commit c9fc4213198b67c1c06fba04e4bb8eb0250e8235
parent b9c6050de5c03c55b7b1adb6626c01743abcea41
Author: Evgeny Grin (Karlson2k) <k2k@drgrin.dev>
Date: Sat, 30 Aug 2025 18:24:19 +0200
Implemented macros for compiler hints
Diffstat:
3 files changed, 249 insertions(+), 0 deletions(-)
diff --git a/configure.ac b/configure.ac
@@ -3142,6 +3142,55 @@ AC_CACHE_CHECK([[whether __builtin_bswap64() is available]],
])
AS_IF([[test "x$mhd_cv_func___builtin_bswap64_avail" = "xyes"]],
[AC_DEFINE([[MHD_HAVE___BUILTIN_BSWAP64]], [[1]], [Define to 1 if you have __builtin_bswap64() builtin function])])
+
+AC_CACHE_CHECK([whether $CC supports __builtin_expect_with_probability()],[mhd_cv_cc___builtin_expect_with_probability_avail],
+ [
+ AC_LINK_IFELSE(
+ [
+ AC_LANG_SOURCE(
+ [[
+int main(int argc, char *argv[])
+{
+ (void) argv;
+ if (__builtin_expect_with_probability(argc < 1, 0, 0.9999)) return 1;
+ return 0;
+}
+ ]]
+ )
+ ],
+ [[mhd_cv_cc___builtin_expect_with_probability_avail="yes"]],
+ [[mhd_cv_cc___builtin_expect_with_probability_avail="no"]]
+ )
+ ]
+)
+AS_VAR_IF([mhd_cv_cc___builtin_expect_with_probability_avail],["yes"],
+ [AC_DEFINE([[MHD_HAVE___BUILTIN_EXPECT_WITH_PROBABILITY]], [[1]], [Define to 1 if compiler supports __builtin_expect_with_probability() builtin function])],
+ [
+ AC_CACHE_CHECK([whether $CC supports __builtin_expect()],[mhd_cv_cc___builtin_expect_avail],
+ [
+ AC_LINK_IFELSE(
+ [
+ AC_LANG_SOURCE(
+ [[
+int main(int argc, char *argv[])
+{
+ (void) argv;
+ if (__builtin_expect(argc < 1, 0)) return 1;
+ return 0;
+}
+ ]]
+ )
+ ],
+ [[mhd_cv_cc___builtin_expect_avail="yes"]],
+ [[mhd_cv_cc___builtin_expect_avail="no"]]
+ )
+ ]
+ )
+ AS_VAR_IF([mhd_cv_cc___builtin_expect_avail],["yes"],
+ [AC_DEFINE([[MHD_HAVE___BUILTIN_EXPECT]], [[1]], [Define to 1 if compiler supports __builtin_expect() builtin function])]
+ )
+ ]
+)
AC_CHECK_PROG([HAVE_CURL_BINARY],[curl],[yes],[no])
AM_CONDITIONAL([HAVE_CURL_BINARY],[test "x$HAVE_CURL_BINARY" = "xyes"])
diff --git a/src/mhd2/Makefile.am b/src/mhd2/Makefile.am
@@ -36,6 +36,7 @@ libmicrohttpd2_la_SOURCES = \
sys_w32_ver.h \
mhd_align.h mhd_bithelpers.h mhd_byteorder.h \
mhd_constexpr.h mhd_assert.h mhd_unreachable.h \
+ mhd_predict.h \
mhd_cntnr_ptr.h mhd_arr_num_elems.h \
mhd_tristate.h mhd_status_code_int.h \
mhd_socket_type.h mhd_sockets_macros.h \
diff --git a/src/mhd2/mhd_predict.h b/src/mhd2/mhd_predict.h
@@ -0,0 +1,199 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later OR (GPL-2.0-or-later WITH eCos-exception-2.0) */
+/*
+ This file is part of GNU libmicrohttpd.
+ Copyright (C) 2025 Evgeny Grin (Karlson2k)
+
+ GNU libmicrohttpd is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ GNU libmicrohttpd is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ Alternatively, you can redistribute GNU libmicrohttpd and/or
+ modify it under the terms of the GNU General Public License as
+ published by the Free Software Foundation; either version 2 of
+ the License, or (at your option) any later version, together
+ with the eCos exception, as follows:
+
+ As a special exception, if other files instantiate templates or
+ use macros or inline functions from this file, or you compile this
+ file and link it with other works to produce a work based on this
+ file, this file does not by itself cause the resulting work to be
+ covered by the GNU General Public License. However the source code
+ for this file must still be made available in accordance with
+ section (3) of the GNU General Public License v2.
