Add -Zmips-nan2008 to enable IEEE 754-2008 NaN encoding on MIPS

compiler/rustc_codegen_llvm/src/errors.rs

@@ -210,6 +210,12 @@ pub(crate) struct FixedX18InvalidArch<'a> {
     pub arch: &'a str,
 }
 
+#[derive(Diagnostic)]
+#[diag("the `-Zmips-nan2008` flag is not supported on the `{$arch}` architecture")]
+pub(crate) struct MipsNan2008InvalidArch<'a> {
+    pub arch: &'a str,
+}
+
 #[derive(Diagnostic)]
 #[diag("`-Zpacked-stack` is incompatible with `backchain` target feature")]
 #[note(

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -639,6 +639,19 @@ fn llvm_features_by_flags(sess: &Session, features: &mut Vec<String>) {
             features.push("+reserve-x18".into());
         }
     }
+
+    // -Zmips-nan2008
+    if sess.opts.unstable_opts.mips_nan2008 {
+        match sess.target.arch {
+            Arch::Mips | Arch::Mips32r6 | Arch::Mips64 | Arch::Mips64r6 => {
+                features.push("+nan2008".into());
+            }
+            _ => {
+                sess.dcx()
+                    .emit_fatal(errors::MipsNan2008InvalidArch { arch: sess.target.arch.desc() });
+            }
+        }
+    }
 }
 
 /// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,

compiler/rustc_interface/src/tests.rs

@@ -846,6 +846,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(merge_functions, Some(MergeFunctions::Disabled));
     tracked!(min_function_alignment, Some(Align::EIGHT));
     tracked!(min_recursion_limit, Some(256));
+    tracked!(mips_nan2008, true);
     tracked!(mir_enable_passes, vec![("DestProp".to_string(), false)]);
     tracked!(mir_opt_level, Some(4));
     tracked!(mir_preserve_ub, true);

compiler/rustc_session/src/options.rs

@@ -2579,6 +2579,8 @@ options! {
         "align all functions to at least this many bytes. Must be a power of 2"),
     min_recursion_limit: Option<usize> = (None, parse_opt_number, [TRACKED],
         "set a minimum recursion limit (final limit = max(this, recursion_limit_from_crate))"),
+    mips_nan2008: bool = (false, parse_bool, [TRACKED] { TARGET_MODIFIER: MipsNan2008 },
+        "enable the nan2008 target feature (IEEE 754-2008 NaN encoding) on MIPS targets (default: no)"),
     mir_enable_passes: Vec<(String, bool)> = (Vec::new(), parse_list_with_polarity, [TRACKED],
         "use like `-Zmir-enable-passes=+DestinationPropagation,-InstSimplify`. Forces the \
         specified passes to be enabled, overriding all other checks. In particular, this will \

src/doc/unstable-book/src/compiler-flags/mips-nan2008.md

@@ -0,0 +1,24 @@
+# `mips-nan2008`
+
+This option enables the LLVM `nan2008` subtarget feature on MIPS targets. It is
+only supported on `mips`, `mips32r6`, `mips64`, and `mips64r6` architectures.
+Using this flag on other targets is an error.
+
+When enabled, this flag causes the compiler to select the IEEE 754-2008 NaN
+encoding for MIPS targets, as opposed to the legacy MIPS NaN encoding. This is
+useful when linking against C libraries and toolchains built with GCC's
+`-mnan=2008` flag, or when the system ABI mandates the 2008 NaN encoding.
+
+This flag is a target modifier. All crates in the crate graph must agree on
+whether the `mips-nan2008` flag is used. The compiler will emit an error if
+crates with conflicting settings are linked together. This error can be bypassed
+at your own risk using the `-Cunsafe-allow-abi-mismatch=mips-nan2008` flag,
+though this may cause runtime failures if the ABIs are truly incompatible.
+
+## Example
+
+To compile code for MIPS with the 2008 NaN encoding:
+
+```sh
+rustc -Zmips-nan2008 --target mips-unknown-linux-gnu example.rs
+```

tests/codegen-llvm/mips-nan2008.rs

@@ -0,0 +1,23 @@
+// Test that the `nan2008` target feature is (not) emitted when
+// the `-Zmips-nan2008` flag is (not) set.
+
+//@ add-minicore
+//@ revisions: unset set
+//@ needs-llvm-components: mips
+//@ compile-flags: --target mips-unknown-linux-gnu
+//@ [set] compile-flags: -Zmips-nan2008
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+#[no_mangle]
+pub fn foo() {
+    // CHECK: @foo() unnamed_addr #0
+
+    // unset-NOT: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+nan2008{{.*}} }
+    // set: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+nan2008{{.*}} }
+}

tests/ui/target_modifiers/auxiliary/mips_nan2008.rs

@@ -0,0 +1,7 @@
+//@ no-prefer-dynamic
+//@ compile-flags: --target mips-unknown-linux-gnu -Zmips-nan2008
+//@ needs-llvm-components: mips
+
+#![feature(no_core)]
+#![crate_type = "rlib"]
+#![no_core]

tests/ui/target_modifiers/incompatible_mips_nan2008.error_generated.stderr

@@ -0,0 +1,13 @@
+error: mixing `-Zmips-nan2008` will cause an ABI mismatch in crate `incompatible_mips_nan2008`
+  --> $DIR/incompatible_mips_nan2008.rs:13:1
+   |
+LL | #![feature(no_core)]
+   | ^
+   |
+   = help: the `-Zmips-nan2008` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
+   = note: unset `-Zmips-nan2008` in this crate is incompatible with `-Zmips-nan2008=` in dependency `mips_nan2008`
+   = help: set `-Zmips-nan2008=` in this crate or unset `-Zmips-nan2008` in `mips_nan2008`
+   = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=mips-nan2008` to silence this error
+
+error: aborting due to 1 previous error
+

tests/ui/target_modifiers/incompatible_mips_nan2008.rs

@@ -0,0 +1,18 @@
+//@ aux-build:mips_nan2008.rs
+//@ compile-flags: --target mips-unknown-linux-gnu
+//@ needs-llvm-components: mips
+
+//@ revisions:allow_match allow_mismatch error_generated
+//@[allow_match] compile-flags: -Zmips-nan2008
+//@[allow_mismatch] compile-flags: -Cunsafe-allow-abi-mismatch=mips-nan2008
+// [error_generated] no extra compile-flags
+//@[allow_mismatch] check-pass
+//@[allow_match] check-pass
+//@ ignore-backends: gcc
+
+#![feature(no_core)]
+//[error_generated]~^ ERROR mixing `-Zmips-nan2008` will cause an ABI mismatch in crate `incompatible_mips_nan2008`
+#![crate_type = "rlib"]
+#![no_core]
+
+extern crate mips_nan2008;