core crate build fails for mips16 instruction set

[athei]

I am writing bare metal code for a mipsel cpu. Since there is no readily available target for hat in rustc I use cargo xbuild to build one from a target specification that I derived from the mipsel linux target. This works great and the resulting code runs on the CPU. The CPU I am using supports mips16 ISA to improve code density. However, when I enable the feature in my target spec I get the follwing error when compiling the core crate:

LLVM ERROR: Not supported instr: <MCInst 351 <MCOperand Reg:321> <MCOperand Imm:2147483647> <MCOperand Imm:-1>>
error: Could not compile `core`.

To learn more, run the command again with --verbose.
error: `"/Users/alexander/.rustup/toolchains/nightly-x86_64-apple-darwin/bin/cargo" "rustc" "-p" "alloc" "--release" "--manifest-path" "/var/folders/8g/_rkmq65n0qnf70c41xvp174h0000gn/T/xargo.cfTitdX9TS2Q/Cargo.toml" "--target" "mipsel-unknown-none" "--" "-Z" "force-unstable-if-unmarked"` failed with exit code: Some(101)
make: *** [target/mipsel-unknown-none/release/mipswififw] Error 1

This is my spec:

{
  "llvm-target": "mipsel-unknown-none",
  "arch": "mips",
  "cpu": "mips32r2",
  "data-layout": "e-m:m-p:32:32-i8:8:32-i16:16:32-i64:64-n32-S64",
  "executables": true,
  "features": "+mips32r2,+mips16,+soft-float",
  "linker-flavor": "ld.lld",
  "linker": "rust-lld",
  "panic-strategy": "abort",
  "max-atomic-width": 32,
  "os": "none",
  "target-c-int-width": "32",
  "target-endian": "little",
  "target-pointer-width": "32"
}

When I remove the mips16 feature the error is gone.

[nagisa]

Would help if you had minimal reproduction, would make reporting the issue upstream easier.

[athei]

Sure. Just to be on the same page what that means. A repo where you basically type cargo xbuild to reproduce this?

[nagisa]

A single file which could be compiled with

rustc --target=path_to_target.json file.rs

to reproduce would be ideal, but a repository with a leaf (dependency-free) crate is fine too.

[nikic]

Just the LLVM bitcode file produced with --emit=llvm-bc would also work.

[draganmladjenovic]

Is this ok for a reproducer on linux? :

cat mipsel-unknown-none.json 
{
  "llvm-target": "mipsel-unknown-none",
  "arch": "mips",
  "cpu": "mips32r2",
  "data-layout": "e-m:m-p:32:32-i8:8:32-i16:16:32-i64:64-n32-S64",
  "executables": true,
  "features": "+mips32r2,+mips16,+soft-float",
  "linker-flavor": "gcc",
  "linker": "gcc",
  "panic-strategy": "abort",
  "max-atomic-width": 0,
  "os": "none",
  "target-c-int-width": "32",
  "target-endian": "little",
  "target-pointer-width": "32"
}

~ rustup default nightly
~ rustup component add rust-src
~ rustc --edition=2018 --crate-name core ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/lib.rs --color always --crate-type lib --emit=link,llvm-bc -C opt-level=0 --out-dir /tmp/ --target mipsel-unknown-none

I've used max-atomic-width of 0 because back-end doesn't use lib-calls for mips16 atomic while it should, but otherwise I wasn't able to reproduce the crash by feeding the resulting .bc or .ll to llvm-8.0's llc.

[workingjubilee]

Requires a change to --edition=2021 due to some subtleties about editions relaxing certain borrowing logic.

opt0:

LLVM ERROR: Not supported instr: <MCInst 491 <MCOperand Reg:23> <MCOperand Imm:2139095040> <MCOperand Imm:-1>>

opt3:

LLVM ERROR: Not supported instr: <MCInst 491 <MCOperand Reg:346> <MCOperand Imm:2147483647> <MCOperand Imm:-1>>

Interesting.

@nikic Gist with base64'd .bc and the resulting .ll https://gist.github.com/workingjubilee/01e2245ff31ba2c5c89e67613fbb8695

[nikic]

Upstream issue: llvm/llvm-project#61055

At least that's the first one hit with current LLVM. It's possible that there are more issues after that.