->m23 * cy + data->m13 * cx + data->m33;
 
-        ushort distxs[BufferSize / 2];
-        ushort distys[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distxs[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distys[BufferSize / 2];
 
         while (length) {
             const int len = qMin(length, BufferSize / 2);
@@ -2884,10 +2884,10 @@ static const QRgba64 *QT_FASTCALL fetchTransformedBilinear64_f32x4(QRgba64 *buff
     const qreal cx = x + qreal(0.5);
     const qreal cy = y + qreal(0.5);
 
-    QRgbaFloat32 sbuf1[BufferSize];
-    QRgbaFloat32 sbuf2[BufferSize];
-    alignas(8) QRgba64 buf1[BufferSize];
-    alignas(8) QRgba64 buf2[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 sbuf1[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 sbuf2[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buf1[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buf2[BufferSize];
     QRgba64 *b = buffer;
 
     if (canUseFastMatrixPath(cx, cy, length, data)) {
@@ -2935,8 +2935,8 @@ static const QRgba64 *QT_FASTCALL fetchTransformedBilinear64_f32x4(QRgba64 *buff
         qreal fy = data->m22 * cy + data->m12 * cx + data->dy;
         qreal fw = data->m23 * cy + data->m13 * cx + data->m33;
 
-        ushort distxs[BufferSize / 2];
-        ushort distys[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distxs[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distys[BufferSize / 2];
 
         while (length) {
             const int len = qMin(length, BufferSize / 2);
@@ -3010,10 +3010,10 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedBilinearFP_uint32(QRgbaFl
     const qreal cx = x + qreal(0.5);
     const qreal cy = y + qreal(0.5);
 
-    uint sbuf1[BufferSize];
-    uint sbuf2[BufferSize];
-    QRgbaFloat32 buf1[BufferSize];
-    QRgbaFloat32 buf2[BufferSize];
+    Q_DECL_UNINITIALIZED uint sbuf1[BufferSize];
+    Q_DECL_UNINITIALIZED uint sbuf2[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 buf1[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 buf2[BufferSize];
     QRgbaFloat32 *b = buffer;
 
     if (canUseFastMatrixPath(cx, cy, length, data)) {
@@ -3058,8 +3058,8 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedBilinearFP_uint32(QRgbaFl
         qreal fx = data->m21 * cy + data->m11 * cx + data->dx;
         qreal fy = data->m22 * cy + data->m12 * cx + data->dy;
         qreal fw = data->m23 * cy + data->m13 * cx + data->m33;
-        ushort distxs[BufferSize / 2];
-        ushort distys[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distxs[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distys[BufferSize / 2];
 
         while (length) {
             const int len = qMin(length, BufferSize / 2);
@@ -3086,10 +3086,10 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedBilinearFP_uint64(QRgbaFl
     const qreal cx = x + qreal(0.5);
     const qreal cy = y + qreal(0.5);
 
-    quint64 sbuf1[BufferSize];
-    quint64 sbuf2[BufferSize];
-    QRgbaFloat32 buf1[BufferSize];
-    QRgbaFloat32 buf2[BufferSize];
+    Q_DECL_UNINITIALIZED quint64 sbuf1[BufferSize] ;
+    Q_DECL_UNINITIALIZED quint64 sbuf2[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 buf1[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 buf2[BufferSize];
     QRgbaFloat32 *b = buffer;
 
     if (canUseFastMatrixPath(cx, cy, length, data)) {
@@ -3129,8 +3129,8 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedBilinearFP_uint64(QRgbaFl
         qreal fy = data->m22 * cy + data->m12 * cx + data->dy;
         qreal fw = data->m23 * cy + data->m13 * cx + data->m33;
 
-        ushort distxs[BufferSize / 2];
-        ushort distys[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distxs[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distys[BufferSize / 2];
 
         while (length) {
             const int len = qMin(length, BufferSize / 2);
@@ -3158,8 +3158,8 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedBilinearFP(QRgbaFloat32 *
     const qreal cx = x + qreal(0.5);
     const qreal cy = y + qreal(0.5);
 
