[PATCH] lib: Declare loop variable as volatile before setjmp

* [PATCH] lib: Declare loop variable as volatile before setjmp
@ 2016-04-15 12:00 Tomeu Vizoso
  2016-04-20 12:53 ` Daniel Vetter
  0 siblings, 1 reply; 2+ messages in thread
From: Tomeu Vizoso @ 2016-04-15 12:00 UTC (permalink / raw)
  To: Intel GFX discussion; +Cc: Tomeu Vizoso

The variable used as loop counter in the igt_fixture macro had
unspecified value from the setjmp(3) man page quoted below. Because of
that, in certain circumstances and with -O2 and -Os, the initialization
of that variable would be eliminated and the compiler would complain of
uninitialized usage. Below can be found a snippet that reproduces the
problem with GCC 5.3.1 and 4.9.3 and the errors as printed by 5.3.1.

"The compiler may optimize variables into registers, and longjmp() may
restore the values of other registers in addition to the stack pointer
and program  counter. Consequently, the values of automatic variables
are unspecified after a call to longjmp() if they meet all the following
criteria:

·  they are local to the function that made the corresponding setjmp(3)
call;

·  their values are changed between the calls to setjmp(3) and
longjmp(); and

·  they are not declared as volatile."

static void test(void)
{
	igt_subtest_group {
		igt_fixture {
		}

		igt_subtest("foo") {
		}

		igt_fixture {
		}
	}
}

In file included from lib/intel_batchbuffer.h:8:0,
                 from lib/drmtest.h:39,
                 from lib/igt.h:27,
                 from tests/kms_addfb_basic.c:28:
tests/kms_addfb_basic.c: In function 'tiling_tests.isra.0':
lib/igt_core.h:110:43: warning: '__tmpint245' is used uninitialized in
this function [-Wuninitialized]
 #define igt_fixture for (int igt_tokencat(__tmpint,__LINE__) = 0; \
                                           ^
lib/igt_core.h:110:43: note: '__tmpint245' was declared here
 #define igt_fixture for (int igt_tokencat(__tmpint,__LINE__) = 0; \
                                           ^
lib/igt_core.h:148:31: note: in definition of macro '__igt_tokencat2'
 #define __igt_tokencat2(x, y) x ## y
                               ^
lib/igt_core.h:110:30: note: in expansion of macro 'igt_tokencat'
 #define igt_fixture for (int igt_tokencat(__tmpint,__LINE__) = 0; \
                              ^
tests/kms_addfb_basic.c:245:3: note: in expansion of macro 'igt_fixture'
   igt_fixture {

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
---
 lib/igt_core.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/igt_core.h b/lib/igt_core.h
index b3fa7356e473..1b62371a7138 100644
--- a/lib/igt_core.h
+++ b/lib/igt_core.h
@@ -107,7 +107,7 @@ void __igt_fixture_end(void) __attribute__((noreturn));
  * enumeration (e.g. when enumerating on systems without an intel gpu) such
  * blocks should be annotated with igt_fixture.
  */
-#define igt_fixture for (int igt_tokencat(__tmpint,__LINE__) = 0; \
+#define igt_fixture for (volatile int igt_tokencat(__tmpint,__LINE__) = 0; \
 			 igt_tokencat(__tmpint,__LINE__) < 1 && \
 			 __igt_fixture() && \
 			 (sigsetjmp(igt_subtest_jmpbuf, 1) == 0); \
-- 
2.5.5

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