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 `target_has_reliable_f16_f128` configuration option, gated
behind `cfg_target_has_reliable_f16_f128`, as an indicator for whether
or not the backend supports these types to a point that they can and
should be tested. The configuration mostly follows the logic used by
`target_feature`, and similarly takes a value:

    #[cfg(target_has_reliable_f16_f128 = "f16")]

Accepted values are:

* `f16`
* `f16-math`
* `f128`
* `f128-math`

`f16` and `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 intended to ever become stable, it is
only intended to replace the build scripts for `std` tests and in
`compiler-builtins` that don't have any way to set configuration based
on the codegen backend.

MCP: rust-lang/compiler-team#866
Closes: rust-lang/compiler-team#866

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

Author: tgross35

compiler/rustc_codegen_cranelift/src/lib.rs

@@ -41,8 +41,8 @@ use std::sync::Arc;
 
 use cranelift_codegen::isa::TargetIsa;
 use cranelift_codegen::settings::{self, Configurable};
-use rustc_codegen_ssa::CodegenResults;
 use rustc_codegen_ssa::traits::CodegenBackend;
+use rustc_codegen_ssa::{CodegenResults, TargetFeatureCfg};
 use rustc_metadata::EncodedMetadata;
 use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_session::Session;
@@ -178,7 +178,7 @@ impl CodegenBackend for CraneliftCodegenBackend {
         }
     }
 
-    fn target_features_cfg(&self, sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
+    fn target_features_cfg(&self, sess: &Session) -> TargetFeatureCfg {
         // FIXME return the actually used target features. this is necessary for #[cfg(target_feature)]
         let target_features = if sess.target.arch == "x86_64" && sess.target.os != "none" {
             // x86_64 mandates SSE2 support and rustc requires the x87 feature to be enabled
@@ -197,7 +197,13 @@ impl CodegenBackend for CraneliftCodegenBackend {
         };
         // FIXME do `unstable_target_features` properly
         let unstable_target_features = target_features.clone();
-        (target_features, unstable_target_features)
+
+        TargetFeatureCfg {
+            target_features,
+            unstable_target_features,
+            // Cranelift does not yet support f16 or f128
+            reliable_unstable_float: vec![],
+        }
     }
 
     fn print_version(&self) {

compiler/rustc_codegen_gcc/src/lib.rs

@@ -102,7 +102,7 @@ use rustc_codegen_ssa::back::write::{
 };
 use rustc_codegen_ssa::base::codegen_crate;
 use rustc_codegen_ssa::traits::{CodegenBackend, ExtraBackendMethods, WriteBackendMethods};
-use rustc_codegen_ssa::{CodegenResults, CompiledModule, ModuleCodegen};
+use rustc_codegen_ssa::{CodegenResults, CompiledModule, ModuleCodegen, TargetFeatureCfg};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync::IntoDynSyncSend;
 use rustc_errors::DiagCtxtHandle;
@@ -112,8 +112,8 @@ use rustc_middle::ty::TyCtxt;
 use rustc_middle::util::Providers;
 use rustc_session::Session;
 use rustc_session::config::{OptLevel, OutputFilenames};
-use rustc_span::Symbol;
 use rustc_span::fatal_error::FatalError;
+use rustc_span::{Symbol, sym};
 use rustc_target::spec::RelocModel;
 use tempfile::TempDir;
 
@@ -260,7 +260,7 @@ impl CodegenBackend for GccCodegenBackend {
             .join(sess)
     }
 
-    fn target_features_cfg(&self, sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
+    fn target_features_cfg(&self, sess: &Session) -> TargetFeatureCfg {
         target_features_cfg(sess, &self.target_info)
     }
 }
@@ -485,10 +485,7 @@ fn to_gcc_opt_level(optlevel: Option<OptLevel>) -> OptimizationLevel {
 }
 
 /// Returns the features that should be set in `cfg(target_feature)`.
-fn target_features_cfg(
-    sess: &Session,
-    target_info: &LockedTargetInfo,
-) -> (Vec<Symbol>, Vec<Symbol>) {
+fn target_features_cfg(sess: &Session, target_info: &LockedTargetInfo) -> TargetFeatureCfg {
     // TODO(antoyo): use global_gcc_features.
     let f = |allow_unstable| {
         sess.target
@@ -523,5 +520,10 @@ fn target_features_cfg(
 
     let target_features = f(false);
     let unstable_target_features = f(true);
-    (target_features, unstable_target_features)
+
+    // There are no known bugs with GCC support for f16 or f128
+    let reliable_unstable_float =
+        [sym::f16, sym::f16_dash_math, sym::f128, sym::f128_dash_math].to_vec();
+
+    TargetFeatureCfg { target_features, unstable_target_features, reliable_unstable_float }
 }