-    QRgbaFloat32 buf1[BufferSize];
-    QRgbaFloat32 buf2[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 buf1[BufferSize];
+    Q_DECL_UNINITIALIZED QRgbaFloat32 buf2[BufferSize];
     QRgbaFloat32 *b = buffer;
 
     if (canUseFastMatrixPath(cx, cy, length, data)) {
@@ -3199,8 +3199,8 @@ static const QRgbaFloat32 *QT_FASTCALL fetchTransformedBilinearFP(QRgbaFloat32 *
         qreal fy = data->m22 * cy + data->m12 * cx + data->dy;
         qreal fw = data->m23 * cy + data->m13 * cx + data->m33;
 
-        ushort distxs[BufferSize / 2];
-        ushort distys[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distxs[BufferSize / 2];
+        Q_DECL_UNINITIALIZED ushort distys[BufferSize / 2];
 
         while (length) {
             const int len = qMin(length, BufferSize / 2);
@@ -3995,7 +3995,7 @@ static void blend_color_generic(int count, const QT_FT_Span *spans, void *userDa
     const QPixelLayout::BPP bpp = qPixelLayouts[data->rasterBuffer->format].bpp;
 
     auto function = [=] (int cStart, int cEnd) {
-        alignas(16) uint buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED uint buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             int x = spans[c].x;
             int length = spans[c].len;
@@ -4073,7 +4073,7 @@ static void blend_color_generic_rgb64(int count, const QT_FT_Span *spans, void *
 
     auto function = [=, &op] (int cStart, int cEnd)
     {
-        alignas(16) QRgba64 buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgba64 buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             int x = spans[c].x;
             int length = spans[c].len;
@@ -4119,7 +4119,7 @@ static void blend_color_generic_fp(int count, const QT_FT_Span *spans, void *use
 
     auto function = [=, &op] (int cStart, int cEnd)
     {
-        alignas(16) QRgbaFloat32 buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgbaFloat32 buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             int x = spans[c].x;
             int length = spans[c].len;
@@ -4155,7 +4155,7 @@ void handleSpans(int count, const QT_FT_Span *spans, const QSpanData *data, cons
 
     auto function = [=, &op] (int cStart, int cEnd)
     {
-        T handler(data, op);
+        T Q_DECL_UNINITIALIZED handler(data, op);
         int coverage = 0;
         for (int c = cStart; c < cEnd;) {
             if (!spans[c].len) {
@@ -4370,8 +4370,8 @@ static void blend_untransformed_generic(int count, const QT_FT_Span *spans, void
 
     auto function = [=, &op] (int cStart, int cEnd)
     {
-        alignas(16) uint buffer[BufferSize];
-        alignas(16) uint src_buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED uint buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED uint src_buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             if (!spans[c].len)
                 continue;
@@ -4428,8 +4428,8 @@ static void blend_untransformed_generic_rgb64(int count, const QT_FT_Span *spans
 
     auto function = [=, &op] (int cStart, int cEnd)
     {
-        alignas(16) QRgba64 buffer[BufferSize];
-        alignas(16) QRgba64 src_buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgba64 buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgba64 src_buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             if (!spans[c].len)
                 continue;
@@ -4486,8 +4486,8 @@ static void blend_untransformed_generic_fp(int count, const QT_FT_Span *spans, v
 
     auto function = [=, &op] (int cStart, int cEnd)
     {
-        alignas(16) QRgbaFloat32 buffer[BufferSize];
-        alignas(16) QRgbaFloat32 src_buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgbaFloat32 buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgbaFloat32 src_buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             if (!spans[c].len)
                 continue;
@@ -4698,8 +4698,8 @@ static void blend_tiled_generic(int count, const QT_FT_Span *spans, void *userDa
 
