qt6-build.eclass: use different workaround for cpu flags issues

This also replaces the nasty workaround from qtbase's ebuild
on top of the function here.

Should "hopefully" be far less error prone, while still allowing
-march=native for people who aren't affected. Does mean slightly
worse optimizations for those affected, but this still tries
to use the highest x86-64-v* and should be insignificant.

Tentative, can't fully test without having an affected cpu so
may still need work.

Closes: https://bugs.gentoo.org/933374
Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

eclass/qt6-build.eclass

@@ -122,7 +122,7 @@ qt6-build_src_prepare() {
 	fi
 
 	_qt6-build_prepare_env
-	_qt6-build_match_cpu_flags
+	_qt6-build_sanitize_cpu_flags
 
 	# LTO cause test failures in several components (e.g. qtcharts,
 	# multimedia, scxml, wayland, webchannel, ...).
@@ -235,34 +235,6 @@ _qt6-build_create_user_facing_links() {
 	done < "${BUILD_DIR}"/user_facing_tool_links.txt || die
 }
 
-# @FUNCTION: _qt6-build_match_cpu_flags
-# @INTERNAL
-# @DESCRIPTION:
-# Try to adjust -m* cpu CXXFLAGS so that they match a configuration
-# accepted by Qt's headers, see bug #908420.
-_qt6-build_match_cpu_flags() {
-	use amd64 || use x86 || return 0
-
-	local flags=() intrin intrins
-	while IFS=' ' read -ra intrins; do
-		[[ ${intrins[*]} == *=[^_]* && ${intrins[*]} == *=_* ]] &&
-			for intrin in "${intrins[@]%=*}"; do
-				[[ ${intrin} ]] && flags+=( -mno-${intrin} )
-			done
-	done < <(
-		$(tc-getCXX) -E -P ${CXXFLAGS} ${CPPFLAGS} - <<-EOF | tail -n 2
-			avx2=__AVX2__ =__BMI__ =__BMI2__ =__F16C__ =__FMA__ =__LZCNT__ =__POPCNT__
-			avx512f=__AVX512F__ avx512bw=__AVX512BW__ avx512cd=__AVX512CD__ avx512dq=__AVX512DQ__ avx512vl=__AVX512VL__
-		EOF
-		assert
-	)
-
-	if (( ${#flags[@]} )); then
-		einfo "Adjusting CXXFLAGS for https://bugs.gentoo.org/908420 with: ${flags[*]}"
-		append-cxxflags "${flags[@]}"
-	fi
-}
-
 # @FUNCTION: _qt6-build_prepare_env
 # @INTERNAL
 # @DESCRIPTION:
@@ -289,6 +261,63 @@ _qt6-build_prepare_env() {
 	readonly QT6_TRANSLATIONDIR=${QT6_DATADIR}/translations
 }
 
+# @FUNCTION: _qt6-build_sanitize_cpu_flags
+# @INTERNAL
+# @DESCRIPTION:
+# Qt hardly support use of -mno-* or -march=native for unusual CPUs
+# (or VMs) that support incomplete x86-64 feature levels, and attempts
+# to allow this anyway has worked poorly.  This instead tries to detect
+# unusual configurations and fallbacks to generic -march=x86-64* if so
+# (bug #898644,#908420,#913400,#933374).
+_qt6-build_sanitize_cpu_flags() {
+	# less of an issue with non-amd64, will revisit only if needed
+	use amd64 || return 0
+
+	local cpuflags=(
+		# list of checked cpu features by qtbase in configure.cmake
+		aes avx avx2 avx512{bw,cd,dq,er,f,ifma,pf,vbmi,vbmi2,vl}
+		f16c rdrnd rdseed sha sse2 sse3 sse4_1 sse4_2 ssse3 vaes
+
+		# extras checked by qtbase's qsimd_p.h
+		bmi bmi2 f16c fma lzcnt popcnt
+	)
+
+	# check if any known problematic -mno-* C(XX)FLAGS
+	if ! is-flagq "@($(IFS='|'; echo "${cpuflags[*]/#/-mno-}"))"; then
+		# check if qsimd_p.h (search for "enable all") will accept -march
+		: "$($(tc-getCXX) -E -P ${CXXFLAGS} ${CPPFLAGS} - <<-EOF | tail -n 1
+				#if (defined(__AVX2__) && (__BMI__ + __BMI2__ + __F16C__ + __FMA__ + __LZCNT__ + __POPCNT__) != 6) || \
+					(defined(__AVX512F__) && (__AVX512BW__ + __AVX512CD__ + __AVX512DQ__ + __AVX512VL__) != 4)
+				bad
+				#endif
+			EOF
+			assert
+		)"
+		[[ ${_} == bad ]] || return 0 # *should* be fine as-is
+	fi
+
+	# determine highest(known) usable x86-64 feature level
+	local march=$(
+		$(tc-getCXX) -E -P ${CXXFLAGS} ${CPPFLAGS} - <<-EOF | tail -n 1
+			default
+			#if (__CRC32__ + __LAHF_SAHF__ + __POPCNT__ + __SSE3__ + __SSE4_1__ + __SSE4_2__ + __SSSE3__) == 7
+			x86-64-v2
+			#  if (__AVX__ + __AVX2__ + __BMI__ + __BMI2__ + __F16C__ + __FMA__ + __LZCNT__ + __MOVBE__ + __XSAVE__) == 9
+			x86-64-v3
+			#    if (__AVX512BW__ + __AVX512CD__ + __AVX512DQ__ + __AVX512F__ + __AVX512VL__ + __EVEX256__ + __EVEX512__) == 7
+			x86-64-v4
+			#    endif
+			#  endif
+			#endif
+		EOF
+		assert
+	)
+
+	filter-flags '-march=*' "${cpuflags[@]/#/-m}" "${cpuflags[@]/#/-mno-}"
+	[[ ${march} == x86-64* ]] && append-flags $(test-flags-CXX -march=${march})
+	einfo "C(XX)FLAGS were adjusted due to Qt limitations: ${CXXFLAGS}"
+}
+
 fi
 
 EXPORT_FUNCTIONS src_unpack src_prepare src_configure src_test src_install