compiler/rustc_codegen_llvm/src/lib.rs

@@ -37,7 +37,7 @@ use rustc_codegen_ssa::back::write::{
     CodegenContext, FatLtoInput, ModuleConfig, TargetMachineFactoryConfig, TargetMachineFactoryFn,
 };
 use rustc_codegen_ssa::traits::*;
-use rustc_codegen_ssa::{CodegenResults, CompiledModule, ModuleCodegen};
+use rustc_codegen_ssa::{CodegenResults, CompiledModule, ModuleCodegen, TargetFeatureCfg};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::{DiagCtxtHandle, FatalError};
 use rustc_metadata::EncodedMetadata;
@@ -338,7 +338,7 @@ impl CodegenBackend for LlvmCodegenBackend {
         llvm_util::print_version();
     }
 
-    fn target_features_cfg(&self, sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
+    fn target_features_cfg(&self, sess: &Session) -> TargetFeatureCfg {
         target_features_cfg(sess)
     }
 

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -6,6 +6,7 @@ use std::sync::Once;
 use std::{ptr, slice, str};
 
 use libc::c_int;
+use rustc_codegen_ssa::TargetFeatureCfg;
 use rustc_codegen_ssa::base::wants_wasm_eh;
 use rustc_codegen_ssa::codegen_attrs::check_tied_features;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
@@ -15,7 +16,7 @@ use rustc_fs_util::path_to_c_string;
 use rustc_middle::bug;
 use rustc_session::Session;
 use rustc_session::config::{PrintKind, PrintRequest};
-use rustc_span::Symbol;
+use rustc_span::{Symbol, sym};
 use rustc_target::spec::{MergeFunctions, PanicStrategy, SmallDataThresholdSupport};
 use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES};
 
@@ -302,7 +303,7 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFea
 /// Must express features in the way Rust understands them.
 ///
 /// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled outside codegen.
-pub(crate) fn target_features_cfg(sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
+pub(crate) fn target_features_cfg(sess: &Session) -> TargetFeatureCfg {
     // Add base features for the target.
     // We do *not* add the -Ctarget-features there, and instead duplicate the logic for that below.
     // The reason is that if LLVM considers a feature implied but we do not, we don't want that to
@@ -402,7 +403,100 @@ pub(crate) fn target_features_cfg(sess: &Session) -> (Vec<Symbol>, Vec<Symbol>)
 
