| Issue |
174601
|
| Summary |
RISC-V clz uses legacy encoding
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
nazar-pc
|
I'm using the following Rust toolchain:
```
rustc 1.94.0-nightly (8d670b93d 2025-12-31)
binary: rustc
commit-hash: 8d670b93d40737e1b320fd892c6f169ffa35e49e
commit-date: 2025-12-31
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8
```
When compiling one of the binaries with the following target specification:
```json
{
"arch": "riscv64",
"code-model": "medium",
"cpu": "generic-rv64",
"crt-objects-fallback": "false",
"data-layout": "e-m:e-p:64:64-i64:64-i128:128-n32:64-S64",
"only-cdylib": true,
"executables": false,
"eh-frame-header": false,
"emit-debug-gdb-scripts": false,
"features": "+e,+m,+b,+zbc",
"linker": "rust-lld",
"linker-flavor": "ld.lld",
"llvm-abiname": "lp64e",
"llvm-target": "riscv64",
"max-atomic-width": 64,
"panic-strategy": "immediate-abort",
"position-independent-executables": true,
"relro-level": "full",
"static-position-independent-executables": true,
"target-pointer-width": 64,
"relocation-model": "pie",
"singlethread": true,
"dynamic-linking": true,
"default-visibility": "hidden",
"pre-link-args": {
"ld": [
"-Bsymbolic"
]
},
"dll-prefix": "",
"dll-suffix": ".contract.so",
"env": "abundance"
}
```
I hit an instruction `0x60051513`.
`objdump` decodes it to `clz a0, a0`, but apparently it is not a legal instruction in ratified version of Zbb extension, it was used by earlier drafts.
Is there a reason for this or just an oversight?
I suspect `ctz` and `cpop` might be affected in a similar way.
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