Implement the internal feature `cfg_target_has_reliable_f16_f128`

Support for `f16` and `f128` is varied across targets, backends, and
backend versions. Eventually we would like to reach a point where all
backends support these approximately equally, but until then we have to
work around some of these nuances of support being observable.

Introduce the `cfg_target_has_reliable_f16_f128` internal feature, which
provides the following new configuration gates:

* `cfg(target_has_reliable_f16)`
* `cfg(target_has_reliable_f16_math)`
* `cfg(target_has_reliable_f128)`
* `cfg(target_has_reliable_f128_math)`

`reliable_f16` and `reliable_f128` indicate that basic arithmetic for
the type works correctly. The `_math` versions indicate that anything
relying on `libm` works correctly, since sometimes this hits a separate
class of codegen bugs.

These options match configuration set by the build script at [1]. The
logic for LLVM support is duplicated as-is from the same script. There
are a few possible updates that will come as a follow up.

The config introduced here is not planned to ever become stable, it is
only intended to replace the build scripts for `std` tests and
`compiler-builtins` that don't have any way to configure based on the
codegen backend.

MCP: https://github.com/rust-lang/compiler-team/issues/866
Closes: https://github.com/rust-lang/compiler-team/issues/866

[1]: https://github.com/rust-lang/rust/blob/555e1d0386f024a8359645c3217f4b3eae9be042/library/std/build.rs#L84-L186

1 files changed, 16 insertions, 5 deletions

diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index c3a939f1ab0..4d346b50c80 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -38,14 +38,25 @@ pub(crate) fn add_configuration(
     codegen_backend: &dyn CodegenBackend,
 ) {
     let tf = sym::target_feature;
+    let tf_cfg = codegen_backend.target_config(sess);
 
-    let (target_features, unstable_target_features) = codegen_backend.target_features_cfg(sess);
+    sess.unstable_target_features.extend(tf_cfg.unstable_target_features.iter().copied());
+    sess.target_features.extend(tf_cfg.target_features.iter().copied());
 
-    sess.unstable_target_features.extend(unstable_target_features.iter().copied());
+    cfg.extend(tf_cfg.target_features.into_iter().map(|feat| (tf, Some(feat))));
 
-    sess.target_features.extend(target_features.iter().copied());
-
-    cfg.extend(target_features.into_iter().map(|feat| (tf, Some(feat))));
+    if tf_cfg.has_reliable_f16 {
+        cfg.insert((sym::target_has_reliable_f16, None));
+    }
+    if tf_cfg.has_reliable_f16_math {
+        cfg.insert((sym::target_has_reliable_f16_math, None));
+    }
+    if tf_cfg.has_reliable_f128 {
+        cfg.insert((sym::target_has_reliable_f128, None));
+    }
+    if tf_cfg.has_reliable_f128_math {
+        cfg.insert((sym::target_has_reliable_f128_math, None));
+    }
 
     if sess.crt_static(None) {
         cfg.insert((tf, Some(sym::crt_dash_static)));