Merge #20447: depends: Patch qt_intersect_spans to avoid non-deterministic behavior in LLVM 8

8f7d1b39efbe65ab2747c593cc3560d4a449a333 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.

  Potential alternative to #20436 and #20440

ACKs for top commit:
  hebasto:
    re-ACK 8f7d1b39efbe65ab2747c593cc3560d4a449a333 ~for merging into the 0.21 branch, but [not into the master](https://github.com/bitcoin/bitcoin/pull/20454) branch.~
  fanquake:
    ACK 8f7d1b39efbe65ab2747c593cc3560d4a449a333

Tree-SHA512: b0d00a77643554021736524fb64611462ef2ec849a220543c12d99edb0f52f2e8128d2cc61fa82176b7e13b294574774a92d6b649badf8b7630c6d6a7e70ce10

2 files changed, 65 insertions, 1 deletions

diff --git a/depends/packages/qt.mk b/depends/packages/qt.mk
index 6c77dca8d5..6b062d4515 100644
--- a/depends/packages/qt.mk
+++ b/depends/packages/qt.mk
@@ -12,7 +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_mingw_cross_compile.patch
+$(package)_patches+= fix_mingw_cross_compile.patch fix_qpainter_non_determinism.patch
 
 # Update OSX_QT_TRANSLATIONS when this is updated
 $(package)_qttranslations_file_name=qttranslations-$($(package)_suffix)
@@ -231,6 +231,7 @@ define $(package)_preprocess_cmds
   patch -p1 -i $($(package)_patch_dir)/fix_riscv64_arch.patch && \
   patch -p1 -i $($(package)_patch_dir)/no-xlib.patch && \
   patch -p1 -i $($(package)_patch_dir)/fix_mingw_cross_compile.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 && \
   mkdir -p qtbase/mkspecs/macx-clang-linux &&\
   cp -f qtbase/mkspecs/macx-clang/qplatformdefs.h qtbase/mkspecs/macx-clang-linux/ &&\
diff --git a/depends/patches/qt/fix_qpainter_non_determinism.patch b/depends/patches/qt/fix_qpainter_non_determinism.patch
new file mode 100644
index 0000000000..3cfcc22f03
--- /dev/null
+++ b/depends/patches/qt/fix_qpainter_non_determinism.patch
@@ -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;
+