Auto merge of #877 - christianpoveda:ldexp-shim, r=RalfJung

Add shim for ldexp

Fixes #821

r? @RalfJung

Author: bors

src/shims/foreign_items.rs

@@ -602,6 +602,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 };
                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
             }
+            // underscore case for windows
+            "_ldexp" | "ldexp" => {
+                // FIXME: Using host floats.
+                let x = f64::from_bits(this.read_scalar(args[0])?.to_u64()?);
+                let exp = this.read_scalar(args[1])?.to_i32()?;
+                // FIXME: We should use cmath if there are any imprecisions.
+                let n = x * 2.0f64.powi(exp);
+                this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
+            }
 
             // Some things needed for `sys::thread` initialization to go through.
             "signal" | "sigaction" | "sigaltstack" => {
@@ -974,4 +983,4 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         }
         return Ok(None);
     }
-}
+}

tests/run-pass/intrinsics-math.rs

@@ -85,4 +85,8 @@ pub fn main() {
     assert_approx_eq!(1.0f32.tan(), 1.557408f32);
     assert_approx_eq!(1.0f64.tan(), 1.557408f64);
 
+    extern {
+        fn ldexp(x: f64, n: i32) -> f64;
+    }
+    unsafe { assert_approx_eq!(ldexp(0.65f64, 3i32), 5.2f64); }
 }