LTO can cause GCC to inline some functions which have attributes set. The act of inlining the functions can lead to GCC forgetting about the attributes which leads to incorrect tests. Notable example being: __attribute__((__target__("no-vsx"))) LTO can also interact strangely with custom assembly functions and cause tests to intermittently fail.
Both these cases are hard to detect and require manual inspection of binaries which is unlikely to happen for all tests. Furthermore, LTO optimisations are not necessary for selftests and correctness is paramount and as such it is best to disable LTO. LTO can be enabled on a per test basis. A pseries_le_defconfig kernel on a POWER8 was used to determine that the same subset of selftests pass and fail with and without -flto in the common Makefile. These tests always fail: selftests: per_event_excludes [FAIL] selftests: event_attributes_test [FAIL] selftests: ebb_vs_cpu_event_test [FAIL] selftests: cpu_event_vs_ebb_test [FAIL] selftests: cpu_event_pinned_vs_ebb_test [FAIL] selftests: ipc_unmuxed [FAIL] And the remaining tests PASS. Signed-off-by: Suraj Jitindar Singh <sjitindarsi...@gmail.com> --- tools/testing/selftests/powerpc/Makefile | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/testing/selftests/powerpc/Makefile b/tools/testing/selftests/powerpc/Makefile index 0c2706b..7925a96 100644 --- a/tools/testing/selftests/powerpc/Makefile +++ b/tools/testing/selftests/powerpc/Makefile @@ -8,7 +8,7 @@ ifeq ($(ARCH),powerpc) GIT_VERSION = $(shell git describe --always --long --dirty || echo "unknown") -CFLAGS := -Wall -O2 -flto -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(CURDIR) $(CFLAGS) +CFLAGS := -Wall -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(CURDIR) $(CFLAGS) export CFLAGS -- 2.5.0 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev