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Fw: strange behavior in GCC for use of uninitialized variables

by Pinski, Andrew

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.liljedahl(a)linaro.org> Sent: Monday, November 24, 2014 2:31 PM To: lng-odp(a)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) == wasless) ^ However the proper code seems to be generated in both cases (there is a "pause" instruction inlined or a call to odp_spin). So odp_spin() is not without side effects and is not optimized away. This contradicts my hypothesis. Consider this minimalistic example: olli@macmini:~/hacking/gcc-wunit$ cat wunit.c #include <stdlib.h> void test(void) { int wasless; int wasmore; if (wasless) (void)0; if (wasmore) abort(); } olli@macmini:~/hacking/gcc-wunit$ gcc -O0 -Wall -c wunit.c wunit.c: In function ‘test’: wunit.c:9:5: warning: ‘wasmore’ is used uninitialized in this function [-Wuninitialized] if (wasmore) abort(); ^ olli@macmini:~/hacking/gcc-wunit$ gcc -O2 -Wall -c wunit.c wunit.c: In function ‘test’: wunit.c:9:5: warning: ‘wasmore’ is used uninitialized in this function [-Wuninitialized] if (wasmore) abort(); ^ Here GCC warns when used with both -O0 and -O2 but only for the usage where there is a side effect. The use of 'wasless' that does not lead to any side-effects is ignored (and possibly rightly so, I can imagine this is undefined behavior, fortunately I did not attempt to run this program or my computer could have melted). It is a bit worrying to me that instances of use of initialized variables is sometimes missed by GCC. Both because of lack of diagnostics for what is most likely a bug but also because I don't understand why GCC does this and the implications of that (at least it is a known unknown now). -- Ola

10 years, 7 months

[ACTIVITY] 17th - 21st November

by Bernie Ogden

cbuild2/ABE benchmarking - TCWG-360 [1/10] * Attempted to use LAVA for benchmarks ** Fell over on lack of TCWG machines in same network libm exercising - TCWG-558 [6/10] * Much fiddling with chroots * Some fiddling with benchfft * Little actual progress Meetings/mail/etc - [3/10] =Plan= libm exercising * Run benchfft in chroots * Investigate Graph500 * Validate existing results with consistent methodology ** Hopefully this is reaching the point of handle-turning ABE benchmarking * Port 'cbuild2' benchmarking to ABE repository * Test ABE benchmarking in TCWG lab (and LAVA?) * Test repeatability (assuming the above go well) * Get storage/automation started, if Rob has time Holiday *next* week (Tuesday and Wednesday, perhaps Monday as well)

10 years, 7 months

[ACTIVITY] 17 - 21 November 2014

by Will Newton

== Progress == * Building an ILP32 toolchain for AArch64 (4/10, TCWG-559) - Applied a few simple ILP32 patches to glibc - Rebasing and reworking existing ILP32 glibc patches * LLD for ARM and AArch64 (5/10) - Submit first set of reloc cleanups for LLVM * Email, meetings, etc. (1/10) == Issues == * None == Plan == * Submit more ILP32 patches for glibc * Further LLVM/LLD changes -- Will Newton Toolchain Working Group, Linaro