     auto function = [=, &op](int cStart, int cEnd)
     {
-        alignas(16) uint buffer[BufferSize];
-        alignas(16) uint src_buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED uint buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED uint src_buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             int x = spans[c].x;
             int length = spans[c].len;
@@ -4755,7 +4755,7 @@ static void blend_tiled_generic_rgb64(int count, const QT_FT_Span *spans, void *
     bool isBpp32 = qPixelLayouts[data->rasterBuffer->format].bpp == QPixelLayout::BPP32;
     bool isBpp64 = qPixelLayouts[data->rasterBuffer->format].bpp == QPixelLayout::BPP64;
     if (op.destFetch64 == destFetch64Undefined && image_width <= BufferSize && (isBpp32 || isBpp64)) {
-        alignas(16) QRgba64 src_buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgba64 src_buffer[BufferSize];
         // If destination isn't blended into the result, we can do the tiling directly on destination pixels.
         while (count--) {
             int x = spans->x;
@@ -4805,8 +4805,8 @@ static void blend_tiled_generic_rgb64(int count, const QT_FT_Span *spans, void *
 
     auto function = [=, &op](int cStart, int cEnd)
     {
-        alignas(16) QRgba64 buffer[BufferSize];
-        alignas(16) QRgba64 src_buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgba64 buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgba64 src_buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             int x = spans[c].x;
             int length = spans[c].len;
@@ -4864,8 +4864,8 @@ static void blend_tiled_generic_fp(int count, const QT_FT_Span *spans, void *use
 
     auto function = [=, &op](int cStart, int cEnd)
     {
-        alignas(16) QRgbaFloat32 buffer[BufferSize];
-        alignas(16) QRgbaFloat32 src_buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgbaFloat32 buffer[BufferSize];
+        alignas(16) Q_DECL_UNINITIALIZED QRgbaFloat32 src_buffer[BufferSize];
         for (int c = cStart; c < cEnd; ++c) {
             int x = spans[c].x;
             int length = spans[c].len;
@@ -5498,7 +5498,7 @@ static void qt_alphamapblit_generic(QRasterBuffer *rasterBuffer,
     if (colorProfile && color.isOpaque())
         srcColor = colorProfile->toLinear(srcColor);
 
-    alignas(8) QRgba64 buffer[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buffer[BufferSize];
     const DestFetchProc64 destFetch64 = destFetchProc64[rasterBuffer->format];
     const DestStoreProc64 destStore64 = destStoreProc64[rasterBuffer->format];
 
@@ -5768,7 +5768,7 @@ static void qt_alphamapblit_nonpremul_argb32(QRasterBuffer *rasterBuffer,
     if (colorProfile && color.isOpaque())
         srcColor = colorProfile->toLinear(srcColor);
 
-    alignas(8) QRgba64 buffer[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buffer[BufferSize];
     const DestFetchProc64 destFetch64 = destFetchProc64[rasterBuffer->format];
     const DestStoreProc64 destStore64 = destStoreProc64[rasterBuffer->format];
 
@@ -5933,7 +5933,7 @@ static void qt_alphargbblit_generic(QRasterBuffer *rasterBuffer,
     if (colorProfile && color.isOpaque())
         srcColor = colorProfile->toLinear(srcColor);
 
-    alignas(8) QRgba64 buffer[BufferSize];
+    alignas(8) Q_DECL_UNINITIALIZED QRgba64 buffer[BufferSize];
     const DestFetchProc64 destFetch64 = destFetchProc64[rasterBuffer->format];
     const DestStoreProc64 destStore64 = destStoreProc64[rasterBuffer->format];
 
@@ -6006,7 +6006,7 @@ static void qt_alphargbblit_generic(QRasterBuffer *rasterBuffer,
     if (colorProfile && color.isOpaque())
         srcColor = colorProfile->toLinear(srcColor);
 
