ab23a83400
Aplies a patch to Qt that fixes the non-determinism by modifying Qt. The source of the non-determinism is how LLVM 8 optimizes qt_intersect_spans when compiling. The particular optimization that seems to be causing the problems is that a temp variable is being added for spans->y. For some reason, when it does this, it chooses different instructions to use when making that variable. We bypass this problem by patching qt_intersect_spans to always make and use this local variable. Github-Pull: #20447 Rebased-From: 8f7d1b39efbe65ab2747c593cc3560d4a449a333 Tree-SHA512: 558da5c2bb0373e2a89f2c219170f802036e0e87cc8e808336b23d074152cb893007a440f46ec957156b0921355cd18502710f2d224f27bc26e934c50ebebc41
64 lines
2.7 KiB
Diff
64 lines
2.7 KiB
Diff
commit 2a8f7dc6ddfc414a66491522501c1574a1343ee1
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Author: Andrew Chow <achow101-github@achow101.com>
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Date: Sat Nov 21 01:11:04 2020 -0500
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build: Fix determinism issue when building with Clang 8
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When building Qt with LLVM/Clang 8 under -O3 (the default), we run into
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a determinism issue in `qt_interset_spans`. The issue has been fixed for
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LLVM/Clang 9, see
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https://github.com/llvm/llvm-project/commit/db101864bdc938deb1d63fe4f7da761bd38e5cae
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and https://reviews.llvm.org/D64601, however this fix was not backported
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to 8.x. Once LLVM/Clang 9 is used, this patch can be dropped.
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The particular issue appears to be an optimization done by -O3 which
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adds a temporary variable for `spans->y` in `qt_intersect_spans`. When
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it does this, sometimes it chooses to use a 32-bit movs instruction
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(movswl), and other times it chooses a 64-bit movs instruction (movswq).
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By patching `qt_intersect_spans` to always make a temporary variable for
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`spans->y`, we are able to sidestep this problem.
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diff --git a/qtbase/src/gui/painting/qpaintengine_raster.cpp b/qtbase/src/gui/painting/qpaintengine_raster.cpp
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index 92ab6e8375..f018009e0b 100644
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--- a/qtbase/src/gui/painting/qpaintengine_raster.cpp
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+++ b/qtbase/src/gui/painting/qpaintengine_raster.cpp
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@@ -3971,22 +3971,23 @@ static const QSpan *qt_intersect_spans(const QClipData *clip, int *currentClip,
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const QSpan *clipEnd = clip->m_spans + clip->count;
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while (available && spans < end ) {
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+ const short spans_y = spans->y;
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if (clipSpans >= clipEnd) {
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spans = end;
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break;
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}
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- if (clipSpans->y > spans->y) {
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+ if (clipSpans->y > spans_y) {
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++spans;
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continue;
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}
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- if (spans->y != clipSpans->y) {
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- if (spans->y < clip->count && clip->m_clipLines[spans->y].spans)
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- clipSpans = clip->m_clipLines[spans->y].spans;
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+ if (spans_y != clipSpans->y) {
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+ if (spans_y < clip->count && clip->m_clipLines[spans_y].spans)
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+ clipSpans = clip->m_clipLines[spans_y].spans;
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else
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++clipSpans;
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continue;
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}
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- Q_ASSERT(spans->y == clipSpans->y);
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+ Q_ASSERT(spans_y == clipSpans->y);
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int sx1 = spans->x;
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int sx2 = sx1 + spans->len;
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@@ -4005,7 +4006,7 @@ static const QSpan *qt_intersect_spans(const QClipData *clip, int *currentClip,
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if (len) {
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out->x = qMax(sx1, cx1);
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out->len = qMin(sx2, cx2) - out->x;
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- out->y = spans->y;
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+ out->y = spans_y;
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out->coverage = qt_div_255(spans->coverage * clipSpans->coverage);
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++out;
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--available;
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