toolchain.eclass: add USE=custom-cflags for gcc-11

Sometimes it's useful to build gcc quickly with CFLAGS=-O0
as a smoke test for sanity. CFLAGS=-O3 should also generally
work as a code generation stres test. But not in general case.

Some options like -flto would probably not work as is.

Let's allow users to experiment with gcc flags if they really
want to.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

1 files changed, 13 insertions, 6 deletions

diff --git a/eclass/toolchain.eclass b/eclass/toolchain.eclass
index 32996a2f8f71..59f8660a3d90 100644
--- a/eclass/toolchain.eclass
+++ b/eclass/toolchain.eclass
@@ -181,6 +181,7 @@ if [[ ${PN} != "kgcc64" && ${PN} != gcc-* ]] ; then
 	tc_version_is_at_least 9.1 && IUSE+=" lto"
 	tc_version_is_at_least 10 && IUSE+=" zstd" TC_FEATURES+=(zstd)
 	tc_version_is_at_least 11 && IUSE+=" valgrind" TC_FEATURES+=(valgrind)
+	tc_version_is_at_least 11 && IUSE+=" custom-cflags"
 fi
 
 if tc_version_is_at_least 10; then
@@ -1391,12 +1392,18 @@ downgrade_arch_flags() {
 }
 
 gcc_do_filter_flags() {
-	# Be conservative here:
-	# - don't allow -O3 and like to over-optimize libgcc # 701786
-	# - don't allow -O0 to generate potentially invalid startup code
-	strip-flags
-	filter-flags '-O?'
-	append-flags -O2
+	# Allow users to explicitly avoid flag sanitization via
+	# USE=custom-cflags.
+	if ! _tc_use_if_iuse custom-cflags; then
+		# Over-zealous CFLAGS can often cause problems.  What may work for one
+		# person may not work for another.  To avoid a large influx of bugs
+		# relating to failed builds, we strip most CFLAGS out to ensure as few
+		# problems as possible.
+		strip-flags
+		# Lock gcc at -O2; we want to be conservative here.
+		filter-flags '-O?'
+		append-flags -O2
+	fi
 
 	# dont want to funk ourselves
 	filter-flags '-mabi*' -m31 -m32 -m64