make miri usable as stage 0/1 tool

[RalfJung]

Moving miri to the compiler made development with miri significantly harder. To avoid rebuilding the compiler twice on every change in librustc_mir/interpret, I usually do ./x.py build src/rustc --keep-stage 0. This still does unnecessary work, however, when changing libstd (but that is likely hard to avoid?). Also --keep-stage is somewhat fragile. So overall, developing miri is much more cumbersome than previously (and that's not even talking about it being much harder to land patches, and much more likely to get merge conflicts, but that part is unavoidable).

It would be great if one could build, test and run miri in earlier stages. The building part actually works, but only with a trick:

$ CARGO_CFG_REGEX_DISABLE_AUTO_OPTIMIZATIONS=1 ./x.py --stage 0 build src/tools/miri

The env var is needed because stage 0 libstd does not contain std::arch. Could we change that?

Running works but is not very straight-forward

$ cp build/x86_64-unknown-linux-gnu/stage0-tools-bin/miri build/x86_64-unknown-linux-gnu/stage1/bin/
$ MIRI_SYSROOT=build/x86_64-unknown-linux-gnu/stage1/ build/x86_64-unknown-linux-gnu/stage1/bin/miri src/tools/miri/tests/run-pass/products.rs

Looks like RUST_SYSROOT is not properly set during build, so I have to give the sysroot manually?

Running the test suite, however, fails:

$ CARGO_CFG_REGEX_DISABLE_AUTO_OPTIMIZATIONS=1 ./x.py --stage 0 test src/tools/miri -v
[...]
running: "/home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-j" "8" "--release" "--manifest-path" "/home/r/src/rust/rustc/src/tools/miri/Cargo.toml"
[...]
error: failed to find a `codegen-backends` folder in the sysroot candidates:
* /home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu
* /home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu
* /home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu/stage0-sysroot

How could we do better (other than maybe enabling std::arch in stage 0)? @eddyb suggested that test src/tools/miri should behave more like test src/test/run-pass, and test the stuff produced by the previous stage, but I find that somewhat strange. The compiler itself has strange "leveling issues" in a staged build, but the tools should be more consistent.

I'd expect miri to behave more like libstd: build --stage 1 src/libstd builds using the compiler produced in stage 0, and test --stage 1 src/libstd --no-doc tests with that same compiler. So maybe build --stage 1 src/tools/miri should be built with the compiler produced in stage 0 and the libstd produced in stage 1 (similar to build --stage 1 src/rustc). build --stage 0 src/tools/miri would use the bootstrap compiler to build miri, which will likely not work but that's okay. test --stage 1 src/tools/miri would first do build --stage 1 src/tools/miri and then run the tests in the stage1 sysroot.
Or does that not work because miri depends on librustc?

Cc @oli-obk @eddyb (and maybe we should get a bootstrap expert in the loop?)

[eddyb]

cc @Mark-Simulacrum

[Mark-Simulacrum]

The env var is needed because stage 0 libstd does not contain std::arch. Could we change that?

Not really -- this is done so that the compiler can be more easily changed in how it handles SIMD. It would also be a degradation in overall compile times since stage 0 builds will take longer.

Looks like RUST_SYSROOT is not properly set during build, so I have to give the sysroot manually?

We do not set RUST_SYSROOT at all today -- I'm certain a patch to do so can be accepted, though. Should be relatively easy.

Running the test suite, however, fails: [...]

It looks like miri needs the codegen backends to function -- this is rather unexpected; the reason the test suite fails is that the codegen backends aren't built/linked into the right place. This should also be relatively easy to fix.

---

I'm not clear on what MIRI needs to work properly, but I suspect that we can make the situation better here. I think what would be most useful at this point is a summary of what you expect to happen after changing std, test, and rustc_mir, for example (i.e., what needs to be rebuilt). Does miri use proc macros?

[RalfJung]

It looks like miri needs the codegen backends to function -- this is rather unexpected; the reason the test suite fails is that the codegen backends aren't built/linked into the right place. This should also be relatively easy to fix.

I am also surprised that miri needs the codegen backends. It shouldn't codegen anything.

Does miri use proc macros?

miri itself does not (AFAIK), but cargo-miri depends on cargo_metadata which depends on serde. Somehow cargo test builds that, I am not sure why. I would be fine with cargo-miri not working in stage 0/1.

EDIT: Ah, miri's test suite needs compiletest which depends on serde as well, for some reason.

EDIT: I managed to get rid of the regex dependency, so SIMD should not be a problem any more. Never mind, env_logger still needs it.

Not really -- this is done so that the compiler can be more easily changed in how it handles SIMD. It would also be a degradation in overall compile times since stage 0 builds will take longer.

Compile time impact is <10s for me, but while I do not understand what the first part of your sentence means, I get that it might be annoying for compiler/SIMD development in the future.

[RalfJung]

Ah dang compiletest-rs uses serde to parse rustc JSON output. So I guess yes we need proc macros. That's a no-go currently for stages <2, right?

---

I think what would be most useful at this point is a summary of what you expect to happen after changing std, test, and rustc_mir, for example (i.e., what needs to be rebuilt).

Ideally on a rustc_mir change, it should rebuild the stage0 compiler artifacts and then build a stage1 libstd with that compiler and then build miri with that.

When changing std, it'd start with step 2 above, i.e. not touch the compiler at all. I guess that needs a --keep-stage 0 as otherwise it'd also recompile the libstd used by the compiler. Not sure what you mean by changing "test", but I guess libtest works like libstd in that regard?

I realize this is off-by-1-stage compared to now: I am imagining stage 1 miri to be built with the stage 0 compiler artifacts. This is in line with how @eddyb told me a "stage" is defined; it is consistent e.g. with --stage 1 test src/run-pass which tests the stage 0 compiler artifacts.

[RalfJung]

@Mark-Simulacrum said the situation changed, so I re-tested this.

My first command to run was ./x.py build --stage 1 src/tools/miri, but interestingly this builds the stage 1 compiler artifacts (unlike build --stage 1 src/libstd), so I cancelled that as that's not what I wanted. I thought I understood how our stages work but it seems I still do not... Cc #59864

So I went for ./x.py build --stage 0 src/tools/miri instead. And that passes! And it does actually seem to use the compiler produced by the stage 0 compiler artifacts, i.e., the compiler used to build stage 1, even though the flag seems to say otherwise. Weird, but whatever.

Unfortunately, ./x.py test --stage 0 src/tools/miri fails. And this probably has to do with this message:

     Running `build/x86_64-unknown-linux-gnu/stage0-tools/x86_64-unknown-linux-gnu/release/cargo-miri miri setup`
WARNING: the sysroot can't be built for the Beta channel. Switch to nightly.

This messages comes from xargo, which is not happy about the compiler we are using... which is strange because to build Miri it clearly used the compiler produced by the stage 0 compiler artifacts, but now xargo seems to pick up the bootstrap compiler and then bails out.

[oli-obk]

it uses the compiler produced by stage 0 (as a library), but it's not compiled by that compiler. Maybe we need to teach xargo about RUST_BOOTSTRAP=1?

[RalfJung]

it uses the compiler produced by stage 0 (as a library), but it's not compiled by that compiler.

Oh... 🤯
I... think that makes sense... maybe?

Maybe we need to teach xargo about RUST_BOOTSTRAP=1?

Or can we make xargo use the compiler produced by stage 0 to build libstd? Not sure if that was already assembled at this point.

[oli-obk]

Or can we make xargo use the compiler produced by stage 0 to build libstd? Not sure if that was already assembled at this point.

I think that will be even more confusing, as everything else is built by the stage 0 compiler (the beta compiler using RUST_BOOTSTRAP)

[RalfJung]

With this xargo patch, it succeeds at building a libstd for Miri.

However, later all tests fail when it tries to use that libstd:

error[E0514]: found crate `std` compiled by an incompatible version of rustc
  |
  = help: please recompile that crate using this compiler (rustc 1.45.0-dev)
  = note: the following crate versions were found:
          crate `std` compiled by rustc 1.44.0-beta.752 (a7d891e31 2020-04-21): /home/r/.cache/miri/HOST/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-b28cadfa61171505.rmeta

error: aborting due to previous error

That's expected I guess -- the libstd is loaded by the rustc built into Miri which is the one created by "stage 0 compiler artifacts", but that is not the rustc that built this libstd.

So I think we have to make xargo use the compiler produced by stage 0 to build libstd. I do not know how to achieve that though.

[oli-obk]

ah yes, that makes sense. Can we make xargo use the miri binary to produce the artifactsß

[RalfJung]

uh...
We'd need some magic flag or so to tell Miri to be a normal compiler and not an interpreter.

[RalfJung]

But yeah that is probably the cleanest solution. What is the best way to just say "behave like the rustc binary would"?

The main thing I am worried about is that it means all the codegen stuff in Miri will be reachable, so the binary might become bigger. But OTOH I doubt that the linker currently is clever enough to actually remove all that stuff, so... whatever.