EXP-063: test the mode-independent mantissa bound before the rounds_to_nearest probe

Author: fcostaoliveira

include/fast_float/parse_number.h

@@ -198,7 +198,14 @@ clinger_fast_path_impl(uint64_t mantissa, int64_t exponent, bool is_negative,
   // We proceed optimistically, assuming that detail::rounds_to_nearest()
   // returns true.
   if (binary_format<T>::min_exponent_fast_path() <= exponent &&
-      exponent <= binary_format<T>::max_exponent_fast_path()) {
+      exponent <= binary_format<T>::max_exponent_fast_path() &&
+      mantissa <= binary_format<T>::max_mantissa_fast_path()) {
+    // The mantissa bound above is a necessary condition for BOTH branches
+    // below: the rounding-mode-dependent branch checks the tighter
+    // max_mantissa_fast_path(exponent) <= max_mantissa_fast_path(). Testing
+    // it before detail::rounds_to_nearest() spares long-mantissa inputs
+    // (which can never take the fast path) the volatile-float probe.
+    //
     // Unfortunately, the conventional Clinger's fast path is only possible
     // when the system rounds to the nearest float.
     //
@@ -209,18 +216,16 @@ clinger_fast_path_impl(uint64_t mantissa, int64_t exponent, bool is_negative,
     if (!cpp20_and_in_constexpr() && detail::rounds_to_nearest()) {
       // We have that fegetround() == FE_TONEAREST.
       // Next is Clinger's fast path.
-      if (mantissa <= binary_format<T>::max_mantissa_fast_path()) {
-        value = T(mantissa);
-        if (exponent < 0) {
-          value = value / binary_format<T>::exact_power_of_ten(-exponent);
-        } else {
-          value = value * binary_format<T>::exact_power_of_ten(exponent);
-        }
-        if (is_negative) {
-          value = -value;
-        }
-        return true;
+      value = T(mantissa);
+      if (exponent < 0) {
+        value = value / binary_format<T>::exact_power_of_ten(-exponent);
+      } else {
+        value = value * binary_format<T>::exact_power_of_ten(exponent);
+      }
+      if (is_negative) {
+        value = -value;
       }
+      return true;
     } else {
       // We do not have that fegetround() == FE_TONEAREST.
       // Next is a modified Clinger's fast path, inspired by Jakub Jelínek's