Add a patch to use _Float16 only when SSE2 is enabled.

Closes: #1076986.

debian/changelog

@@ -1,5 +1,7 @@
 qt6-base (6.6.2+dfsg-11) UNRELEASED; urgency=medium
 
+  [ Dmitry Shachnev ]
+  * Add a patch to use _Float16 only when SSE2 is enabled (closes: #1076986).
 
  -- Debian Qt/KDE Maintainers <debian-qt-kde@lists.debian.org>  Thu, 01 Aug 2024 23:45:39 +0300
 

debian/patches/series

@@ -8,6 +8,7 @@ Add-SH-detection.patch
 
 # Pushed to gerrit
 be_verbose_on_plugin_inclusion.patch
+use_float16_only_with_sse2.patch
 
 # Debian specific
 remove_privacy_breaches.diff

debian/patches/use_float16_only_with_sse2.patch

@@ -0,0 +1,39 @@
+Description: use _Float16 only when SSE2 is enabled
+ The GCC documentation [1] says: “On x86 targets with SSE2 enabled, GCC
+ supports half-precision (16-bit) floating point via the _Float16 type”.
+ .
+ On non-SSE2 x86 (such as Debian i386 baseline [2]), __FLT16_MAX__ is
+ defined starting with GCC 14 [3], however any non-trivial use of the
+ _Float16 type results in an error:
+ .
+ error: operation not permitted on type ‘_Float16’ without option ‘-msse2’
+ .
+ which makes some packages fail to build on i386 architecture [4].
+ .
+ [1]: https://gcc.gnu.org/onlinedocs/gcc/Half-Precision.html
+ [2]: https://wiki.debian.org/ArchitectureSpecificsMemo#i386-1
+ [3]: https://gcc.gnu.org/g:9a19fa8b616f83474c35cc5b34a3865073ced829
+ [4]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1076986
+Author: Dmitry Shachnev <mitya57@debian.org>
+Forwarded: https://codereview.qt-project.org/c/qt/qtbase/+/579205
+Last-Update: 2024-08-01
+
+--- a/src/corelib/global/qtypes.h
++++ b/src/corelib/global/qtypes.h
+@@ -263,13 +263,12 @@ using NativeFloat16Type = std::float16_t
+ // disabled due to https://github.com/llvm/llvm-project/issues/56963
+ #  define QFLOAT16_IS_NATIVE        1
+ using NativeFloat16Type = decltype(__FLT16_MAX__);
+-#elif defined(Q_CC_GNU_ONLY) && defined(__FLT16_MAX__)
++#elif defined(Q_CC_GNU_ONLY) && defined(__FLT16_MAX__) && defined(__ARM_FP16_FORMAT_IEEE)
+ #  define QFLOAT16_IS_NATIVE        1
+-#  ifdef __ARM_FP16_FORMAT_IEEE
+ using NativeFloat16Type = __fp16;
+-#  else
++#elif defined(Q_CC_GNU_ONLY) && defined(__FLT16_MAX__) && defined(__SSE2__)
++#  define QFLOAT16_IS_NATIVE        1
+ using NativeFloat16Type = _Float16;
+-#  endif
+ #else
+ #  define QFLOAT16_IS_NATIVE        0
+ using NativeFloat16Type = void;