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

2 files changed, 28 insertions, 3 deletions

diff --git a/compiler/rustc_codegen_ssa/src/lib.rs b/compiler/rustc_codegen_ssa/src/lib.rs
index c927aae2c4c..b67c871cac9 100644
--- a/compiler/rustc_codegen_ssa/src/lib.rs
+++ b/compiler/rustc_codegen_ssa/src/lib.rs
@@ -235,6 +235,24 @@ pub struct CrateInfo {
     pub lint_levels: CodegenLintLevels,
 }
 
+/// Target-specific options that get set in `cfg(...)`.
+///
+/// RUSTC_SPECIFIC_FEATURES should be skipped here, those are handled outside codegen.
+pub struct TargetConfig {
+    /// Options to be set in `cfg(target_features)`.
+    pub target_features: Vec<Symbol>,
+    /// Options to be set in `cfg(target_features)`, but including unstable features.
+    pub unstable_target_features: Vec<Symbol>,
+    /// Option for `cfg(target_has_reliable_f16)`, true if `f16` basic arithmetic works.
+    pub has_reliable_f16: bool,
+    /// Option for `cfg(target_has_reliable_f16_math)`, true if `f16` math calls work.
+    pub has_reliable_f16_math: bool,
+    /// Option for `cfg(target_has_reliable_f128)`, true if `f128` basic arithmetic works.
+    pub has_reliable_f128: bool,
+    /// Option for `cfg(target_has_reliable_f128_math)`, true if `f128` math calls work.
+    pub has_reliable_f128_math: bool,
+}
+
 #[derive(Encodable, Decodable)]
 pub struct CodegenResults {
     pub modules: Vec<CompiledModule>,
diff --git a/compiler/rustc_codegen_ssa/src/traits/backend.rs b/compiler/rustc_codegen_ssa/src/traits/backend.rs
index 65fd843e7a5..e6b50cc7c0c 100644
--- a/compiler/rustc_codegen_ssa/src/traits/backend.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/backend.rs
@@ -18,7 +18,7 @@ use super::write::WriteBackendMethods;
 use crate::back::archive::ArArchiveBuilderBuilder;
 use crate::back::link::link_binary;
 use crate::back::write::TargetMachineFactoryFn;
-use crate::{CodegenResults, ModuleCodegen};
+use crate::{CodegenResults, ModuleCodegen, TargetConfig};
 
 pub trait BackendTypes {
     type Value: CodegenObject;
@@ -50,8 +50,15 @@ pub trait CodegenBackend {
     /// - The second is like the first, but also includes unstable features.
     ///
     /// RUSTC_SPECIFIC_FEATURES should be skipped here, those are handled outside codegen.
-    fn target_features_cfg(&self, _sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
-        (vec![], vec![])
+    fn target_config(&self, _sess: &Session) -> TargetConfig {
+        TargetConfig {
+            target_features: vec![],
+            unstable_target_features: vec![],
+            has_reliable_f16: true,
+            has_reliable_f16_math: true,
+            has_reliable_f128: true,
+            has_reliable_f128_math: true,
+        }
     }
 
     fn print_passes(&self) {}