Different C-compilers find different features during Configure

[jkeenan]

While I would expect that configuring with different C compilers would affect the decisions that ./Configure makes with respect to how to subsequently build perl, I would expect, cet. par., that the probes the different compilers use would find the same features present on a given machine.

Today, however, I configured on the same machine with clang10, gcc9 and g++9 and got different findings on each, some of which were unexpected. Here I present excerpts from diffs of the output of ./Configure on FreeBSD-12. For clarity, in each case I configured with sh ./Configure -des -Dusedevel, adding only the appropriate -Dcc=XXX for the respective compilers.

clang10 versus gcc9 (excerpts):

$ diff -w configure.simple.clang10.output.txt configure.simple.gcc9.output.txt
55c55
< Use which C compiler? [clang10]  
---
> Use which C compiler? [gcc9]  
...
530c529
< <quadmath.h> NOT found.
---
> <quadmath.h> found.
776c775
< You seem not to have gcc 4.* or later, not adding -D_FORTIFY_SOURCE.
---
> Adding -D_FORTIFY_SOURCE=2 to ccflags...
...

1. I would have expected either compiler to locate F<quadmath.h>; clang did not.
2. The clang run tells me that I do not have "gcc 4.* or later", but I clearly have gcc 9. Does that not count?

gcc9 versus g++9 (excerpts):

$ diff -w configure.simple.gcc9.output.txt configure.simple.g++9.output.txt 
55c55
< Use which C compiler? [gcc9]  
---
> Use which C compiler? [g++9]  
136c136,139
< dlopen() found.
---
> dlopen() NOT found.
> Hmm.  Based on the hints in hints/freebsd.sh, 
> the recommended value for $d_dlopen on this machine was "define"!
>     Keep the recommended value? [y]  
140c143
...
282c285,286
< dlsym doesn't need a leading underscore.
---
> I can't compile and run the test program.
> I'm guessing that dlsym doesn't need a leading underscore.
...

1. Why should gcc find dl_open() while g++ does not?
2. Why should g++ be unable to compile the test program for dlsym?

clang10 versus g++9 (excerpts)

< Use which C compiler? [clang10]  
---
> Use which C compiler? [g++9]  
...
137c136,139
< dlopen() found.
---
> dlopen() NOT found.
> Hmm.  Based on the hints in hints/freebsd.sh, 
> the recommended value for $d_dlopen on this machine was "define"!
>     Keep the recommended value? [y]  
...
283c285,286
< dlsym doesn't need a leading underscore.
---
> I can't compile and run the test program.
> I'm guessing that dlsym doesn't need a leading underscore.
...
530c530
< <quadmath.h> NOT found.
---
> <quadmath.h> found.
...
776c777
< You seem not to have gcc 4.* or later, not adding -D_FORTIFY_SOURCE.
---
> Adding -D_FORTIFY_SOURCE=2 to ccflags...
...

[Same questions as in each of the two other comparisons above.]

1. Why should gcc find dl_open() while g++ does not?
2. Why should g++ be unable to compile the test program for dlsym?
3. I would have expected either compiler to locate F<quadmath.h>; clang did not.
4. The clang run tells me that I do not have "gcc 4.* or later", but I clearly have gcc 9. Does that not count?

While I doubt these anomalies are important with such simple config_args, they may be more significant with more complex configurations.

Any thoughts?

Thank you very much.
Jim Keenan

[Leont]

I would have expected either compiler to locate F<quadmath.h>; clang did not.

quadmath is essentially a compiler feature, so that makes sense.

The clang run tells me that I do not have "gcc 4.* or later", but I clearly have gcc 9. Does that not count?

It probably needs some clang specific detection instead.

Why should gcc find dl_open() while g++ does not?

That is weird, I can't reproduce that

Why should g++ be unable to compile the test program for dlsym?

I'm guessing some of the linker flags end up wrong. Can you post a diff of the config.sh for both configurations?

[jkeenan]

Can you post a diff of the config.sh for both configurations?

Here's gcc9 versus g++9:

$ diff -w --ignore-matching-lines Configuration \
> --ignore-matching-lines cftime  \
> cb222c40b5.gcc9.config.sh cb222c40b5.g++9.config.sh
17c17
< config_args='-des -Dusedevel -Dcc=gcc9'
---
> config_args='-des -Dusedevel -Dcc=g++9'
21c21
< config_arg3='-Dcc=gcc9'
---
> config_arg3='-Dcc=g++9'
63c63
< cc='gcc9'
---
> cc='g++9'
92,93c92,93
< cpprun='gcc9  -E'
< cppstdin='gcc9  -E'
---
> cpprun='g++9  -E'
> cppstdin='g++9  -E'
142c142
< d_builtin_choose_expr='define'
---
> d_builtin_choose_expr='undef'
159c159
< d_cplusplus='undef'
---
> d_cplusplus='define'
211c211
< d_faststdio='define'
---
> d_faststdio='undef'
577,579c577,579
< d_stdio_cnt_lval='define'
< d_stdio_ptr_lval='define'
< d_stdio_ptr_lval_nochange_cnt='define'
---
> d_stdio_cnt_lval='undef'
> d_stdio_ptr_lval='undef'
> d_stdio_ptr_lval_nochange_cnt='undef'
582,583c582,583
< d_stdiobase='define'
< d_stdstdio='define'
---
> d_stdiobase='undef'
> d_stdstdio='undef'
696c696
< extern_C='extern'
---
> extern_C='extern "C"'
851c851
< incpth='/usr/local/lib/gcc9/gcc/x86_64-portbld-freebsd12.1/9.3.0/include /usr/local/include /usr/include'
---
> incpth='/usr/local/lib/gcc9/include/c++ /usr/local/lib/gcc9/include/c++/x86_64-portbld-freebsd12.1 /usr/local/lib/gcc9/include/c++/backward /usr/local/lib/gcc9/gcc/x86_64-portbld-freebsd12.1/9.3.0/include /usr/local/include /usr/include'
889c889
< ld='gcc9'
---
> ld='g++9'
1012c1012
< ranlib=':'
---
> ranlib='/usr/bin/ranlib'

[jkeenan]

One additional twist ... When I configure with threads (which I did not do in the above), g++ segfaults on one of the probes run within ./Configure.

$ grep -n -A4 nl_langinfo_l configure.gcc9.output.txt configure.g++9.output.t
xt
configure.gcc9.output.txt:532:Your system has nl_langinfo_l()...
configure.gcc9.output.txt-533-define
configure.gcc9.output.txt-534-and it is thread-safe (just as I'd hoped).
configure.gcc9.output.txt-535-define
configure.gcc9.output.txt-536- 
--
configure.g++9.output.txt:533:Your system has nl_langinfo_l()...
configure.g++9.output.txt-534-define
configure.g++9.output.txt-535-Segmentation fault (core dumped)
configure.g++9.output.txt-536-but it isn't thread-safe.
configure.g++9.output.txt-537-