Issue #7632: Fix a bug in dtoa.c that could lead to incorrectly-rounded results.
diff --git a/Python/dtoa.c b/Python/dtoa.c
index 4d64be5..1fe20f4 100644
--- a/Python/dtoa.c
+++ b/Python/dtoa.c
@@ -1130,6 +1130,26 @@
return q;
}
+/* version of ulp(x) that takes bc.scale into account.
+
+ Assuming that x is finite and nonzero, and x / 2^bc.scale is exactly
+ representable as a double, sulp(x) is equivalent to 2^bc.scale * ulp(x /
+ 2^bc.scale). */
+
+static double
+sulp(U *x, BCinfo *bc)
+{
+ U u;
+
+ if (bc->scale && 2*P + 1 - ((word0(x) & Exp_mask) >> Exp_shift) > 0) {
+ /* rv/2^bc->scale is subnormal */
+ word0(&u) = (P+2)*Exp_msk1;
+ word1(&u) = 0;
+ return u.d;
+ }
+ else
+ return ulp(x);
+}
/* return 0 on success, -1 on failure */
@@ -1289,12 +1309,12 @@
else if (dd < 0) {
if (!dsign) /* does not happen for round-near */
retlow1:
- dval(rv) -= ulp(rv);
+ dval(rv) -= sulp(rv, bc);
}
else if (dd > 0) {
if (dsign) {
rethi1:
- dval(rv) += ulp(rv);
+ dval(rv) += sulp(rv, bc);
}
}
else {