-    quint32 buffer[BufferSize];
+    Q_DECL_UNINITIALIZED quint32 buffer[BufferSize];
     const DestFetchProc destFetch = destFetchProc[rasterBuffer->format];
     const DestStoreProc destStore = destStoreProc[rasterBuffer->format];
 
diff --git a/src/gui/painting/qdrawhelper_avx2.cpp b/src/gui/painting/qdrawhelper_avx2.cpp
index 34de69ecf45..967abb8b66e 100644
--- a/src/gui/painting/qdrawhelper_avx2.cpp
+++ b/src/gui/painting/qdrawhelper_avx2.cpp
@@ -763,7 +763,7 @@ void QT_FASTCALL fetchTransformedBilinearARGB32PM_simple_scale_helper_avx2(uint
     const int offset = (fx + adjust) >> 16;
     int x = offset;
 
-    IntermediateBuffer intermediate;
+    Q_DECL_UNINITIALIZED IntermediateBuffer intermediate;
     // count is the size used in the intermediate_buffer.
     int count = (qint64(length) * qAbs(fdx) + FixedScale - 1) / FixedScale + 2;
     // length is supposed to be <= BufferSize either because data->m11 < 1 or
diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 64e056a65dc..92f21d3aaf4 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -1410,7 +1410,7 @@ static void fillRect_normalized(const QRect &r, QSpanData *data,
     ProcessSpans blend = isUnclipped ? data->unclipped_blend : data->blend;
 
     const int nspans = 512;
-    QT_FT_Span spans[nspans];
+    Q_DECL_UNINITIALIZED QT_FT_Span spans[nspans];
 
     Q_ASSERT(data->blend);
     int y = y1;
@@ -3590,7 +3590,7 @@ void QRasterPaintEnginePrivate::rasterize(QT_FT_Outline *outline,
     // raster pool is changed for lower value, reallocations will
     // occur normally.
     int rasterPoolSize = MINIMUM_POOL_SIZE;
-    uchar rasterPoolOnStack[MINIMUM_POOL_SIZE + 0xf];
+    Q_DECL_UNINITIALIZED uchar rasterPoolOnStack[MINIMUM_POOL_SIZE + 0xf];
     uchar *rasterPoolBase = alignAddress(rasterPoolOnStack, 0xf);
     uchar *rasterPoolOnHeap = nullptr;
 
@@ -4080,7 +4080,7 @@ static void qt_span_fill_clipped(int spanCount, const QT_FT_Span *spans, void *u
     Q_ASSERT(fillData->blend && fillData->unclipped_blend);
 
     const int NSPANS = 512;
-    QT_FT_Span cspans[NSPANS];
+    Q_DECL_UNINITIALIZED QT_FT_Span cspans[NSPANS];
     int currentClip = 0;
     const QT_FT_Span *end = spans + spanCount;
     while (spans < end) {
diff --git a/src/gui/painting/qpainter.cpp b/src/gui/painting/qpainter.cpp
index 45ea98e5373..4e8e1d65ecf 100644
--- a/src/gui/painting/qpainter.cpp
+++ b/src/gui/painting/qpainter.cpp
@@ -5628,7 +5628,7 @@ void QPainter::drawText(const QPointF &p, const QString &str, int tf, int justif
     if (!d->engine || str.isEmpty() || pen().style() == Qt::NoPen)
         return;
 
-    QStackTextEngine engine(str, d->state->font);
+    Q_DECL_UNINITIALIZED QStackTextEngine engine(str, d->state->font);
     engine.option.setTextDirection(d->state->layoutDirection);
     if (tf & (Qt::TextForceLeftToRight|Qt::TextForceRightToLeft)) {
         engine.ignoreBidi = true;
@@ -7260,7 +7260,7 @@ start_lengthVariant:
     qreal width = 0;
 
