…hton

Enable ThinLTO with incremental compilation.

This is an updated version of #52309. This PR allows `rustc` to use (local) ThinLTO and incremental compilation at the same time. In theory this should allow for getting compile-time improvements for small changes while keeping the runtime performance of the generated code roughly the same as when compiling non-incrementally.

The difference to #52309 is that this version also caches the pre-LTO version of LLVM bitcode. This allows for another layer of caching:
1. if the module itself has changed, we have to re-codegen and re-optimize.
2. if the module itself has not changed, but a module it imported from during ThinLTO has, we don't need to re-codegen and don't need to re-run the first optimization phase. Only the second (i.e. ThinLTO-) optimization phase is re-run.
3. if neither the module itself nor any of its imports have changed then we can re-use the final, post-ThinLTO version of the module. (We might have to load its pre-ThinLTO version though so it's available for other modules to import from)

…nerics-for-incr-comp, r=alexcrichton

incr.comp.: Don't automatically enable -Zshare-generics for incr. comp. builds.

So far the compiler would automatically enable sharing of monomorphizations for incremental builds. That was OK because without (Thin)LTO this could have very little impact on the runtime performance of the generated code. However, since rust-lang#53673, ThinLTO and incr. comp. can be combined, so the trade-off is not as clear anymore.

This PR removes the automatic tie between the two options. Whether monomorphizations are shared between crates or not now _only_ depends on the optimization level.

r? @alexcrichton

…t-lto-imports, r=michaelwoerister

save LTO import info and check it when trying to reuse build products

Fix rust-lang#59535

Previous runs of LTO optimization on the previous incremental build can import larger portions of the dependence graph into a codegen unit than the current compilation run is choosing to import. We need to take that into account when we choose to reuse PostLTO-optimization object files from previous compiler invocations.

This PR accomplishes that by serializing the LTO import information on each incremental build. We load up the previous LTO import data as well as the current LTO import data. Then as we decide whether to reuse previous PostLTO objects or redo LTO optimization, we check whether the LTO import data matches. After we finish with this decision process for every object, we write the LTO import data back to disk.

----

What is the scenario where comparing against past LTO import information is necessary?

I've tried to capture it in the comments in the regression test, but here's yet another attempt from me to summarize the situation:

 1. Consider a call-graph like `[A] -> [B -> D] <- [C]` (where the letters are functions and the modules are enclosed in `[]`)
 2. In our specific instance, the earlier compilations were inlining the call to`B` into `A`; thus `A` ended up with a external reference to the symbol `D` in its object code, to be resolved at subsequent link time. The LTO import information provided by LLVM for those runs reflected that information: it explicitly says during those runs, `B` definition and `D` declaration were imported into `[A]`.
 3. The change between incremental builds was that the call `D <- C` was removed.
 4. That change, coupled with other decisions within `rustc`, made the compiler decide to make `D` an internal symbol (since it was no longer accessed from other codegen units, this makes sense locally). And then the definition of `D` was inlined into `B` and `D` itself was eliminated entirely.
  5. The current LTO import information reported that `B` alone is imported into `[A]` for the *current compilation*. So when the Rust compiler surveyed the dependence graph, it determined that nothing `[A]` imports changed since the last build (and `[A]` itself has not changed either), so it chooses to reuse the object code generated during the previous compilation.
  6. But that previous object code has an unresolved reference to `D`, and that causes a link time failure!

----

The interesting thing is that its quite hard to actually observe the above scenario arising, which is probably why no one has noticed this bug in the year or so since incremental LTO support landed (PR rust-lang#53673).

I've literally spent days trying to observe the bug on my local machine, but haven't managed to find the magic combination of factors to get LLVM and `rustc` to do just the right set of the inlining and `internal`-reclassification choices that cause this particular problem to arise.

----

Also, I have tried to be careful about injecting new bugs with this PR. Specifically, I was/am worried that we could get into a scenario where overwriting the current LTO import data with past LTO import data would cause us to "forget" a current import. ~~To guard against this, the PR as currently written always asserts, at overwrite time, that the past LTO import-set is a *superset* of the current LTO import-set. This way, the overwriting process should always be safe to run.~~
 * The previous note was written based on the first version of this PR. It has since been revised to use a simpler strategy, where we never attempt to merge the past LTO import information into the current one. We just *compare* them, and act accordingly.
 * Also, as you can see from the comments on the PR itself, I was quite right to be worried about forgetting past imports; that scenario was observable via a trivial transformation of the regression test I had devised.

…ts, r=mw

save LTO import info and check it when trying to reuse build products

Fix #59535

Previous runs of LTO optimization on the previous incremental build can import larger portions of the dependence graph into a codegen unit than the current compilation run is choosing to import. We need to take that into account when we choose to reuse PostLTO-optimization object files from previous compiler invocations.

This PR accomplishes that by serializing the LTO import information on each incremental build. We load up the previous LTO import data as well as the current LTO import data. Then as we decide whether to reuse previous PostLTO objects or redo LTO optimization, we check whether the LTO import data matches. After we finish with this decision process for every object, we write the LTO import data back to disk.

----

What is the scenario where comparing against past LTO import information is necessary?

I've tried to capture it in the comments in the regression test, but here's yet another attempt from me to summarize the situation:

 1. Consider a call-graph like `[A] -> [B -> D] <- [C]` (where the letters are functions and the modules are enclosed in `[]`)
 2. In our specific instance, the earlier compilations were inlining the call to`B` into `A`; thus `A` ended up with a external reference to the symbol `D` in its object code, to be resolved at subsequent link time. The LTO import information provided by LLVM for those runs reflected that information: it explicitly says during those runs, `B` definition and `D` declaration were imported into `[A]`.
 3. The change between incremental builds was that the call `D <- C` was removed.
 4. That change, coupled with other decisions within `rustc`, made the compiler decide to make `D` an internal symbol (since it was no longer accessed from other codegen units, this makes sense locally). And then the definition of `D` was inlined into `B` and `D` itself was eliminated entirely.
  5. The current LTO import information reported that `B` alone is imported into `[A]` for the *current compilation*. So when the Rust compiler surveyed the dependence graph, it determined that nothing `[A]` imports changed since the last build (and `[A]` itself has not changed either), so it chooses to reuse the object code generated during the previous compilation.
  6. But that previous object code has an unresolved reference to `D`, and that causes a link time failure!

----

The interesting thing is that its quite hard to actually observe the above scenario arising, which is probably why no one has noticed this bug in the year or so since incremental LTO support landed (PR #53673).

I've literally spent days trying to observe the bug on my local machine, but haven't managed to find the magic combination of factors to get LLVM and `rustc` to do just the right set of the inlining and `internal`-reclassification choices that cause this particular problem to arise.

----

Also, I have tried to be careful about injecting new bugs with this PR. Specifically, I was/am worried that we could get into a scenario where overwriting the current LTO import data with past LTO import data would cause us to "forget" a current import. ~~To guard against this, the PR as currently written always asserts, at overwrite time, that the past LTO import-set is a *superset* of the current LTO import-set. This way, the overwriting process should always be safe to run.~~
 * The previous note was written based on the first version of this PR. It has since been revised to use a simpler strategy, where we never attempt to merge the past LTO import information into the current one. We just *compare* them, and act accordingly.
 * Also, as you can see from the comments on the PR itself, I was quite right to be worried about forgetting past imports; that scenario was observable via a trivial transformation of the regression test I had devised.