[PATCH 2/2] selftests/powerpc: benchmarks/context_switch.c use vector instructions/types

[Cyril Bur]

Currently it doesn't appear the resulting binary actually uses any Altivec
or VSX instructions the solution is to explicitly tell GCC to use vector
instructions and use vector types in the code.

Part of this this issue can be GCC version specific:

GCC 4.9.x is happy to use Altivec and VSX instructions if altivec.h is
includedi (and possibly if vector types are used), this also means that
4.9.x will use VSX instructions even if only -maltivec is passed. It is
also possible that Altivec instructions will be used even without -maltivec
or -mabi=altivec.

GCC 5.2.x complains about the lack of -maltivec parameter if altivec.h is
included and will not use VSX unless -mvsx is present on commandline.

GCC 5.3.0 has a regression that means __attribute__((__target__("no-vsx"))
fails to build. A fix is targeted for 5.4.

Furthermore LTO (Link Time Optimisation) doesn't play well with
__attribute__((__target__("no-vsx")), LTO can cause GCC to forget about the
attribute and compile with VSX instructions regardless. Be weary when
enabling -flfo for this test.

Signed-off-by: Cyril Bur <cyrilbur at gmail.com>
---
 tools/testing/selftests/powerpc/benchmarks/Makefile         |  1 +
 tools/testing/selftests/powerpc/benchmarks/context_switch.c | 11 ++++++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/powerpc/benchmarks/Makefile b/tools/testing/selftests/powerpc/benchmarks/Makefile
index 912445f..6816fc2 100644
--- a/tools/testing/selftests/powerpc/benchmarks/Makefile
+++ b/tools/testing/selftests/powerpc/benchmarks/Makefile
@@ -7,6 +7,7 @@ all: $(TEST_PROGS)
 $(TEST_PROGS): ../harness.c
 
 context_switch: ../utils.c
+context_switch: CFLAGS += -maltivec -mvsx -mabi=altivec
 context_switch: LDLIBS += -lpthread
 
 include ../../lib.mk
diff --git a/tools/testing/selftests/powerpc/benchmarks/context_switch.c b/tools/testing/selftests/powerpc/benchmarks/context_switch.c
index e6af382..a36883a 100644
--- a/tools/testing/selftests/powerpc/benchmarks/context_switch.c
+++ b/tools/testing/selftests/powerpc/benchmarks/context_switch.c
@@ -25,7 +25,9 @@
 #include <sys/types.h>
 #include <sys/shm.h>
 #include <linux/futex.h>
-
+#ifdef __powerpc__
+#include <altivec.h>
+#endif
 #include "../utils.h"
 
 static unsigned int timeout = 30;
@@ -37,12 +39,15 @@ static int touch_fp = 1;
 double fp;
 
 static int touch_vector = 1;
-typedef int v4si __attribute__ ((vector_size (16)));
-v4si a, b, c;
+vector int a, b, c;
 
 #ifdef __powerpc__
 static int touch_altivec = 1;
 
+/*
+ * Note: LTO (Link Time Optimisation) doesn't play well with this function
+ * attribute. Be very careful enabling LTO for this test.
+ */
 static void __attribute__((__target__("no-vsx"))) altivec_touch_fn(void)
 {
 	c = a + b;
-- 
2.7.2