     QString finalText = text.mid(old_offset, length);
-    QStackTextEngine engine(finalText, fnt);
+    Q_DECL_UNINITIALIZED QStackTextEngine engine(finalText, fnt);
     if (option) {
         engine.option = *option;
     }
diff --git a/src/gui/painting/qpixellayout.cpp b/src/gui/painting/qpixellayout.cpp
index 6c21218f151..cfe26d6c5d7 100644
--- a/src/gui/painting/qpixellayout.cpp
+++ b/src/gui/painting/qpixellayout.cpp
@@ -2057,7 +2057,7 @@ template<QImage::Format format>
 static void QT_FASTCALL storeGenericFromRGBA64PM(uchar *dest, const QRgba64 *src, int index, int count,
                                                  const QList<QRgb> *clut, QDitherInfo *dither)
 {
-    uint buffer[BufferSize];
+    Q_DECL_UNINITIALIZED uint buffer[BufferSize];
     convertFromRgb64(buffer, src, count);
     qPixelLayouts[format].storeFromARGB32PM(dest, buffer, index, count, clut, dither);
 }
@@ -2242,7 +2242,7 @@ template<QImage::Format format>
 static const QRgbaFloat32 * QT_FASTCALL convertGenericToRGBA32F(QRgbaFloat32 *buffer, const uint *src, int count,
                                                             const QList<QRgb> *clut, QDitherInfo *)
 {
-    uint buffer32[BufferSize];
+    Q_DECL_UNINITIALIZED uint buffer32[BufferSize];
     memcpy(buffer32, src, count * sizeof(uint));
     qPixelLayouts[format].convertToARGB32PM(buffer32, count, clut);
     convertToRgbaF32(buffer, buffer32, count);
@@ -2453,7 +2453,7 @@ template<QImage::Format format>
 static void QT_FASTCALL storeGenericFromRGBA32F(uchar *dest, const QRgbaFloat32 *src, int index, int count,
                                                 const QList<QRgb> *clut, QDitherInfo *dither)
 {
-    uint buffer[BufferSize];
+    Q_DECL_UNINITIALIZED uint buffer[BufferSize];
     convertFromRgba32f(buffer, src, count);
     qPixelLayouts[format].storeFromARGB32PM(dest, buffer, index, count, clut, dither);
 }
diff --git a/src/gui/text/qfontmetrics.cpp b/src/gui/text/qfontmetrics.cpp
index 52a4d6995cf..c02c81f18e2 100644
--- a/src/gui/text/qfontmetrics.cpp
+++ b/src/gui/text/qfontmetrics.cpp
@@ -507,7 +507,7 @@ int QFontMetrics::horizontalAdvance(const QString &text, int len) const
     if (len == 0)
         return 0;
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     return qRound(layout.width(0, len));
 }
 
@@ -533,7 +533,7 @@ int QFontMetrics::horizontalAdvance(const QString &text, const QTextOption &opti
     if (len == 0)
         return 0;
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.option = option;
     return qRound(layout.width(0, len));
 }
@@ -618,7 +618,7 @@ QRect QFontMetrics::boundingRect(const QString &text) const
     if (text.size() == 0)
         return QRect();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.itemize();
     glyph_metrics_t gm = layout.boundingBox(0, text.size());
     return QRect(qRound(gm.x), qRound(gm.y), qRound(gm.width), qRound(gm.height));
@@ -653,7 +653,7 @@ QRect QFontMetrics::boundingRect(const QString &text, const QTextOption &option)
     if (text.size() == 0)
         return QRect();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.option = option;
     layout.itemize();
     glyph_metrics_t gm = layout.boundingBox(0, text.size());
@@ -821,7 +821,7 @@ QRect QFontMetrics::tightBoundingRect(const QString &text) const
     if (text.size() == 0)
         return QRect();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.itemize();
     glyph_metrics_t gm = layout.tightBoundingBox(0, text.size());
     return QRect(qRound(gm.x), qRound(gm.y), qRound(gm.width), qRound(gm.height));
@@ -853,7 +853,7 @@ QRect QFontMetrics::tightBoundingRect(const QString &text, const QTextOption &op
     if (text.size() == 0)
         return QRect();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.option = option;
     layout.itemize();
     glyph_metrics_t gm = layout.tightBoundingBox(0, text.size());
@@ -897,7 +897,7 @@ QString QFontMetrics::elidedText(const QString &text, Qt::TextElideMode mode, in
         }
         _text = _text.mid(posA);
     }
-    QStackTextEngine engine(_text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine engine(_text, QFont(d.data()));
     return engine.elidedText(mode, width, flags);
 }
 
