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Merge #20479: Fix QPainter non-determinism on macOS (0.21 backport)

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ab23a83400 Fix QPainter non-determinism on macOS (Andrew Chow)

Pull request description:

  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

ACKs for top commit:
  jonasschnelli:
    codereview ACK ab23a83400
  achow101:
    ACK ab23a83400

Tree-SHA512: 10991fe2b5452b1393678c315281cfdca011e9bb2cd8094a002746e690890ace148ac2dbf39c5fbe5e7f4cd39eeebfa0a715c065cff150cf70e9733cb0ff32d6
This commit is contained in:
fanquake 2020-11-25 09:50:27 +08:00
parent 3fae2f95f1 ab23a83400
commit 17294c1820
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2 changed files with 65 additions and 0 deletions
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2
depends/packages/qt.mk
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@ -12,6 +12,7 @@ $(package)_patches=fix_qt_pkgconfig.patch mac-qmake.conf fix_configure_mac.patch
$(package)_patches+= fix_rcc_determinism.patch fix_riscv64_arch.patch xkb-default.patch no-xlib.patch
$(package)_patches+= fix_android_qmake_conf.patch fix_android_jni_static.patch dont_hardcode_pwd.patch
$(package)_patches+= freetype_back_compat.patch drop_lrelease_dependency.patch fix_powerpc_libpng.patch
$(package)_patches+= fix_qpainter_non_determinism.patch
# Update OSX_QT_TRANSLATIONS when this is updated
$(package)_qttranslations_file_name=qttranslations-$($(package)_suffix)
@ -195,6 +196,7 @@ endef
define $(package)_preprocess_cmds
patch -p1 -i $($(package)_patch_dir)/freetype_back_compat.patch && \
patch -p1 -i $($(package)_patch_dir)/fix_powerpc_libpng.patch && \
patch -p1 -i $($(package)_patch_dir)/fix_qpainter_non_determinism.patch &&\
sed -i.old "s|updateqm.commands = \$$$$\$$$$LRELEASE|updateqm.commands = $($(package)_extract_dir)/qttools/bin/lrelease|" qttranslations/translations/translations.pro && \
patch -p1 -i $($(package)_patch_dir)/drop_lrelease_dependency.patch && \
patch -p1 -i $($(package)_patch_dir)/dont_hardcode_pwd.patch &&\

63
depends/patches/qt/fix_qpainter_non_determinism.patch Normal file
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@ -0,0 +1,63 @@
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;