save a bit of space with some structure packing

[eriols]

This is an attempt at saving a bit of space by packing some structs (using pahole to find the holes) where it might make sense to do so without losing readability.
I.e., I tried to avoid separating fields that seem grouped together (like the cwd... fields in struct ftp_conn for instance).
Also abstained from touching fields behind conditional macros as that quickly can get complicated.

If this is at all useful (and I doubt it) I'm happy to squash; kept them separate for now to easy be able any unwanted ones.

[jay]

Looks reasonable, please squash

[jay]

These comments should be reversed but IMO are not needed at all because of the comment block above it

[eriols]

Thanks, removing the per-field comments.

[bagder]

Summed up, all these tiny changes add up of course and I don't mind saving this memory at basically no cost.

What's perhaps a little annoying is that the general idea to manually pack structs is to keep them in size order, in a large to small order but this bot only breaks that but it seems that we will add or change entries going forward that will alter this "optimized" state.

@eriols, you mentioned you worked with the pahole tool to figure these out. Can we come up with a way to run that tool automatically somehow to check/update the struct status?

[MarcelRaad]

Is this one OK @bagder @jay ? Isn't this an ABI change in a public struct?

[bagder]

Good catch, I didn't see that. Correct, we must not change this!

[jay]

Yes I missed that

[eriols]

Thanks, dropping that one!

[bagder]

Spotting this change made me think and create #6484 instead

[eriols]

@bagder I'll try to dig around to see if there are any automation possibilities within reach, but I figure it's a bit tricky to do. For projects that come in many flavors and for loads of architectures I figure this would mean running several parallel "packings" on different builds and then only applying the ones that do not mess up any other (important) configuration.
So that would probably be a manual call to make those trade-offs.
Also, running automated re-ordering would not support maintaining grouped variables together for readability as I tried to do here so that's another potential drawback.

[bagder]

A first step could be to just run it automatically and get some sort of daily summary or report or something, to see if we can learn from that and then see how we can proceed and improve. Probably on a chosen "default config" and platform or something.

[eriols]

Right, so just something ultra-crude like pahole --holes=1 lib/.libs/libcurl.a | perl -nle '$sum+=$1 if /XXX (\d+) bytes hole/; END {print $sum}' and if this goes above some threshold one might want to see if any modified or added structs should be reconsidered in terms of field arrangements.

[jay]

Thanks