@@ -1419,7 +1419,7 @@ qreal QFontMetricsF::horizontalAdvance(const QString &text, int length) const
     if (length == 0)
         return 0;
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.itemize();
     return layout.width(0, length).toReal();
 }
@@ -1446,7 +1446,7 @@ qreal QFontMetricsF::horizontalAdvance(const QString &text, const QTextOption &o
     if (length == 0)
         return 0;
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.option = option;
     layout.itemize();
     return layout.width(0, length).toReal();
@@ -1532,7 +1532,7 @@ QRectF QFontMetricsF::boundingRect(const QString &text) const
     if (len == 0)
         return QRectF();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.itemize();
     glyph_metrics_t gm = layout.boundingBox(0, len);
     return QRectF(gm.x.toReal(), gm.y.toReal(),
@@ -1566,7 +1566,7 @@ QRectF QFontMetricsF::boundingRect(const QString &text, const QTextOption &optio
     if (text.size() == 0)
         return QRectF();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.option = option;
     layout.itemize();
     glyph_metrics_t gm = layout.boundingBox(0, text.size());
@@ -1740,7 +1740,7 @@ QRectF QFontMetricsF::tightBoundingRect(const QString &text) const
     if (text.size() == 0)
         return QRectF();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.itemize();
     glyph_metrics_t gm = layout.tightBoundingBox(0, text.size());
     return QRectF(gm.x.toReal(), gm.y.toReal(), gm.width.toReal(), gm.height.toReal());
@@ -1772,7 +1772,7 @@ QRectF QFontMetricsF::tightBoundingRect(const QString &text, const QTextOption &
     if (text.size() == 0)
         return QRectF();
 
-    QStackTextEngine layout(text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine layout(text, QFont(d.data()));
     layout.option = option;
     layout.itemize();
     glyph_metrics_t gm = layout.tightBoundingBox(0, text.size());
@@ -1815,7 +1815,7 @@ QString QFontMetricsF::elidedText(const QString &text, Qt::TextElideMode mode, q
         }
         _text = _text.mid(posA);
     }
-    QStackTextEngine engine(_text, QFont(d.data()));
+    Q_DECL_UNINITIALIZED QStackTextEngine engine(_text, QFont(d.data()));
     return engine.elidedText(mode, QFixed::fromReal(width), flags);
 }
 
diff --git a/src/testlib/qtestcrashhandler.cpp b/src/testlib/qtestcrashhandler.cpp
index 42a715ff865..2a62044b14e 100644
--- a/src/testlib/qtestcrashhandler.cpp
+++ b/src/testlib/qtestcrashhandler.cpp
@@ -478,7 +478,7 @@ LONG WINAPI WindowsFaultHandler::windowsFaultHandler(struct _EXCEPTION_POINTERS
             fprintf(stderr, "Nearby symbol    : %s\n", exceptionSymbol.name);
             delete [] exceptionSymbol.name;
         }
-        void *stack[maxStackFrames];
+        Q_DECL_UNINITIALIZED void *stack[maxStackFrames];
         fputs("\nStack:\n", stderr);
         const unsigned frameCount = CaptureStackBackTrace(0, DWORD(maxStackFrames), stack, NULL);
         for (unsigned f = 0; f < frameCount; ++f)     {
diff --git a/src/testlib/qtestresult.cpp b/src/testlib/qtestresult.cpp
index b770b5a1f26..4a6fb5918d7 100644
--- a/src/testlib/qtestresult.cpp
+++ b/src/testlib/qtestresult.cpp
@@ -302,7 +302,7 @@ bool QTestResult::verify(bool statement, const char *statementStr,
 {
     QTEST_ASSERT(statementStr);
 
