clean up some warnings that appear when building with clang

I built threaded debugging bleads on Linux using clang-15 on one machine and clang-16 on another. (These are the latest versions available through their respective package managers.) I got no build-time warnings on either. So can I assume that you only began to notice these when you started compiling with clang-17?

I built threaded debugging bleads on Linux using clang-15 on one machine and clang-16 on another. (These are the latest versions available through their respective package managers.) I got no build-time warnings on either. So can I assume that you only began to notice these when you started compiling with clang-17?

No, I'd been seeing them for a while, but then didn't see them when following up, probably because the follow-up normally happened some time after seeing it, and I used gcc when testing. As you can see below the warnings are produced as far back as clang-15.

Recently I was debugging #22132 and saw them consistently, so this time I tested with clang and was able to reproduce them and track down the causes.

You won't see the merged warning in debugging builds.

See https://apt.llvm.org/ if you're running Debian or Ubuntu and want to try bleeding edge versions of clang, They don't list them, but I think you can also install clang 15 and 16 from there, in addition to the 17, 18 and 19 they do list.

clang-15 -c -DPERL_CORE -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -std=c99 -O2 -Wall -Werror=pointer-arith -Werror=vla -Wextra -Wno-long-long -Wno-declaration-after-statement -Wc++-compat -Wwrite-strings regcomp_study.c
regcomp_study.c:1067:9: warning: variable 'merged' set but not used [-Wunused-but-set-variable]
    U32 merged = 0;
        ^
1 warning generated.
clang-15 -c -DPERL_CORE -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -std=c99 -O2 -Wall -Werror=pointer-arith -Werror=vla -Wextra -Wno-long-long -Wno-declaration-after-statement -Wc++-compat -Wwrite-strings regcomp_trie.c
regcomp_trie.c:667:13: warning: variable 'wordlen' set but not used [-Wunused-but-set-variable]
        U32 wordlen      = 0;         /* required init */
            ^
1 warning generated.

Analysis:

Now trying non-debugging builds.

$ cat /etc/debian_version 
12.5

$ clang-16 --version
Debian clang version 16.0.6 (15~deb12u1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

blead:

$ sh ./Configure -des -Dusedevel -Duseithreads -Dcc=clang-16 && make test-prep 2>&1 | tee $P5P_DIR/make-output/`extract-sha`.linux.threaded.clang-16.maketp.output.txt

$ report-build-warnings 67164c3968.linux.threaded.clang-16.maketp.output.txt.gz 
File:  67164c3968.linux.threaded.clang-16.maketp.output.txt.gz

  Wunused-but-set-variable                   6

branch:

$ git checkout gh-22154-tonycoz-warning-cleanup-20240418
Switched to branch 'gh-22154-tonycoz-warning-cleanup-20240418'

$ sh ./Configure -des -Dusedevel -Duseithreads -Dcc=clang-16 && make test-prep 2>&1 | tee $P5P_DIR/make-output/`extract-sha`.linux.threaded.clang-16.maketp.output.txt

$ report-build-warnings 2b712a66ef.linux.threaded.clang-16.maketp.output.txt.gz 
File:  2b712a66ef.linux.threaded.clang-16.maketp.output.txt.gz

  Wunused-but-set-variable                   3

$ parse-build-warnings 2b712a66ef.linux.threaded.clang-16.maketp.output.txt.gz 
File:  2b712a66ef.linux.threaded.clang-16.maketp.output.txt.gz

[
  {
    char   => 17,
    group  => "Wunused-but-set-variable",
    line   => 376,
    source => "Bzip2.xs",
    text   => "variable 'flags' set but not used",
  },
  {
    char   => 11,
    group  => "Wunused-but-set-variable",
    line   => 765,
    source => "blocksort.c",
    text   => "variable 'numQSorted' set but not used",
  },
  {
    char   => 30,
    group  => "Wunused-but-set-variable",
    line   => 247,
    source => "compress.c",
    text   => "variable 'totc' set but not used",
  },
]

The 3 remaining unused-but-set-variable warnings observed on Debian 12 using clang-16 are from cpan/ modules and hence outside the scope of this pull request.

Similar results on Ubuntu 22.04 LTS with clang-15. LGTM.

Perl / perl5

clean up some warnings that appear when building with clang #22154