Rollup merge of rust-lang#133395 - calebzulawski:simd_relaxed_fma, r=workingjubilee

Add simd_relaxed_fma intrinsic

Adds compiler support for rust-lang/portable-simd#387 (comment)

r? `@workingjubilee`

cc `@RalfJung` is this kind of nondeterminism a problem for miri/opsem?

Author: matthiaskrgr

core/src/intrinsics/simd.rs

@@ -612,6 +612,20 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn simd_fma<T>(x: T, y: T, z: T) -> T;
 
+    /// Computes `(x*y) + z` for each element, non-deterministically executing either
+    /// a fused multiply-add or two operations with rounding of the intermediate result.
+    ///
+    /// The operation is fused if the code generator determines that target instruction
+    /// set has support for a fused operation, and that the fused operation is more efficient
+    /// than the equivalent, separate pair of mul and add instructions. It is unspecified
+    /// whether or not a fused operation is selected, and that may depend on optimization
+    /// level and context, for example.
+    ///
+    /// `T` must be a vector of floats.
+    #[cfg(not(bootstrap))]
+    #[rustc_nounwind]
+    pub fn simd_relaxed_fma<T>(x: T, y: T, z: T) -> T;
+
     // Computes the sine of each element.
     ///
     /// `T` must be a vector of floats.