gcc-11 C++ Regression on Cygwin

Liu Hao lh_mouse@126.com
Sun May 2 07:10:19 GMT 2021


在 2021-05-02 08:02, James McKelvey 写道:
> My code works fine on Cygwin using C++ 10.2.0. But it fails when I build with the latest gcc-11 snapshot,gcc-11-20210426, with undefined references at link time. It fails under -O3 and -O0, with differenterrors. It failed under earlier snapshots as well.
> 
> 

We have been aware of this issue since a while ago. I had some discussion 
with Jonathan Yong (who is 
notably the Cygwin port maintainer), but there was no conclusion:


```
Apr 01 22:15:51 <lh_mouse>	what's the error?
Apr 01 22:17:40 <jon_y>	strange I am not able to reproduce it now
Apr 01 22:17:50 <jon_y>	oh I think I know what the problem was
Apr 01 22:18:06 <jon_y>	gcc11 was producing broken powf() from the crt
Apr 01 22:18:23 <jon_y>	for some reason it generated undefined internal compiler symbols
Apr 01 22:18:28 <jon_y>	like U LC5
Apr 01 22:18:41 <jon_y>	switching back to gcc-10 fixed it
Apr 01 22:19:03 <lh_mouse>	that is defined in 'powf.c' I think.
Apr 01 22:19:13 <jon_y>	and because I did not rebuild the crt right after, it must be the reason why 
the compiler tries to generate its own
Apr 01 22:19:28 <jon_y>	I mean, there's an inline version in math.h
Apr 01 22:19:38 <jon_y>	and then another copy in libstdc++-v3/src/c++98/math_stubs_float.cc
Apr 01 22:19:54 <jon_y>	powf itself is defined
Apr 01 22:20:04 <jon_y>	but the U LC5 is causing an undefined symbol error
Apr 01 22:20:19 <jon_y>	I have no idea where this LC5 should be coming from
Apr 01 22:20:29 <jon_y>	it only appears in the assembly, not even in the preprocessed file
Apr 01 22:20:35 <lh_mouse>	so linking failed and configure thought `powf` 
wasn't available?
Apr 01 22:21:33 <jon_y>	yes
Apr 01 22:21:50 <jon_y>	the solution was to revert back to gcc10, rebuild 
the crt and try again

```


-- 
Best regards,
Liu Hao

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