clamp ldexp exponent to i16

Author: RalfJung

Diff for: src/shims/foreign_items.rs

@@ -597,7 +597,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             "_ldexp" | "ldexp" | "scalbn" => {
                 let x = this.read_scalar(args[0])?.to_f64()?;
                 let exp = this.read_scalar(args[1])?.to_i32()?;
-                let res = x.scalbn(exp.try_into().unwrap());
+                // Saturating cast to i16. Even those are outside the valid exponent range to
+                // `scalbn` below will to its over/underflow handling.
+                let exp = if exp > i16::max_value() as i32 {
+                    i16::max_value()
+                } else if exp < i16::min_value() as i32 {
+                    i16::min_value()
+                } else {
+                    exp.try_into().unwrap()
+                };
+                let res = x.scalbn(exp);
                 this.write_scalar(Scalar::from_f64(res), dest)?;
             }