Support preloading without changing command line arguments about mold

[nitsky]

@rui314 thanks for the explanation, I clearly misunderstood how preloading is implemented in mold. This will not work with cargo because cargo does not allow the user to access the list of object files prior to linking.

Ideally, mold would have a feature like this:

If I run mold -daemon -o output a.o b.o c.o, mold detects that no daemon is running, starts one up, and performs the first link.
Now, I can run mold -daemon -o output a.o b.o c.o any number of times, and each time mold detects that the daemon is running and the arguments are the same, so it reuses work from the first invocation.

Is this possible? I am not an expert on linking, so I don't know what the limitations are.

Thank you!

[rui314]

Thank you for your suggestion, but I think there are a few practical issues with that approach:

• Preloading is supposed to be used in preload object files, compile, link cycles. So, in a single build (e.g. a single invocation of make or cargo), you are supposed to first run mold with -reload, compile source, and then invoke mold again without -preload. Can you make a change to cargo config to do this in the first place?
• If we allow the exact same command line flags, every number of mold run does preload, and every odd number of mold run creates an output. I think it can easily be out of sync and confuses build systems and users.

[rui314]

The preloading feature can make mold faster, but mold is already pretty fast. Could you share a little bit more about why you wanted to use the preloading feature? How long does your build takes for the linking step?

[vi]

why you wanted to use the preloading feature?

To move from "1.69s" in "mold" column in README to "0.79s" in "mold w/ preloading"?

[gilescope]

If you're using mold --run then that would know when it had started for the first time and could do the --preload and then subsequent mold calls could make use of that for the lifetime that the mold --run is running. I.e. it could act as a short-term daemon. (That said maybe I too have misunderstood)

[rui314]

When mold started as mold run make, it doesn't know the actual linker command. It only gets make as an argument. But we need a list of object files and other command line parameters to start preloading. So it is not feasible.

[gilescope]

Can’t preload but once the first child mold process runs you have enough knowledge to speed up subsequent invocations.

…

On Wed, 1 Sep 2021 at 12:07, Rui Ueyama ***@***.***> wrote: When mold started as mold run make, it doesn't know the actual linker command. It only gets make as an argument. But we need a list of object files and other command line parameters to start preloading. So it is not feasible. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#41 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAGEJCDQGVPG6B7MKOYS6CLT7YCQDANCNFSM45L7ZJ5A> .

[rui314]

Object file preloading doesn't seem to get traction, and it looks to me that making that feature trickier/more implicit isn't a good idea, so I'll close this issue as working-as-intended.