     let target_features = f(false);
     let unstable_target_features = f(true);
-    (target_features, unstable_target_features)
+
+    TargetFeatureCfg {
+        target_features,
+        unstable_target_features,
+        reliable_unstable_float: target_features_cfg_float(sess),
+    }
+}
+
+/// Update whether or not experimental float types are reliable.
+fn target_features_cfg_float(sess: &Session) -> Vec<Symbol> {
+    let mut ret = Vec::new();
+    let target_arch = sess.target.arch.as_ref();
+    let target_os = sess.target.options.os.as_ref();
+    let target_env = sess.target.options.env.as_ref();
+    let target_abi = sess.target.options.abi.as_ref();
+    let target_pointer_width = sess.target.pointer_width;
+
+    let has_reliable_f16 = match (target_arch, target_os) {
+        // Selection failure <https://github.com/llvm/llvm-project/issues/50374>
+        ("s390x", _) => false,
+        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
+        ("arm64ec", _) => false,
+        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
+        ("x86_64", "windows") if target_env == "gnu" && target_abi != "llvm" => false,
+        // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
+        ("csky", _) => false,
+        ("hexagon", _) => false,
+        ("powerpc" | "powerpc64", _) => false,
+        ("sparc" | "sparc64", _) => false,
+        ("wasm32" | "wasm64", _) => false,
+        // `f16` support only requires that symbols converting to and from `f32` are available. We
+        // provide these in `compiler-builtins`, so `f16` should be available on all platforms that
+        // do not have other ABI issues or LLVM crashes.
+        _ => true,
+    };
+
+    let has_reliable_f128 = match (target_arch, target_os) {
+        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
+        ("arm64ec", _) => false,
+        // Selection bug <https://github.com/llvm/llvm-project/issues/96432>
+        ("mips64" | "mips64r6", _) => false,
+        // Selection bug <https://github.com/llvm/llvm-project/issues/95471>
+        ("nvptx64", _) => false,
+        // ABI bugs <https://github.com/rust-lang/rust/issues/125109> et al. (full
+        // list at <https://github.com/rust-lang/rust/issues/116909>)
+        ("powerpc" | "powerpc64", _) => false,
+        // ABI unsupported  <https://github.com/llvm/llvm-project/issues/41838>
+        ("sparc", _) => false,
+        // Stack alignment bug <https://github.com/llvm/llvm-project/issues/77401>. NB: tests may
+        // not fail if our compiler-builtins is linked.
+        ("x86", _) => false,
+        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
+        ("x86_64", "windows") if target_env == "gnu" && target_abi != "llvm" => false,
+        // There are no known problems on other platforms, so the only requirement is that symbols
+        // are available. `compiler-builtins` provides all symbols required for core `f128`
+        // support, so this should work for everything else.
+        _ => true,
+    };
+
+    let has_reliable_f16_math = match (target_arch, target_os) {
+        // x86 has a crash for `powi`: <https://github.com/llvm/llvm-project/issues/105747>
+        ("x86" | "x86_64", _) => false,
+        // Assume that working `f16` means working `f16` math for most platforms, since
+        // operations just go through `f32`.
+        _ => has_reliable_f16,
+    } && has_reliable_f16;
+
+    let has_reliable_f128_math = match (target_arch, target_os) {
+        // LLVM lowers `fp128` math to `long double` symbols even on platforms where
+        // `long double` is not IEEE binary128. See
+        // <https://github.com/llvm/llvm-project/issues/44744>.
+        //
+        // This rules out anything that doesn't have `long double` = `binary128`; <= 32 bits
+        // (ld is `f64`), anything other than Linux (Windows and MacOS use `f64`), and `x86`
+        // (ld is 80-bit extended precision).
+        ("x86_64", _) => false,
+        (_, "linux") if target_pointer_width == 64 => true,
+        _ => false,
+    } && has_reliable_f128;
+
+    if has_reliable_f16 {
+        ret.push(sym::f16);
+    }
+    if has_reliable_f16_math {
+        ret.push(sym::f16_dash_math);
+    }
+    if has_reliable_f128 {
+        ret.push(sym::f128);
+    }
+    if has_reliable_f128_math {
+        ret.push(sym::f128_dash_math);
+    }
+
+    ret
 }
 
 pub(crate) fn print_version() {

compiler/rustc_codegen_ssa/src/lib.rs

@@ -235,6 +235,19 @@ pub struct CrateInfo {
     pub lint_levels: CodegenLintLevels,
 }
 
+/// Target-specific feature support.
+///
+/// RUSTC_SPECIFIC_FEATURES should be skipped here, those are handled outside codegen.
+pub struct TargetFeatureCfg {
+    /// 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>,
+    /// Options to set in `cfg(target_has_reliable_f16_f128)` Valid options are `f16`, `f128`,
+    /// `f16-math`, and `f128-math`.
+    pub reliable_unstable_float: Vec<Symbol>,
+}
+
 #[derive(Encodable, Decodable)]
 pub struct CodegenResults {
     pub modules: Vec<CompiledModule>,

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, TargetFeatureCfg};
 
 pub trait BackendTypes {
     type Value: CodegenObject;
@@ -50,8 +50,12 @@ 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_features_cfg(&self, _sess: &Session) -> TargetFeatureCfg {
+        TargetFeatureCfg {
+            target_features: vec![],
+            unstable_target_features: vec![],
+            reliable_unstable_float: vec![],
+        }
     }
 
     fn print_passes(&self) {}

compiler/rustc_feature/src/builtin_attrs.rs

@@ -40,6 +40,11 @@ const GATED_CFGS: &[GatedCfg] = &[
     // this is consistent with naming of the compiler flag it's for
     (sym::fmt_debug, sym::fmt_debug, Features::fmt_debug),
     (sym::emscripten_wasm_eh, sym::cfg_emscripten_wasm_eh, Features::cfg_emscripten_wasm_eh),
+    (
+        sym::target_has_reliable_f16_f128,
+        sym::cfg_target_has_reliable_f16_f128,
+        Features::cfg_target_has_reliable_f16_f128,
+    ),
 ];
 
 /// Find a gated cfg determined by the `pred`icate which is given the cfg's name.