10 years, 7 months

[ANNOUNCE] Linaro Toolchain Binaries 2013.07 released

by Zhenqiang Chen

The Linaro Toolchain and Platform Working Groups are pleased to announce the 2013.07 release of the Linaro Toolchain Binaries, a pre-built version of Linaro GCC and Linaro GDB that runs on generic Linux or Windows and targets the glibc Linaro Evaluation Build. Uses include: * Cross compiling ARM applications from your laptop * Remote debugging * Build the Linux kernel for your board What's included: * Linaro GCC 4.8 2013.07-1 * Linaro Newlib 2.0 2013.06 * Linaro Binutils 2.23 2013.06 * Linaro Eglibc 2.17-2013.07-2 * Linaro GDB 7.6 2013.05 * A statically linked gdbserver * A system root * Manuals under share/doc/ The system root contains the basic header files and libraries to link your programs against. Interesting changes include: * The sysroot is based on Linaro versions of Eglibc. About details of Linaro Eglibc, please refer https://releases.linaro.org/13.07/components/toolchain/eglibc-linaro. The Linux version is supported on Ubuntu 10.04.3 and 12.04, Debian 6.0.2, Fedora 16, openSUSE 12.1, Red Hat Enterprise Linux Workstation 5.7 and later, and should run on any Linux Standard Base 3.0 compatible distribution. Please see the README about running on x86_64 hosts. The Windows version is supported on Windows XP Pro SP3, Windows Vista Business SP2, and Windows 7 Pro SP1. The binaries and build scripts are available from: https://launchpad.net/linaro-toolchain-binaries/trunk/2013.07 Need help? Ask a question on https://ask.linaro.org/ Already on Launchpad? Submit a bug at https://bugs.launchpad.net/linaro-toolchain-binaries On IRC? See us on #linaro on Freenode. Other ways that you can contact us or get involved are listed at https://wiki.linaro.org/GettingInvolved. Know issues: * Some version information in README are incorrect. * gdb can not backtrace into libc. Notes: * To use all of the features of Linaro eglibc, the sysroot in 32-bit toolchain release is updated to Linaro eglibc, which is 2.17. If you get runtime errors about libc version, please get the sysroot from the release tarball (gcc-linaro-arm-linux-gnueabihf-4.8-2013.07-1_linux/arm-linux-gnueabihf/libc/) or download from launchpad https://launchpad.net/linaro-toolchain-binaries/support/01/+download/linaro… If you do not want to use Linaro sysroot, you'd add option to gcc to find your sysroot: --sysroot=<directory> * To run 32-bit application built from arm-linux-gnueabihf toolchain in aarch64 system, you'd copy sysroot and runtime from release package to your root of aarch64 system. i,e. scp -r gcc-linaro-arm-linux-gnueabihf-4.8-2013.07-1_linux/arm-linux-gnueabihf/libc/* AARCH64-SYSTEM:/ scp -r gcc-linaro-arm-linux-gnueabihf-4.8-2013.07-1_runtime/* AARCH64-SYSTEM:/

10 years, 7 months

[ACTIVITY] Week 47

by Yvan Roux

== Issues == * none. == Progress == * GCC 4.9 2014.12 (2/10) - Back on Backports. - Around 30 revisions backported are under review. * AArch64 libunwind/ptrace (6/10) - Remote unwinding is not working because of a wrong ptrace invocation. - Implement registers access with PTRACE_GETREGSET * Misc: (2/10) - GCC git mirror migration. - Various meetings. == Plan == * Backports * Libunwind

10 years, 7 months

[Weekly] 17-21 NOV 2014

by Michael Collison

== This week == * GCC Modularization Project (7/10) - Flattening header files - Submitted gimple-streamer.h, tree-streamer.h, lto-streamer.h patches to gcc-patches list - tree-core.h - Several rounds of changes based on review and bootstraps - final fifty target bootstrap in progress - df.h - Initial round of changes - cfgloop.h - Initial patch created by Parthamesh * Bug fixing (2/10) - Investigation of bug 539 - .LTHUNK symbols are surviving * Misc (1/10) - Conference calls - Worked with Prathamesh to bootstrap him on GCC modularization project == Next week == - Submit tree-core.h patches - Complete df.h pathces - Review Prathamesh's patches to cfgloop.h - Thanksgiving holiday, November 27-28

10 years, 7 months

[ACTIVITY] 17 - 21 November 2014

by Kugan

== Progress == * Zero/sign extension elimination with widening types (2/10) - Addressing comments from the review * Improve block memory operations by GCC (TCWG-142 - 3/10) -Looked at ARM vs AArch64 * BUG #880 (3/10) - Analysed tree dumps. - Updated bug report with the findings. * MISC (2/10) - Looked at git and stg documents == Plan == * Continue with Zero/sign extension. == Planned Leave == * 27/11/2014 to 28/11/2014 - 2 days * 11/12/2014 to 24/12/2014 - 10 days

10 years, 7 months

[ACTIVITY] Week 47

by Renato Golin

== Progress == Lab move (for the last 11 days non-stop) == Plan == Sleep

10 years, 7 months

[ACTIVITY] 10 - 14 November 2014

by Omair Javaid

== Progress == Environment setup, installations, board setup etc [4/10] -- Prepared a local dev machine to work with in absence of lab. -- Prepare panda board for gdb on android testing. -- Figure out SSH failure issues with gdb testsuite runs. -- Try out foundation model for android testing. -- Getting to know CBuildv2 tried to set it up locally. On travel to submit Hong Kong visa application [5/10] Miscellaneous [1/10] -- Meetings, Emails etc == Plan == Try android boot with foundation model some more and hope that it works. Resume AArch64 work with lab availability. Review and update GDB related cards.

10 years, 7 months

[Activity] Week 46

by Charles Baylis

== Progress == bug #403/418 [5/10] . submitted partial patch to list for core fix for Aarch64 . needs reworking of expansion for a bunch of builtins + corresponding patterns . discussion/review of related upstream patches bug #868 - brief investigation [1/10] Misc [4/10] == Plan == On holiday on Friday and following Monday morning

10 years, 7 months

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