How to know where moves are memcpys?

help
1

Is there a way to get information about where in a Rust app moves are likely to be implemented by memory copy instead of a no-op? I know that the Rust compiler reserves the right to decide this differently based on optimization level and whim. But I'd like to know which cases I can expect to not be memcpys in a release, so that I can address those that might be memcpys by re-organizing the code. I'm only concerned with a few large-ish arrays I have.

I'm trying to get away from the C-style pass a pointer to raw memory and let the array get initialized there idiom and use the Rust preferred call-and-return-by-value style. But for some arrays that's making me wonder about memcpys I should try to avoid.

2

Inspect the assembly. Really, there is no other way (but godbolt is a goldmine).

Also, moves doesn't have to be memcpys, they can also be direct moving instructions in assembly.

5 Likes
3

@chrefr is right that the only truly reliable way is to look at the emitted code.

There's this (de-facto unstable) lint https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#large-assignments, but it will also warn about things that are actually not an issue at all, just happen to be big.

1 Like
4

That would be helpful. But, for some reason, I'm not getting that lint warning in my IDE (emacs rustic mode).

5

As others have said, read the assembly, but also the compiler is free to optimize the return value so even returning a large object doesn't necessarily entail a memcpy.

2 Likes
6

How do I turn on the unstable large assignments lint?

I can't get it to work in the playground:

This playground, when disassembled, shows a large memcpy in foo. But the large assignment lint doesn't show up - it builds without any warnings at all.

7

Is there a way to annotate a spot in Rust code so as to make it easy to find that spot in optimized assembly output, without impacting the Rust compiler's optimization decisions?

8

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.