Using unlikely markers for PR386

include/fast_float/ascii_number.h

@@ -330,10 +330,17 @@ report_parse_error(UC const *p, parse_error error) {
 
 // Assuming that you use no more than 19 digits, this will
 // parse an ASCII string.
+//
+// store_spans is a *runtime* flag (not a template parameter, deliberately: a
+// template would create a second instantiation of this whole function and the
+// extra icache pressure wipes out the gain). When false, the integer/fraction
+// spans (read only by the rare digit_comp slow path) are not materialized,
+// which keeps the fat parsed_number_string_t off the hot path. The caller
+// re-parses with store_spans=true if the slow path is actually reached.
 template <bool basic_json_fmt, typename UC>
 fastfloat_really_inline FASTFLOAT_CONSTEXPR20 parsed_number_string_t<UC>
-parse_number_string(UC const *p, UC const *pend,
-                    parse_options_t<UC> options) noexcept {
+parse_number_string(UC const *p, UC const *pend, parse_options_t<UC> options,
+                    bool store_spans = true) noexcept {
   chars_format const fmt = detail::adjust_for_feature_macros(options.format);
   UC const decimal_point = options.decimal_point;
 
@@ -402,7 +409,9 @@ parse_number_string(UC const *p, UC const *pend,
   }
   UC const *const end_of_integer_part = p;
   int64_t digit_count = int64_t(end_of_integer_part - start_digits);
-  answer.integer = span<UC const>(start_digits, size_t(digit_count));
+  if (store_spans) {
+    answer.integer = span<UC const>(start_digits, size_t(digit_count));
+  }
   FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
     // at least 1 digit in integer part, without leading zeros
     if (digit_count == 0) {
@@ -429,7 +438,9 @@ parse_number_string(UC const *p, UC const *pend,
       i = i * 10 + digit; // in rare cases, this will overflow, but that's ok
     }
     exponent = before - p;
-    answer.fraction = span<UC const>(before, size_t(p - before));
+    if (store_spans) {
+      answer.fraction = span<UC const>(before, size_t(p - before));
+    }
     digit_count -= exponent;
   }
   FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
@@ -514,29 +525,35 @@ parse_number_string(UC const *p, UC const *pend,
 
     if (digit_count > 19) {
       answer.too_many_digits = true;
-      // Let us start again, this time, avoiding overflows.
-      // We don't need to call if is_integer, since we use the
-      // pre-tokenized spans from above.
-      i = 0;
-      p = answer.integer.ptr;
-      UC const *int_end = p + answer.integer.len();
-      uint64_t const minimal_nineteen_digit_integer{1000000000000000000};
-      while ((i < minimal_nineteen_digit_integer) && (p != int_end)) {
-        i = i * 10 + uint64_t(*p - UC('0'));
-        ++p;
-      }
-      if (i >= minimal_nineteen_digit_integer) { // We have a big integer
-        exponent = end_of_integer_part - p + exp_number;
-      } else { // We have a value with a fractional component.
-        p = answer.fraction.ptr;
-        UC const *frac_end = p + answer.fraction.len();
-        while ((i < minimal_nineteen_digit_integer) && (p != frac_end)) {
+      // The truncation recompute below reads the integer/fraction spans. When
+      // store_spans is false we didn't materialize them, so just flag
+      // too_many_digits; the caller re-parses with store_spans=true to obtain
+      // the corrected mantissa/exponent before taking the slow path.
+      if (store_spans) {
+        // Let us start again, this time, avoiding overflows.
+        // We don't need to call if is_integer, since we use the
+        // pre-tokenized spans from above.
+        i = 0;
+        p = answer.integer.ptr;
+        UC const *int_end = p + answer.integer.len();
+        uint64_t const minimal_nineteen_digit_integer{1000000000000000000};
+        while ((i < minimal_nineteen_digit_integer) && (p != int_end)) {
           i = i * 10 + uint64_t(*p - UC('0'));
           ++p;
         }
-        exponent = answer.fraction.ptr - p + exp_number;
+        if (i >= minimal_nineteen_digit_integer) { // We have a big integer
+          exponent = end_of_integer_part - p + exp_number;
+        } else { // We have a value with a fractional component.
+          p = answer.fraction.ptr;
+          UC const *frac_end = p + answer.fraction.len();
+          while ((i < minimal_nineteen_digit_integer) && (p != frac_end)) {
+            i = i * 10 + uint64_t(*p - UC('0'));
+            ++p;
+          }
+          exponent = answer.fraction.ptr - p + exp_number;
+        }
+        // We have now corrected both exponent and i, to a truncated value
       }
-      // We have now corrected both exponent and i, to a truncated value
     }
   }
   answer.exponent = exponent;

include/fast_float/float_common.h

@@ -197,6 +197,39 @@ using parse_options = parse_options_t<char>;
 #define fastfloat_really_inline inline __attribute__((always_inline))
 #endif
 