-    char msg[maxMsgLen];
+    Q_DECL_UNINITIALIZED char msg[maxMsgLen];
     msg[0] = '\0';
 
     if (QTestLog::verboseLevel() >= 2) {
@@ -431,7 +431,7 @@ static bool compareHelper(bool success, const char *failureMsg,
                           const char *file, int line,
                           bool hasValues = true)
 {
-    char msg[maxMsgLen];
+    Q_DECL_UNINITIALIZED char msg[maxMsgLen];
     msg[0] = '\0';
 
     QTEST_ASSERT(expected);
@@ -473,7 +473,7 @@ static bool compareHelper(bool success, const char *failureMsg,
                           const char *file, int line)
 {
     const size_t maxMsgLen = 1024;
-    char msg[maxMsgLen];
+    Q_DECL_UNINITIALIZED char msg[maxMsgLen];
     msg[0] = '\0';
 
     QTEST_ASSERT(expected);
@@ -695,7 +695,7 @@ bool QTestResult::reportResult(bool success, const void *lhs, const void *rhs,
                                QTest::ComparisonOperation op, const char *file, int line,
                                const char *failureMessage)
 {
-    char msg[maxMsgLen];
+    Q_DECL_UNINITIALIZED char msg[maxMsgLen];
     msg[0] = '\0';
 
     QTEST_ASSERT(lhsExpr);
diff --git a/src/widgets/graphicsview/qgraphicsitem.cpp b/src/widgets/graphicsview/qgraphicsitem.cpp
index 8c7b349012b..b55bea7e305 100644
--- a/src/widgets/graphicsview/qgraphicsitem.cpp
+++ b/src/widgets/graphicsview/qgraphicsitem.cpp
@@ -10515,7 +10515,7 @@ void QGraphicsSimpleTextItemPrivate::updateBoundingRect()
     } else {
         QString tmp = text;
         tmp.replace(u'\n', QChar::LineSeparator);
-        QStackTextEngine engine(tmp, font);
+        Q_DECL_UNINITIALIZED QStackTextEngine engine(tmp, font);
         QTextLayout layout(&engine);
         br = setupTextLayout(&layout);
     }
@@ -10676,7 +10676,7 @@ void QGraphicsSimpleTextItem::paint(QPainter *painter, const QStyleOptionGraphic
 
     QString tmp = d->text;
     tmp.replace(u'\n', QChar::LineSeparator);
-    QStackTextEngine engine(tmp, d->font);
+    Q_DECL_UNINITIALIZED QStackTextEngine engine(tmp, d->font);
     QTextLayout layout(&engine);
 
     QPen p;
diff --git a/src/widgets/styles/qcommonstyle.cpp b/src/widgets/styles/qcommonstyle.cpp
index d1f17ff2832..7980d1d2105 100644
--- a/src/widgets/styles/qcommonstyle.cpp
+++ b/src/widgets/styles/qcommonstyle.cpp
@@ -938,7 +938,7 @@ QString QCommonStylePrivate::calculateElidedText(const QString &text, const QTex
                     text.chop(1);
                 text += QChar(0x2026);
             }
-            const QStackTextEngine engine(text, font);
+            Q_DECL_UNINITIALIZED const QStackTextEngine engine(text, font);
             ret += engine.elidedText(textElideMode, textRect.width(), flags);
 
             // no newline for the last line (last visible or real)