squiid/rust
1
0
Fork 0
Code Issues Pull requests Projects Activity
rust/compiler/rustc_codegen_llvm
History
bors dae9d6ac3e Auto merge of #84004 - mattico:print-target-features-improvements, r=petrochenkov 
Categorize and explain target features support

There are 3 different uses of the `-C target-feature` args passed to rustc:
1. All of the features are passed to LLVM, which uses them to configure code-generation. This is sort-of stabilized since 1.0 though LLVM does change/add/remove target features regularly.
2. Target features which are in [the compiler's allowlist](69e1d22ddb/compiler/rustc_codegen_ssa/src/target_features.rs (L12-L34)) can be used in `cfg!(target_feature)` etc. These may have different names than in LLVM and are renamed before passing them to LLVM.
3. Target features which are in the allowlist and which are stabilized or feature-gate-enabled can be used in `#[target_feature]`.

It can be confusing that `rustc --print target-features` just prints out the LLVM features without separating out the rustc features or even mentioning that the dichotomy exists.

This improves the situation by separating out the rustc and LLVM target features and adding a brief explanation about the difference.

Abbreviated Example Output:
```
$ rustc --print target-features
Features supported by rustc for this target:
    adx                         - Support ADX instructions.
    aes                         - Enable AES instructions.
...
    xsaves                      - Support xsaves instructions.
    crt-static                  - Enables libraries with C Run-time Libraries(CRT) to be statically linked.

Code-generation features supported by LLVM for this target:
    16bit-mode                  - 16-bit mode (i8086).
    32bit-mode                  - 32-bit mode (80386).
...
    x87                         - Enable X87 float instructions.
    xop                         - Enable XOP instructions.

Use +feature to enable a feature, or -feature to disable it.
For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2

Code-generation features cannot be used in cfg or #[target_feature],
and may be renamed or removed in a future version of LLVM or rustc.

```

Motivated by #83975.
CC https://github.com/rust-lang/rust/issues/49653
2021-04-09 21:14:50 +00:00
..
src Auto merge of #84004 - mattico:print-target-features-improvements, r=petrochenkov 2021-04-09 21:14:50 +00:00
Cargo.toml Rollup merge of #82537 - wesleywiser:update_measureme, r=oli-obk 2021-02-27 21:56:20 +01:00
README.md

README.md

The codegen crate contains the code to convert from MIR into LLVM IR, and then from LLVM IR into machine code. In general it contains code that runs towards the end of the compilation process.

For more information about how codegen works, see the rustc dev guide.