libstdc++: Another merge from fast_float upstream [PR107468]

Upstream fast_float came up with a cheaper test for
fegetround () == FE_TONEAREST using one float addition, one subtraction
and one comparison.  If we know we are rounding to nearest, we can use
fast path in more cases as before.
The following patch merges those changes into libstdc++.

2022-11-24  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/107468
	* src/c++17/fast_float/fast_float.h: Partial merge from fast_float
	2ef9abbcf6a11958b6fa685a89d0150022e82e78 commit.

(cherry picked from commit ec73b55c75baa16c1cf7482fa65928a8d45598d4)

diff --git a/libstdc++-v3/src/c++17/fast_float/fast_float.h b/libstdc++-v3/src/c++17/fast_float/fast_float.h
index 5e58468d996e5a2cd1c51af0132eb33453d189b2..18aadb18c573fb246834639c6148140e5d53d3c7 100644 (file)
--- a/libstdc++-v3/src/c++17/fast_float/fast_float.h
+++ b/libstdc++-v3/src/c++17/fast_float/fast_float.h
@@ -357,16 +357,34 @@ template <typename T> struct binary_format {
   static inline constexpr int minimum_exponent();
   static inline constexpr int infinite_power();
   static inline constexpr int sign_index();
+  static inline constexpr int min_exponent_fast_path(); // used when fegetround() == FE_TONEAREST
   static inline constexpr int max_exponent_fast_path();
   static inline constexpr int max_exponent_round_to_even();
   static inline constexpr int min_exponent_round_to_even();
   static inline constexpr uint64_t max_mantissa_fast_path(int64_t power);
+  static inline constexpr uint64_t max_mantissa_fast_path(); // used when fegetround() == FE_TONEAREST
   static inline constexpr int largest_power_of_ten();
   static inline constexpr int smallest_power_of_ten();
   static inline constexpr T exact_power_of_ten(int64_t power);
   static inline constexpr size_t max_digits();
 };
 
+template <> inline constexpr int binary_format<double>::min_exponent_fast_path() {
+#if (FLT_EVAL_METHOD != 1) && (FLT_EVAL_METHOD != 0)
+  return 0;
+#else
+  return -22;
+#endif
+}
+
+template <> inline constexpr int binary_format<float>::min_exponent_fast_path() {
+#if (FLT_EVAL_METHOD != 1) && (FLT_EVAL_METHOD != 0)
+  return 0;
+#else
+  return -10;
+#endif
+}
+
 template <> inline constexpr int binary_format<double>::mantissa_explicit_bits() {
   return 52;
 }
@@ -413,13 +431,18 @@ template <> inline constexpr int binary_format<double>::max_exponent_fast_path()
 template <> inline constexpr int binary_format<float>::max_exponent_fast_path() {
   return 10;
 }
-
+template <> inline constexpr uint64_t binary_format<double>::max_mantissa_fast_path() {
+  return uint64_t(2) << mantissa_explicit_bits();
+}
 template <> inline constexpr uint64_t binary_format<double>::max_mantissa_fast_path(int64_t power) {
   // caller is responsible to ensure that
   // power >= 0 && power <= 22
   //
   return max_mantissa_double[power];
 }
+template <> inline constexpr uint64_t binary_format<float>::max_mantissa_fast_path() {
+  return uint64_t(2) << mantissa_explicit_bits();
+}
 template <> inline constexpr uint64_t binary_format<float>::max_mantissa_fast_path(int64_t power) {
   // caller is responsible to ensure that
   // power >= 0 && power <= 10
@@ -2876,6 +2899,48 @@ from_chars_result parse_infnan(const char *first, const char *last, T &value)  n
   return answer;
 }
 
