miri: algebraic intrinsics: bring back float non-determinism

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -178,8 +178,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
                 let res = self.binary_op(op, &a, &b)?;
                 // `binary_op` already called `generate_nan` if needed.
-
-                // FIXME: Miri should add some non-determinism to the result here to catch any dependences on exact computations. This has previously been done, but the behaviour was removed as part of constification.
+                let res = M::apply_float_nondet(self, res)?;
                 self.write_immediate(*res, dest)?;
             }
 

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -276,6 +276,14 @@ pub trait Machine<'tcx>: Sized {
         F2::NAN
     }
 
+    /// Apply non-determinism to float operations that do not return a precise result.
+    fn apply_float_nondet(
+        _ecx: &mut InterpCx<'tcx, Self>,
+        val: ImmTy<'tcx, Self::Provenance>,
+    ) -> InterpResult<'tcx, ImmTy<'tcx, Self::Provenance>> {
+        interp_ok(val)
+    }
+
     /// Determines the result of `min`/`max` on floats when the arguments are equal.
     fn equal_float_min_max<F: Float>(_ecx: &InterpCx<'tcx, Self>, a: F, _b: F) -> F {
         // By default, we pick the left argument.