[lttng-dev] with -Og option, lttng-ust compile failed on 32bit arm

[Changqing Li]

On 9/30/21 9:14 PM, Mathieu Desnoyers wrote:
>
> **[Please note: This e-mail is from an EXTERNAL e-mail address]
>
> ----- On Sep 29, 2021, at 11:47 PM, lttng-dev 
> <lttng-dev at lists.lttng.org> wrote:
>
>     Hi,
>
>     The problem happened after upgrade to lttng-ust 2.13.0.  2.12.0
>     don't have this issue.
>
>     liburcu version: 0.13.0
>
>     gcc: 11.2.0
>
>     This is my reproduce steps, it is cross compile enviroment based
>     on yocto project.
>
>      1. git clone git://git.yoctoproject.org/poky
>      2. . oe-init-build-env
>      3. echo "MACHINE='qemuarm'" >> conf/local.conf
>      4. echo "DEBUG_BUILD='1'" >> conf/local.conf
>      5. bitbake lttng-ust
>
>     compile failed with error:
>
>     |
>     /work/cortexa15t2hf-neon-poky-linux-gnueabi/lttng-ust/2_2.13.0-r0/recipe-sysroot-native/usr/bin/arm-poky-linux-gnueabi/../../libexec/arm-poky-linux-gnueabi/gcc/arm-poky-linux-gnueabi/11.2.0/ld:
>     ../../../src/lib/lttng-ust/.libs/liblttng-ust.so
>     <https://urldefense.com/v3/__http://liblttng-ust.so__;!!AjveYdw8EvQ!LLTouw-mBEBAY8hObF75ydNA3D3KMc0Hla12A4D4cQ-hm-F5lidnggb4zDVbAwolB77-gw$>:
>     undefined reference to `_uatomic_link_error'
>     | collect2: error: ld returned 1 exit status
>     | Makefile:399: recipe for target 'test_ust_error' failed
>
>     checked with "nm ../../../src/lib/lttng-ust/.libs/liblttng-ust.so
>     <https://urldefense.com/v3/__http://liblttng-ust.so__;!!AjveYdw8EvQ!LLTouw-mBEBAY8hObF75ydNA3D3KMc0Hla12A4D4cQ-hm-F5lidnggb4zDVbAwolB77-gw$>"
>     | grep atomic,   we can see 'U _uatomic_link_error',  but since -Og
>
>     is used,  liburcu don't define this function.
>
>     [snip]
>     #if !defined __OPTIMIZE__  || defined UATOMIC_NO_LINK_ERROR
>     static inline __attribute__((always_inline, noreturn))
>     void _uatomic_link_error(void)
>     {
>     #ifdef ILLEGAL_INSTR
>             /*
>              * generate an illegal instruction. Cannot catch this with
>              * linker tricks when optimizations are disabled.
>              */
>             __asm__ __volatile__(ILLEGAL_INSTR);
>     #else
>             __builtin_trap();
>     #endif
>     }
>     #else /* #if !defined __OPTIMIZE__  || defined
>     UATOMIC_NO_LINK_ERROR */
>     extern void _uatomic_link_error(void);
>     #endif /* #else #if !defined __OPTIMIZE__  || defined
>     UATOMIC_NO_LINK_ERROR */
>
>     [snip]
>
>     we cannot see 'U _uatomic_link_error' in following conditions, so
>     compile successed:
>
>     1.  without -Og(using -O2),  + 32bit arm
>
>     2. -Og + 64bit arm
>
>     3. -Og + x86/x86-64
>
>
>     Do you have any idea about how to fix this? I don't understand why
>     only "-Og + 32bit arm" will call function _uatomic_link_error.
>
>
> I suspect it depends on which optimizations are being enabled at -Og 
> on each architecture.
> The "_uatomic_link_error()" trick indeed depends on the compiler 
> optimizing away
> unreachable calls.
>
> If you really intend on using "-Og" on arm32, trying building with 
> "-DUATOMIC_NO_LINK_ERROR".
> It should take care of making sure to generate an illegal instruction 
> rather than rely on the linker
> error.
>
> Thanks,
>
> Mathieu

Thanks.  I have tried with "-DUATOMIC_NO_LINK_ERROR".  With this,  it 
can compile successfully.

Changqing

>
>     Thanks
>
>     //Changqing
>
>
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>
>
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com 
> <https://urldefense.com/v3/__http://www.efficios.com__;!!AjveYdw8EvQ!LLTouw-mBEBAY8hObF75ydNA3D3KMc0Hla12A4D4cQ-hm-F5lidnggb4zDVbAwo9O85jiQ$>
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