Hi Ola, I can confirm this problem with latest linaro-4.9-branch build. However, this is target-independent problem (reproducible on, at least, x86 and aarch64), so it is best to be tracked and addressed via upstream bugzilla.
Would you submit a PR on http://gcc.gnu.org/bugzilla ? Thank you, -- Maxim Kuvyrkov www.linaro.org On Nov 25, 2014, at 1:34 AM, Pinski, Andrew <andrew.pin...@caviumnetworks.com> wrote: > Forwarding this message to the linaro toolchain list instead. I am not the > person who should be supporting the Linaro ODP project with GCC questiosn; > the toolchain team inside Linaro should be instead. > > Thanks, > Andrew Pinski > > ________________________________________ > From: Ola Liljedahl <ola.liljed...@linaro.org> > Sent: Monday, November 24, 2014 2:31 PM > To: lng-...@lists.linaro.org; Pinski, Andrew > Subject: strange behavior in GCC for use of uninitialized variables > > Consider the following code fragment (from real life): > > #include <stdint.h> > > typedef volatile uint32_t odp_atomic_u32_t; > > static inline uint32_t odp_atomic_fetch_inc_u32(odp_atomic_u32_t *ptr) > { > return __sync_fetch_and_add(ptr, 1); > } > > static inline void odp_spin(void) > { > #ifdef __SSE2__ > __asm__ __volatile__ ("pause"); > #else > __asm__ __volatile__ ("rep; nop"); > #endif > } > > typedef struct { > int count; > odp_atomic_u32_t bar; > } odp_barrier_t; > > void odp_barrier_wait(odp_barrier_t *barrier) > { > uint32_t count; > int wasless; > > // wasless = barrier->bar < barrier->count; <<<lost on git add -p > __atomic_thread_fence(__ATOMIC_SEQ_CST); > count = odp_atomic_fetch_inc_u32(&barrier->bar); > > if (count == 2*barrier->count-1) { > barrier->bar = 0; > } else { > while ((barrier->bar < barrier->count) == wasless) > odp_spin(); > } > > __atomic_thread_fence(__ATOMIC_SEQ_CST); > } > > While fixing and cleaning up this code, the indicated line that > initializes 'wasless' was dropped (because it reappears in a later > patch in the patch set after the odp_atomic_fetch_inc call). To my > surprise, GCC did not complain when compiling this file (using -O2 > -Wall). But it does complain when compiling with -O0 -Wall. With some > investigation, it seems like GCC understands that if a statement does > not have any side effects so it can optimize away everything, > including the usage of the uninitialized variable and thus also the > corresponding warning. > > olli@macmini:~/hacking/gcc-wunit$ gcc -O2 -Wall -c odp_barrier.c > olli@macmini:~/hacking/gcc-wunit$ gcc -O0 -Wall -c odp_barrier.c > odp_barrier.c: In function ‘odp_barrier_wait’: > odp_barrier.c:42:9: warning: ‘wasless’ may be used uninitialized in > this function [-Wmaybe-uninitialized] > while ((barrier->bar < barrier->count) == was less) _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
