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Auto merge of #83084 - nagisa:nagisa/features-native, r=petrochenkov

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Adjust `-Ctarget-cpu=native` handling in cg_llvm

When cg_llvm encounters the `-Ctarget-cpu=native` it computes an
explciit set of features that applies to the target in order to
correctly compile code for the host CPU (because e.g. `skylake` alone is
not sufficient to tell if some of the instructions are available or
not).

However there were a couple of issues with how we did this. Firstly, the
order in which features were overriden wasn't quite right – conceptually
you'd expect `-Ctarget-cpu=native` option to override the features that
are implicitly set by the target definition. However due to how other
`-Ctarget-cpu` values are handled we must adopt the following order
of priority:

* Features from -Ctarget-cpu=*; are overriden by
* Features implied by --target; are overriden by
* Features from -Ctarget-feature; are overriden by
* function specific features.

Another problem was in that the function level `target-features`
attribute would overwrite the entire set of the globally enabled
features, rather than just the features the
`#[target_feature(enable/disable)]` specified. With something like
`-Ctarget-cpu=native` we'd end up in a situation wherein a function
without `#[target_feature(enable)]` annotation would have a broader
set of features compared to a function with one such attribute. This
turned out to be a cause of heavy run-time regressions in some code
using these function-level attributes in conjunction with
`-Ctarget-cpu=native`, for example.

With this PR rustc is more careful about specifying the entire set of
features for functions that use `#[target_feature(enable/disable)]` or
`#[instruction_set]` attributes.

Sadly testing the original reproducer for this behaviour is quite
impossible – we cannot rely on `-Ctarget-cpu=native` to be anything in
particular on developer or CI machines.

cc https://github.com/rust-lang/rust/issues/83027 `@BurntSushi`
This commit is contained in:
bors 2021-03-17 05:46:08 +00:00
commit 0c341226ad
7 changed files with 155 additions and 52 deletions
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5
compiler/rustc_codegen_llvm/src/back/write.rs
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@ -1,4 +1,3 @@
use crate::attributes;
use crate::back::lto::ThinBuffer;
use crate::back::profiling::{
selfprofile_after_pass_callback, selfprofile_before_pass_callback, LlvmSelfProfiler,
@ -166,8 +165,6 @@ pub fn target_machine_factory(
let code_model = to_llvm_code_model(sess.code_model());
let mut features = llvm_util::handle_native_features(sess);
features.extend(attributes::llvm_target_features(sess).map(|s| s.to_owned()));
let mut singlethread = sess.target.singlethread;
// On the wasm target once the `atomics` feature is enabled that means that
@ -182,7 +179,7 @@ pub fn target_machine_factory(
let triple = SmallCStr::new(&sess.target.llvm_target);
let cpu = SmallCStr::new(llvm_util::target_cpu(sess));
let features = features.join(",");
let features = llvm_util::llvm_global_features(sess).join(",");
let features = CString::new(features).unwrap();
let abi = SmallCStr::new(&sess.target.llvm_abiname);
let trap_unreachable =