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

[Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>]

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----- On Sep 29, 2021, at 11:47 PM, lttng-dev <lttng-dev@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: 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" | 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

> //Changqing

> _______________________________________________
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> lttng-dev@lists.lttng.org
> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

-- 
Mathieu Desnoyers 
EfficiOS Inc. 
http://www.efficios.com 

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