compiler/rustc_feature/src/unstable.rs

@@ -205,6 +205,8 @@ declare_features! (
     (unstable, anonymous_lifetime_in_impl_trait, "1.63.0", None),
     /// Allows access to the emscripten_wasm_eh config, used by panic_unwind and unwind
     (internal, cfg_emscripten_wasm_eh, "1.86.0", None),
+    /// Allows checking whether or not the backend correctly supports unstable float types.
+    (internal, cfg_target_has_reliable_f16_f128, "CURRENT_RUSTC_VERSION", None),
     /// Allows identifying the `compiler_builtins` crate.
     (internal, compiler_builtins, "1.13.0", None),
     /// Allows writing custom MIR

compiler/rustc_interface/src/util.rs

@@ -5,6 +5,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::{env, iter, thread};
 
 use rustc_ast as ast;
+use rustc_codegen_ssa::TargetFeatureCfg;
 use rustc_codegen_ssa::traits::CodegenBackend;
 use rustc_data_structures::sync;
 use rustc_metadata::{DylibError, load_symbol_from_dylib};
@@ -39,13 +40,19 @@ pub(crate) fn add_configuration(
 ) {
     let tf = sym::target_feature;
 
-    let (target_features, unstable_target_features) = codegen_backend.target_features_cfg(sess);
+    let TargetFeatureCfg { target_features, unstable_target_features, reliable_unstable_float } =
+        codegen_backend.target_features_cfg(sess);
 
     sess.unstable_target_features.extend(unstable_target_features.iter().copied());
 
     sess.target_features.extend(target_features.iter().copied());
 
     cfg.extend(target_features.into_iter().map(|feat| (tf, Some(feat))));
+    cfg.extend(
+        reliable_unstable_float
+            .into_iter()
+            .map(|feat| (sym::target_has_reliable_f16_f128, Some(feat))),
+    );
 
     if sess.crt_static(None) {
         cfg.insert((tf, Some(sym::crt_dash_static)));

compiler/rustc_session/src/config/cfg.rs

@@ -142,6 +142,7 @@ pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {
             | (sym::target_has_atomic, Some(_))
             | (sym::target_has_atomic_equal_alignment, Some(_))
             | (sym::target_has_atomic_load_store, Some(_))
+            | (sym::target_has_reliable_f16_f128, Some(_))
             | (sym::target_thread_local, None) => disallow(cfg, "--target"),
             (sym::fmt_debug, None | Some(_)) => disallow(cfg, "-Z fmt-debug"),
             (sym::emscripten_wasm_eh, None | Some(_)) => disallow(cfg, "-Z emscripten_wasm_eh"),
@@ -463,6 +464,9 @@ impl CheckCfg {
             ins!(sym, no_values).extend(atomic_values);
         }
 
+        let reliable_float_vals = [sym::f16, sym::f128, sym::f16_dash_math, sym::f128_dash_math];
+        ins!(sym::target_has_reliable_f16_f128, no_values).extend(reliable_float_vals);
+
         ins!(sym::target_thread_local, no_values);
 
         ins!(sym::ub_checks, no_values);

compiler/rustc_span/src/symbol.rs

@@ -623,6 +623,7 @@ symbols! {
         cfg_target_feature,
         cfg_target_has_atomic,
         cfg_target_has_atomic_equal_alignment,
+        cfg_target_has_reliable_f16_f128,
         cfg_target_thread_local,
         cfg_target_vendor,
         cfg_trace: "<cfg>", // must not be a valid identifier
@@ -937,8 +938,10 @@ symbols! {
         external_doc,
         f,
         f128,
+        f128_dash_math: "f128-math",
         f128_nan,
         f16,
+        f16_dash_math: "f16-math",
         f16_nan,
         f16c_target_feature,
         f32,
@@ -2073,6 +2076,7 @@ symbols! {
         target_has_atomic,
         target_has_atomic_equal_alignment,
         target_has_atomic_load_store,
+        target_has_reliable_f16_f128,
         target_os,
         target_pointer_width,
         target_thread_local,

tests/ui/feature-gates/feature-gate-cfg-target-has-reliable-f16-f128.rs

@@ -0,0 +1,10 @@
+fn main() {
+    cfg!(target_has_reliable_f16_f128 = "f16");
+    //~^ ERROR `cfg(target_has_reliable_f16_f128)` is experimental and subject to change
+    cfg!(target_has_reliable_f16_f128 = "f16-math");
+    //~^ ERROR `cfg(target_has_reliable_f16_f128)` is experimental and subject to change
+    cfg!(target_has_reliable_f16_f128 = "f128");
+    //~^ ERROR `cfg(target_has_reliable_f16_f128)` is experimental and subject to change
+    cfg!(target_has_reliable_f16_f128 = "f128-math");
+    //~^ ERROR `cfg(target_has_reliable_f16_f128)` is experimental and subject to change
+}