rust/compiler/rustc_target
Dylan DPC 67d6cc6ef3
Rollup merge of #91608 - workingjubilee:fold-neon-fp, r=nagisa,Amanieu
Fold aarch64 feature +fp into +neon

Arm's FEAT_FP and Feat_AdvSIMD describe the same thing on AArch64:
The Neon unit, which handles both floating point and SIMD instructions.
Moreover, a configuration for AArch64 must include both or neither.
Arm says "entirely proprietary" toolchains may omit floating point:
https://developer.arm.com/documentation/102374/0101/Data-processing---floating-point
In the Programmer's Guide for Armv8-A, Arm says AArch64 can have
both FP and Neon or neither in custom implementations:
https://developer.arm.com/documentation/den0024/a/AArch64-Floating-point-and-NEON

In "Bare metal boot code for Armv8-A", enabling Neon and FP
is just disabling the same trap flag:
https://developer.arm.com/documentation/dai0527/a

In an unlikely future where "Neon and FP" become unrelated,
we can add "[+-]fp" as its own feature flag.
Until then, we can simplify programming with Rust on AArch64 by
folding both into "[+-]neon", which is valid as it supersets both.

"[+-]neon" is retained for niche uses such as firmware, kernels,
"I just hate floats", and so on.

I am... pretty sure no one is relying on this.

An argument could be made that, as we are not an "entirely proprietary" toolchain, we should not support AArch64 without floats at all. I think that's a bit excessive. However, I want to recognize the intent: programming for AArch64 should be simplified where possible. For x86-64, programmers regularly set up illegal feature configurations because it's hard to understand them, see https://github.com/rust-lang/rust/issues/89586. And per the above notes, plus the discussion in https://github.com/rust-lang/rust/issues/86941, there should be no real use cases for leaving these features split: the two should in fact always go together.

- Fixes rust-lang/rust#95002.
- Fixes rust-lang/rust#95064.
- Fixes rust-lang/rust#95122.
2022-03-23 03:05:28 +01:00
..
src Rollup merge of #91608 - workingjubilee:fold-neon-fp, r=nagisa,Amanieu 2022-03-23 03:05:28 +01:00
Cargo.toml Migrate to 2021 2021-09-20 22:21:42 -04:00
README.md Fix outdated crate names in compiler docs 2021-04-08 11:12:14 -05:00

rustc_target contains some very low-level details that are specific to different compilation targets and so forth.

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