Fix segfaults in FMA for some inputs

Author: gnzlbg

crates/libm-test/tests/unit.rs

@@ -344,3 +344,19 @@ fn j1f_2488() {
 fn y1f_2002() {
     assert_eq!(y1f(2.0000002_f32), -0.10703229_f32);
 }
+
+#[test]
+fn fma_segfault_bug() {
+    // An attempt to substract with overflow was causing a segfault
+    // on FMA for these inputs:
+    assert_eq!(
+        fma(
+            -0.0000000000000002220446049250313,
+            -0.0000000000000002220446049250313,
+            -0.0000000000000002220446049250313
+        ),
+        -0.00000000000000022204460492503126
+    );
+
+    assert_eq!(fma(-0.992, -0.992, -0.992), -0.00793599999988632);
+}

crates/libm/src/math/fma.rs

@@ -126,8 +126,8 @@ pub extern "C" 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;