Add tracing to InterpCx::layout_of()

[Stypox]

This PR adds tracing calls to instantiate_from_frame_and_normalize_erasing_regions and to InterpCx::layout_of(). The latter is done by shadowing LayoutOf's trait method with an inherent method on InterpCx.

Previous attempt by overriding the `layout_of` query (includes downloadable `.diff` patch)

This PR is meant for Miri, but requires a few changes in rustc code, hence why it's here. It adds tracing capabilities to the layout_of function in tcx by overriding the layout_of query (under local_providers) with a wrapper that opens a tracing span and then calls the actual layout_of. To make this possible, I had to make rustc_ty_utils::layout::layout_of public. I added an assert to ensure the providers.layout_of value I am replacing is actually rustc_ty_utils::layout::layout_of, just in case.

I also considered taking the previous value in providers.layout_of and calling that one instead, to avoid making layout_of public. But then the closure would not be castable to a function pointer anymore (providers.layout_of is a function pointer), because it would depend on the local variable storing the previous value of providers.layout_of. Using a global variable would work but would rely on unsafe or on Mutexes, so I wanted to avoid it.

Here is some tracing output when Miri is run on src/tools/miri/tests/pass/hello.rs, visualizable in https://ui.perfetto.dev: trace-1750338860374637.json

Another place where I could have added tracing calls is to the rustc_middle::ty::layout::LayoutCx struct / spanned_layout_of() function, however there is no simple way to disable the tracing calls with compile-time boolean constants there (since LayoutCx::new() is used everywhere and referenced directly), and in any case it seems like spanned_layout_of() just calls tcx.layout_of() anyway. For completeness' sake, here is tracing output for when a tracing call is added to spanned_layout_of(): trace-1750340887920584.json

Patch to override layout_of query: tracing-layout_of-query-override.diff.txt

Note: obtaining tracing output depends on rust-lang/miri#4406, but this PR is standalone and can be merged without waiting for rust-lang/miri#4406.

r? @RalfJung

[RalfJung]

What a hack.^^

But also, this will only measure the time inside the query, not the time spent in the query machinery, so I think this is the wrong place to add the tracing span.

Ultimately we want to catch all the self.layout_of calls in InterpCx. Maybe just add a new inherent method, which should take priority over the trait method, and do the tracing there?

[Stypox]

Ultimately we want to catch all the self.layout_of calls in InterpCx. Maybe just add a new inherent method, which should take priority over the trait method, and do the tracing there?

Oh ok, thanks for clarifying. I pushed a commit that does that, and added a comment on the inherent method hoping it won't cause confusion. Here is a trace collected with both the query override and the inherent method: trace-1751014622990785.json

Should I keep the query override? I think it may still provide useful info, as by looking at the trace it seems like the layout_of query is sometimes called outside of InterpCx::layout_of.

[RalfJung]

I'd prefer not to have the query override -- but it may be good to figure out where those "outside" calls are coming from. They may be coming from other parts of the compiler, not from Miri itself.

[Stypox]

Ok, I removed that commit and added a todo on my side to figure out where the "outside" calls come from

[Stypox]

@rustbot ready

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[RalfJung]

Thanks!
@bors r+ rollup

[bors]

📌 Commit 708dc15 has been approved by RalfJung

It is now in the queue for this repository.