From e91760a3bbc3d9267fb0bfd018b8af834e0b4286 Mon Sep 17 00:00:00 2001
From: Ionen Wolkens <ionen@gentoo.org>
Date: Sat, 19 Oct 2024 08:49:43 -0400
Subject: qt6-build.eclass: extend -m* flags sanitizing

A user on the forums reported a build failure with:
`-march=znver4 -mavx512vp2intersect`

On its own, there is no issue there (expanded from -march=native) but
when Qt passes `-march=haswell` to build certain parts (e.g. avx2 bits
for runtime detection regardless of what CHOST supports), it overrides
znver4 and keeps -mavx512vp2intersect resulting in an incomplete set
and then qsimd_p.h complains

Do not plan to pursue this myself, but this specific issue could
be improved upstream by passing -mavx2 and others rather than
-march=haswell (this would also allow overriding a -mno-avx2 that
we currently need to change), and/or ideally by making qsimd_p.h
be able to deal with these configurations without #error.

If problems keep piling up, could consider always filtering *all*
-m{,no-}<instr> flags and doing x86-64-v* unconditionally. Albeit
that'd still require maintaining a list of these not to filter
unrelated -m* flags which could be important.

Signed-off-by: Ionen Wolkens <ionen@gentoo.org>
---
 eclass/qt6-build.eclass | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

(limited to 'eclass')

diff --git a/eclass/qt6-build.eclass b/eclass/qt6-build.eclass
index c6b8db578f46..2dec4b7ec14e 100644
--- a/eclass/qt6-build.eclass
+++ b/eclass/qt6-build.eclass
@@ -301,18 +301,31 @@ _qt6-build_sanitize_cpu_flags() {
 		fma4 sse4a
 	)
 
+	# extras for which -mno-* does not matter, but can lead to enabling
+	# other flags when set and breaking the -march=haswell case below
+	# (add more as needed if users use these)
+	local cpuflags_filter_only=(
+		avx512vp2intersect
+	)
+
 	# 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
-		)"
-		[[ ${_} == bad ]] || return 0 # *should* be fine as-is
+		# check if qsimd_p.h (search for "enable all") will accept -march, and
+		# further check when -march=haswell is appended (which Qt uses for some
+		# parts) given combination with other -m* could lead to partial support
+		local bad flags
+		for flags in '' '-march=haswell'; do
+			: "$($(tc-getCXX) -E -P ${CXXFLAGS} ${CPPFLAGS} ${flags} - <<-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 ]] && bad=1 && break
+		done
+		[[ -v bad ]] || return 0 # *should* be fine as-is
 	fi
 
 	# determine highest(known) usable x86-64 feature level
@@ -332,6 +345,7 @@ _qt6-build_sanitize_cpu_flags() {
 		assert
 	)
 
+	cpuflags+=("${cpuflags_filter_only[@]}")
 	filter-flags '-march=*' "${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}"
-- 
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