Re: Why the Makefile is so eager to re-build & re-link

> On Thu, Jun 24, 2021 at 03:16:48PM +0200, Ævar Arnfjörð Bjarmason wrote:
>>  * {command,config}-list.h (and in-flight, my hook-list.h): Every time
>>    you touch a Documentation/git-*.txt we need to re-generate these, and
>>    since their mtime changes we re-compile and re-link all the way up to
>>    libgit and our other tools.
>>
>>    I think the best solution here is to make the generate-*.sh
>>    shellscripts faster (just one takes ~300ms of nested shellscripting,
>>    just to grep out the first few lines of every git-*.txt, in e.g. Perl
>>    or a smarter awk script this would be <5ms).
> 
> Yeah, I think Eric mentioned he had looked into doing this in perl, but
> we weren't entirely happy with the dependency. Here's another really odd
> thing I noticed:
> 
>   $ time sh ./generate-cmdlist.sh command-list.txt >one
>   real	0m1.323s
>   user	0m1.531s
>   sys	0m0.834s
> 
>   $ time sh -x ./generate-cmdlist.sh command-list.txt >two
>   [a bunch of trace output]
>   real	0m0.513s
>   user	0m0.754s
>   sys	0m0.168s
> 
>   $ cmp one two
>   [no output]
> 
> Er, what? Running with "-x" makes it almost 3 times faster to generate
> the same output? I'd have said this is an anomaly, but it's repeatable
> (and swapping the order produces the same output, so it's not some weird
> priming thing). And then to top it all off, redirecting the trace is
> slow again:
> 
>   $ time sh -x ./generate-cmdlist.sh command-list.txt >two 2>/dev/null
>   real	0m1.363s
>   user	0m1.538s
>   sys	0m0.902s
> 
> A little mini-mystery that I think I may leave unsolved for now.