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Rollup merge of #91608 - workingjubilee:fold-neon-fp, r=nagisa,Amanieu

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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.
This commit is contained in:
Dylan DPC 2022-03-23 03:05:28 +01:00 • committed by GitHub
commit 67d6cc6ef3
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9 changed files with 155 additions and 12 deletions
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10
compiler/rustc_codegen_llvm/src/llvm_util.rs
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@ -187,7 +187,6 @@ pub fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> SmallVec<[&'a str; 2]
("x86", "avx512vaes") => smallvec!["vaes"],
("x86", "avx512gfni") => smallvec!["gfni"],
("x86", "avx512vpclmulqdq") => smallvec!["vpclmulqdq"],
("aarch64", "fp") => smallvec!["fp-armv8"],
("aarch64", "rcpc2") => smallvec!["rcpc-immo"],
("aarch64", "dpb") => smallvec!["ccpp"],
("aarch64", "dpb2") => smallvec!["ccdp"],
@ -230,6 +229,8 @@ pub fn check_tied_features(
None
}
// Used to generate cfg variables and apply features
// Must express features in the way Rust understands them
pub fn target_features(sess: &Session) -> Vec<Symbol> {
let target_machine = create_informational_target_machine(sess);
let mut features: Vec<Symbol> =
@ -239,13 +240,14 @@ pub fn target_features(sess: &Session) -> Vec<Symbol> {
if sess.is_nightly_build() || gate.is_none() { Some(feature) } else { None }
})
.filter(|feature| {
// check that all features in a given smallvec are enabled
for llvm_feature in to_llvm_features(sess, feature) {
let cstr = SmallCStr::new(llvm_feature);
if unsafe { llvm::LLVMRustHasFeature(target_machine, cstr.as_ptr()) } {
return true;
if !unsafe { llvm::LLVMRustHasFeature(target_machine, cstr.as_ptr()) } {
return false;
}
}
false
true
})
.map(|feature| Symbol::intern(feature))
.collect();