+/**
+ * Returns true if the floating-pointing rounding mode is to 'nearest'.
+ * It is the default on most system. This function is meant to be inexpensive.
+ * Credit : @mwalcott3
+ */
+fastfloat_really_inline bool rounds_to_nearest() noexcept {
+  // See
+  // A fast function to check your floating-point rounding mode
+  // https://lemire.me/blog/2022/11/16/a-fast-function-to-check-your-floating-point-rounding-mode/
+  //
+  // This function is meant to be equivalent to :
+  // prior: #include <cfenv>
+  //  return fegetround() == FE_TONEAREST;
+  // However, it is expected to be much faster than the fegetround()
+  // function call.
+  //
+  // The volatile keywoard prevents the compiler from computing the function
+  // at compile-time.
+  // There might be other ways to prevent compile-time optimizations (e.g., asm).
+  // The value does not need to be std::numeric_limits<float>::min(), any small
+  // value so that 1 + x should round to 1 would do (after accounting for excess
+  // precision, as in 387 instructions).
+  static volatile float fmin = std::numeric_limits<float>::min();
+  float fmini = fmin; // we copy it so that it gets loaded at most once.
+  //
+  // Explanation:
+  // Only when fegetround() == FE_TONEAREST do we have that
+  // fmin + 1.0f == 1.0f - fmin.
+  //
+  // FE_UPWARD:
+  //  fmin + 1.0f > 1
+  //  1.0f - fmin == 1
+  //
+  // FE_DOWNWARD or  FE_TOWARDZERO:
+  //  fmin + 1.0f == 1
+  //  1.0f - fmin < 1
+  //
+  // Note: This may fail to be accurate if fast-math has been
+  // enabled, as rounding conventions may not apply.
+  return (fmini + 1.0f == 1.0f - fmini);
+}
+
 } // namespace detail
 
 template<typename T>
@@ -2903,12 +2968,45 @@ from_chars_result from_chars_advanced(const char *first, const char *last,
   }
   answer.ec = std::errc(); // be optimistic
   answer.ptr = pns.lastmatch;
-  // Next is a modified Clinger's fast path, inspired by Jakub Jelínek's proposal
-  if (pns.exponent >= 0 && pns.exponent <= binary_format<T>::max_exponent_fast_path() && pns.mantissa <=binary_format<T>::max_mantissa_fast_path(pns.exponent) && !pns.too_many_digits) {
-    value = T(pns.mantissa);
-    value = value * binary_format<T>::exact_power_of_ten(pns.exponent);
-    if (pns.negative) { value = -value; }
-    return answer;
+  // The implementation of the Clinger's fast path is convoluted because
+  // we want round-to-nearest in all cases, irrespective of the rounding mode
+  // selected on the thread.
+  // We proceed optimistically, assuming that detail::rounds_to_nearest() returns
+  // true.
+  if (binary_format<T>::min_exponent_fast_path() <= pns.exponent && pns.exponent <= binary_format<T>::max_exponent_fast_path() && !pns.too_many_digits) {
+    // Unfortunately, the conventional Clinger's fast path is only possible
+    // when the system rounds to the nearest float.
+    //
+    // We expect the next branch to almost always be selected.
+    // We could check it first (before the previous branch), but
+    // there might be performance advantages at having the check
+    // be last.
+    if(detail::rounds_to_nearest())  {
+      // We have that fegetround() == FE_TONEAREST.
+      // Next is Clinger's fast path.
+      if (pns.mantissa <=binary_format<T>::max_mantissa_fast_path()) {
+        value = T(pns.mantissa);
+        if (pns.exponent < 0) { value = value / binary_format<T>::exact_power_of_ten(-pns.exponent); }
+        else { value = value * binary_format<T>::exact_power_of_ten(pns.exponent); }
+        if (pns.negative) { value = -value; }
+        return answer;
+      }
+    } else {
+      // We do not have that fegetround() == FE_TONEAREST.
+      // Next is a modified Clinger's fast path, inspired by Jakub Jelínek's proposal
+      if (pns.exponent >= 0 && pns.mantissa <=binary_format<T>::max_mantissa_fast_path(pns.exponent)) {
+#if (defined(_WIN32) && defined(__clang__))
+        // ClangCL may map 0 to -0.0 when fegetround() == FE_DOWNWARD
+        if(pns.mantissa == 0) {
+          value = 0;
+          return answer;
+        }
+#endif
+        value = T(pns.mantissa) * binary_format<T>::exact_power_of_ten(pns.exponent);
+        if (pns.negative) { value = -value; }
+        return answer;
+      }
+    }
   }
   adjusted_mantissa am = compute_float<binary_format<T>>(pns.exponent, pns.mantissa);
   if(pns.too_many_digits && am.power2 >= 0) {