+// Branch-probability hint marking the rare slow-path branches as cold, so the
+// optimizer keeps the out-of-line slow-path re-parse off the hot path (and does
+// not duplicate the force-inlined hot scanner into the caller, which bloated
+// the hot frame and hurt ILP on some targets). Used at the call site as
+//   if fastfloat_unlikely(cond) { ... }
+// (the macro supplies the parentheses). It expands to the standard [[unlikely]]
+// attribute when supported, otherwise to __builtin_expect on GCC/Clang, or
+// to a no-op elsewhere (e.g. pre-C++20 MSVC, which has no equivalent hint).
+#ifdef __has_cpp_attribute
+#if __has_cpp_attribute(unlikely) >= 201803L
+#define FASTFLOAT_USE_UNLIKELY_ATTR 1
+#endif
+#endif
+
+#ifdef FASTFLOAT_USE_UNLIKELY_ATTR
+// We have to disable -Wc++20-extensions for the [[unlikely]] attribute
+// See comment for @jwakely at
+// https://github.com/fastfloat/fast_float/pull/387#discussion_r3366943539
+// This is unfortunate.
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wc++20-extensions"
+#endif
+#define fastfloat_unlikely(x) (x) [[unlikely]]
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+#elif defined(__GNUC__) || defined(__clang__)
+#define fastfloat_unlikely(x) (__builtin_expect(!!(x), 0))
+#else
+#define fastfloat_unlikely(x) (x)
+#endif
+
 #ifndef FASTFLOAT_ASSERT
 #define FASTFLOAT_ASSERT(x)                                                    \
   { ((void)(x)); }

include/fast_float/parse_number.h

@@ -289,6 +289,23 @@ from_chars_advanced(parsed_number_string_t<UC> &pns, T &value) noexcept {
   return answer;
 }
 
+// Slow path: re-parse materializing the integer/fraction spans the hot no-span
+// parse skipped, then run the full algorithm. The two callers reach it only
+// through a fastfloat_unlikely branch, so the optimizer keeps this re-parse off
+// the hot path on its own (no function-level noinline needed).
+// from_chars_advanced already handles both the too_many_digits disambiguation
+// and the am.power2<0 digit_comp recompute, so both slow branches collapse to
+// one helper call.
+template <typename T, typename UC>
+FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
+parse_number_slow_path(UC const *first, UC const *last, T &value,
+                       parse_options_t<UC> options, bool bjf) noexcept {
+  parsed_number_string_t<UC> pns =
+      bjf ? parse_number_string<true, UC>(first, last, options, true)
+          : parse_number_string<false, UC>(first, last, options, true);
+  return from_chars_advanced(pns, value);
+}
+
 template <typename T, typename UC>
 fastfloat_really_inline FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
 from_chars_float_advanced(UC const *first, UC const *last, T &value,
@@ -312,10 +329,15 @@ from_chars_float_advanced(UC const *first, UC const *last, T &value,
     answer.ptr = first;
     return answer;
   }
+  bool const bjf = uint64_t(fmt & detail::basic_json_fmt) != 0;
+
+  // Fast path: parse WITHOUT materializing the integer/fraction spans (read
+  // only by the rare slow paths). Skipping their stores keeps the fat
+  // parsed_number_string_t off the hot path. store_spans is a runtime argument,
+  // so this reuses the single parse_number_string instantiation.
   parsed_number_string_t<UC> pns =
-      uint64_t(fmt & detail::basic_json_fmt)
-          ? parse_number_string<true, UC>(first, last, options)
-          : parse_number_string<false, UC>(first, last, options);
+      bjf ? parse_number_string<true, UC>(first, last, options, false)
+          : parse_number_string<false, UC>(first, last, options, false);
   if (!pns.valid) {
     if (uint64_t(fmt & chars_format::no_infnan)) {
       answer.ec = std::errc::invalid_argument;
@@ -326,8 +348,43 @@ from_chars_float_advanced(UC const *first, UC const *last, T &value,
     }
   }
 
-  // call overload that takes parsed_number_string_t directly.
-  return from_chars_advanced(pns, value);
+  // Slow path A (rare): > 19 significant digits. The no-span parse left the
+  // mantissa un-truncated and skipped the span-based recompute; the cold helper
+  // re-parses with spans and runs the full algorithm.
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wc++20-extensions"
+#endif
+  if fastfloat_unlikely (pns.too_many_digits) {
+    return parse_number_slow_path<T, UC>(first, last, value, options, bjf);
+  }
+
+  answer.ec = std::errc(); // be optimistic
+  answer.ptr = pns.lastmatch;
+
+  if (clinger_fast_path_impl(pns.mantissa, pns.exponent, pns.negative, value)) {
+    return answer;
+  }
+
+  adjusted_mantissa am =
+      compute_float<binary_format<T>>(pns.exponent, pns.mantissa);
+  // Slow path B (rare): Eisel-Lemire could not resolve; digit_comp needs the
+  // integer/fraction spans. Route to the cold helper (clinger there is a
+  // dead-effect since it already failed here; the cold re-parse + digit_comp
+  // via from_chars_advanced reproduces this branch).
+  if fastfloat_unlikely (am.power2 < 0) {
+    return parse_number_slow_path<T, UC>(first, last, value, options, bjf);
+  }
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+  to_float(pns.negative, am, value);
+  // Test for over/underflow.
+  if ((pns.mantissa != 0 && am.mantissa == 0 && am.power2 == 0) ||
+      am.power2 == binary_format<T>::infinite_power()) {
+    answer.ec = std::errc::result_out_of_range;
+  }
+  return answer;
 }
 
 template <typename T, typename UC, typename>