Merge pull request rust-lang/libm#209 from gnzlbg/fma

Fix overflow bugs in fma

library/compiler-builtins/libm/src/math/fma.rs

@@ -123,8 +123,8 @@ pub fn fma(x: f64, y: f64, z: f64) -> f64 {
     } else {
         /* r -= z */
         let t = rlo;
-        rlo -= zlo;
-        rhi = rhi - zhi - (t < rlo) as u64;
+        rlo = rlo.wrapping_sub(zlo);
+        rhi = rhi.wrapping_sub(zhi.wrapping_sub((t < rlo) as u64));
         if (rhi >> 63) != 0 {
             rlo = (-(rlo as i64)) as u64;
             rhi = (-(rhi as i64)) as u64 - (rlo != 0) as u64;
@@ -202,3 +202,22 @@ pub fn fma(x: f64, y: f64, z: f64) -> f64 {
     }
     scalbn(r, e)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn fma_segfault() {
+        // These two inputs cause fma to segfault on release due to overflow:
+        assert_eq!(
+            fma(
+                -0.0000000000000002220446049250313,
+                -0.0000000000000002220446049250313,
+                -0.0000000000000002220446049250313
+            ),
+            -0.00000000000000022204460492503126,
+        );
+
+        assert_eq!(fma(-0.992, -0.992, -0.992), -0.00793599999988632,);
+    }
+}