Bug#939656: dh_strip should strip sections with LTO information from .a and .o files / lintian should warn about these

[Matthias Klose <doko@debian.org>]

Control: tags -1 + patch
Control: tags -1 - moreinfo

Please find attached a patch stripping the LTO information, together with an 
option to explicitly keep it.

lintian probably only needs to warn about the sections found in static archives.

  * dh_strip: Strip LTO sections unless --keep-lto is given.

diff -Nru debhelper-12.6.1/dh_strip debhelper-12.6.1+nmu1/dh_strip
--- debhelper-12.6.1/dh_strip	2019-09-08 06:34:14.000000000 +0000
+++ debhelper-12.6.1+nmu1/dh_strip	2019-09-17 08:41:18.000000000 +0000
@@ -114,6 +114,14 @@
 
 Historical name for B<--dbgsym-migration>.
 
+=item B<--keep-lto>
+
+Sections containing information generated by B<-flto -ffat-lto-objects>
+are normally removed from static archives, because the streaming
+format changes even in minor GCC versions, and because cross package
+link time optimizations are normally not desired.  This option allows
+to keep this information in the static libraries.
+
 =back
 
 =head1 NOTES
@@ -138,6 +146,7 @@
 
 init(options => {
 	'keep-debug|keep|k'  => \$dh{K_FLAG},
+	'keep-lto'           => \$dh{KEEP_LTO},
 	'dbgsym-migration=s' => \$dh{MIGRATE_DBGSYM},
 	'automatic-dbgsym!'  => \$dh{ENABLE_DBGSYM},
     # Deprecated variants
@@ -376,8 +385,15 @@
 		foreach (@static_libs) {
 			# NB: The short variant (-D) is broken in Jessie
 			# (binutils/2.25-3)
-			doit($strip, '--strip-debug', '--remove-section=.comment',
-				 '--remove-section=.note', '--enable-deterministic-archives', $_);
+			if ($dh{KEEP_LTO}) {
+				doit($strip, '--strip-debug', '--remove-section=.comment',
+					 '--remove-section=.note', '--enable-deterministic-archives', $_);
+			} else {
+				doit($strip, '--strip-debug', '--remove-section=.comment',
+					 '--remove-section=.note', '--enable-deterministic-archives',
+					 '-R', '.gnu.lto_*', '-R', '.gnu.debuglto_*',
+					 '-N', '__gnu_lto_slim', '-N', '__gnu_lto_v1', $_);
+			}
 		}
 		if (-d "$tmp/usr/lib/debug/.dwz" and ($use_build_id > 1 or ($dh{DEBUGPACKAGE} and $dh{DEBUGPACKAGE} ne $package))) {
 			my @files = glob_expand(["$tmp/usr/lib/debug/.dwz"], \&glob_expand_error_handler_reject, '*');