commit 2a8f7dc6ddfc414a66491522501c1574a1343ee1
Author: Andrew Chow <achow101-github@achow101.com>
Date:   Sat Nov 21 01:11:04 2020 -0500

    build: Fix determinism issue when building with Clang 8

    When building Qt with LLVM/Clang 8 under -O3 (the default), we run into
    a determinism issue in `qt_interset_spans`. The issue has been fixed for
    LLVM/Clang 9, see
    https://github.com/llvm/llvm-project/commit/db101864bdc938deb1d63fe4f7da761bd38e5cae
    and https://reviews.llvm.org/D64601, however this fix was not backported
    to 8.x. Once LLVM/Clang 9 is used, this patch can be dropped.

    The particular issue appears to be an optimization done by -O3 which
    adds a temporary variable for `spans->y` in `qt_intersect_spans`. When
    it does this, sometimes it chooses to use a 32-bit movs instruction
    (movswl), and other times it chooses a 64-bit movs instruction (movswq).
    By patching `qt_intersect_spans` to always make a temporary variable for
    `spans->y`, we are able to sidestep this problem.

diff --git a/qtbase/src/gui/painting/qpaintengine_raster.cpp b/qtbase/src/gui/painting/qpaintengine_raster.cpp
index 92ab6e8375..f018009e0b 100644
--- a/qtbase/src/gui/painting/qpaintengine_raster.cpp
+++ b/qtbase/src/gui/painting/qpaintengine_raster.cpp
@@ -3971,22 +3971,23 @@ static const QSpan *qt_intersect_spans(const QClipData *clip, int *currentClip,
     const QSpan *clipEnd = clip->m_spans + clip->count;
 
     while (available && spans < end ) {
+        const short spans_y = spans->y;
         if (clipSpans >= clipEnd) {
             spans = end;
             break;
         }
-        if (clipSpans->y > spans->y) {
+        if (clipSpans->y > spans_y) {
             ++spans;
             continue;
         }
-        if (spans->y != clipSpans->y) {
-            if (spans->y < clip->count && clip->m_clipLines[spans->y].spans)
-                clipSpans = clip->m_clipLines[spans->y].spans;
+        if (spans_y != clipSpans->y) {
+            if (spans_y < clip->count && clip->m_clipLines[spans_y].spans)
+                clipSpans = clip->m_clipLines[spans_y].spans;
             else
                 ++clipSpans;
             continue;
         }
-        Q_ASSERT(spans->y == clipSpans->y);
+        Q_ASSERT(spans_y == clipSpans->y);
 
         int sx1 = spans->x;
         int sx2 = sx1 + spans->len;
@@ -4005,7 +4006,7 @@ static const QSpan *qt_intersect_spans(const QClipData *clip, int *currentClip,
         if (len) {
             out->x = qMax(sx1, cx1);
             out->len = qMin(sx2, cx2) - out->x;
-            out->y = spans->y;
+            out->y = spans_y;
             out->coverage = qt_div_255(spans->coverage * clipSpans->coverage);
             ++out;
             --available;