Proposed fix to avoid rounding mantissa down to zero for number on th…

…e denormal boundary and add test cases. Signed-off-by: Alan Bain <alan.bain@gmail.com>
ulfjack · Jun 30, 2020 · abf76d2 · abf76d2
1 parent b38c7cc
commit abf76d2
Show file tree

Hide file tree

Showing 2 changed files with 11 additions and 2 deletions.
diff --git a/ryu/s2d.c b/ryu/s2d.c
@@ -246,11 +246,13 @@ enum Status s2d_n(const char * buffer, const int len, double * result) {
   printf("ieee_m2 = %" PRIu64 "\n", (m2 >> shift) + roundUp);
 #endif
   uint64_t ieee_m2 = (m2 >> shift) + roundUp;
-  if (ieee_m2 == (1ull << (DOUBLE_MANTISSA_BITS + 1))) {
+  assert(ieee_m2 <= (1ull << (DOUBLE_MANTISSA_BITS + 1)));
+  ieee_m2 &= (1ull << DOUBLE_MANTISSA_BITS) - 1;
+  if (ieee_m2 == 0 && roundUp) {
     // Due to how the IEEE represents +/-Infinity, we don't need to check for overflow here.
     ieee_e2++;
   }
-  ieee_m2 &= (1ull << DOUBLE_MANTISSA_BITS) - 1;
+
   uint64_t ieee = (((((uint64_t) signedM) << DOUBLE_EXPONENT_BITS) | (uint64_t)ieee_e2) << DOUBLE_MANTISSA_BITS) | ieee_m2;
   *result = int64Bits2Double(ieee);
   return SUCCESS;

diff --git a/ryu/tests/s2d_test.cc b/ryu/tests/s2d_test.cc
@@ -90,3 +90,10 @@ TEST(S2dTest, TableSizeDenormal) {
 TEST(S2dTest, Issue157) {
   EXPECT_S2D(1.2999999999999999E+154, "1.2999999999999999E+154");
 }
+
+TEST(S2dTest, Issue173) {
+	// Denormal boundary
+	EXPECT_S2D(2.2250738585072012e-308, "2.2250738585072012e-308");
+	EXPECT_S2D(2.2250738585072013e-308, "2.2250738585072013e-308");
+	EXPECT_S2D(2.2250738585072014e-308, "2.2250738585072014e-308");
+}