rust-lang/stdarch

Compile error on RISC-V due to .insn (llvm<=13 only)

XieJiSS opened this issue · 4 comments

XieJiSS commented

I'd like to create this issue to track the status of compiling .insn i 0x0F, 0, x0, x0, 0x010 directives, as well as pause / fence w,0 (these three are aliases)

Currently with clang 13.0.1, this error occurs when compiling rust:

error: unknown directive
   |
note: instantiated into assembly here
  --> <inline asm>:1:2
   |
1  |     .insn i 0x0F, 0, x0, x0, 0x010
   |     ^

error: could not compile `log` due to previous error

This is introduced by 597b8e8. I'm not saying that the commit itself has issues: rust can use this safely because rust-lang/llvm-project has cherry-picked the upstream commit to support .insn: rust-lang/rust#91528. However, for those using clang v13.0.1 (or, let's say, the unmodified llvm toolchain), they may encounter this issue during compiling. Hence, I create this issue as a explanation as well as tracking the status of llvm support for .insn.

The code should compile fine with clang 14.

CoelacanthusHex commented

In my opinion, the problem is that we are using this feature too early, I'm not saying it's wrong to use a new feature, but since almost downstream and users use rust with vanilla LLVM, we should use this feature when LLVM 14 is released.
A new tag waiting-llvm-release can be added to some pull requests that use unreleased LLVM features, we should check them out when LLVM releases a new major version instead of merging it right away.

Amanieu commented

In general the functions in std::arch are only fully supported when using Rust's custom LLVM fork. Normally this isn't a problem unless you specifically use these intrinsics in user code.

However in this case I would accept a PR to change this to a .word or equivalent since this is preventing Rust itself from compiling.

XieJiSS commented

Thanks, I'm closing this issue as LLVM 14 can compile rust now