Sync to ulfjack/ryu@688f43b

README.md

@@ -19,7 +19,7 @@ C, [https://github.com/ulfjack/ryu][upstream].
 uses nothing from the Rust standard library so is usable from no_std crates.*
 
 [paper]: https://dl.acm.org/citation.cfm?id=3192369
-[upstream]: https://github.com/ulfjack/ryu/tree/159ce150fa94e0229f3e02ecc6f4243f21388c0b
+[upstream]: https://github.com/ulfjack/ryu/tree/688f43b62276b400728baad54afc32c3ab9c1a95
 
 ```toml
 [dependencies]

src/d2s.rs

@@ -258,11 +258,10 @@ pub fn d2d(ieee_mantissa: u64, ieee_exponent: u32) -> FloatingDecimal64 {
             }
         } else if q < 63 {
             // TODO(ulfjack): Use a tighter bound here.
-            // We need to compute min(ntz(mv), pow5_factor(mv) - e2) >= q - 1
-            // <=> ntz(mv) >= q - 1  &&  pow5_factor(mv) - e2 >= q - 1
-            // <=> ntz(mv) >= q - 1    (e2 is negative and -e2 >= q)
-            // <=> (mv & ((1 << (q - 1)) - 1)) == 0
-            // We also need to make sure that the left shift does not overflow.
+            // We want to know if the full product has at least q trailing zeros.
+            // We need to compute min(p2(mv), p5(mv) - e2) >= q
+            // <=> p2(mv) >= q && p5(mv) - e2 >= q
+            // <=> p2(mv) >= q (because -e2 >= q)
             vr_is_trailing_zeros = multiple_of_power_of_2(mv, q);
         }
     }