+
+ This exception does not invalidate any other reasons why a work
+ based on this file might be covered by the GNU General Public
+ License.
+
+ You should have received copies of the GNU Lesser General Public
+ License and the GNU General Public License along with this library;
+ if not, see <https://www.gnu.org/licenses/>.
+*/
+
+/**
+ * @file src/mhd2/mhd_predict.h
+ * @brief Macros that provide the compiler with hints about
+ * the anticipated outcomes of conditional expressions.
+ * @author Karlson2k (Evgeny Grin)
+ */
+
+#ifndef MHD_PREDICT_H
+#define MHD_PREDICT_H 1
+
+#include "mhd_sys_options.h"
+
+/**
+ * @defgroup mhd2_predict Branch prediction hints
+ *
+ * Internal helpers to annotate most probable branch outcomes.
+ *
+ * @warning Misusing prediction (marking the uncommon path as common) can harm
+ * performance. Measuring before and after is highly recommended.
+ *
+ * @see MHD_NO_PREDICT_COND
+ *
+ * @{
+ */
+
+
+/**
+ * @def MHD_NO_PREDICT_COND
+ *
+ * When defined, disables all prediction hints regardless of compiler
+ * support, turning the macros into plain boolean expressions.
+ *
+ * This can be useful for benchmarking or debugging to assess the impact of
+ * branch prediction hints on a given workload and compiler.
+ */
+
+#ifdef MHD_NO_PREDICT_COND
+# ifdef MHD_HAVE___BUILTIN_EXPECT_WITH_PROBABILITY
+# undef MHD_HAVE___BUILTIN_EXPECT_WITH_PROBABILITY
+# endif
+# ifdef MHD_HAVE___BUILTIN_EXPECT
+# undef MHD_HAVE___BUILTIN_EXPECT
+# endif
+#else
+# ifdef __has_builtin
+# if ! defined(MHD_HAVE___BUILTIN_EXPECT_WITH_PROBABILITY) && \
+ __has_builtin (__builtin_expect_with_probability)
+# define MHD_HAVE___BUILTIN_EXPECT_WITH_PROBABILITY 1
+# endif
+# if ! defined(MHD_HAVE___BUILTIN_EXPECT) && __has_builtin (__builtin_expect)
+# define MHD_HAVE___BUILTIN_EXPECT 1
+# endif
+# endif
+#endif
+
+
+/*
+ * True/false primitives with direct probability specification
+ */
+
+/**
+ * @def mhd_COND_PRED_TRUE_P(cond, prob)
+ *
+ * Hint that @p cond is expected to be true with probability @p prob.
+ *
+ * @param cond Boolean-like expression; evaluated exactly once.
+ * @param prob Anticipated probability in the range [0.0, 1.0].
+ * Only compile-time constants; avoid 0.0 or 1.0.
+ *
+ * Expands to compiler-specific intrinsics when available; otherwise to
+ * a boolean normalisation of @p cond.
+ *
+ * @see mhd_COND_PRED_FALSE_P
+ */
+
+/**
+ * @def mhd_COND_PRED_FALSE_P(cond, prob)
+ *
+ * Hint that @p cond is expected to be false with probability @p prob.
+ *
+ * @param cond Boolean-like expression; evaluated exactly once.
+ * @param prob Anticipated probability in the range [0.0, 1.0].
+ * Only compile-time constants; avoid 0.0 or 1.0.
+ *
+ * Expands to compiler-specific intrinsics when available; otherwise to
+ * a boolean normalisation of @p cond.
+ *
+ * @see mhd_COND_PRED_TRUE_P
+ */
+
+
+#if defined(MHD_HAVE___BUILTIN_EXPECT_WITH_PROBABILITY)
+# define mhd_COND_PRED_TRUE_P(cond,prob) \
+ __builtin_expect_with_probability (! ! (cond), ! 0, (prob))
+# define mhd_COND_PRED_FALSE_P(cond,prob) \
+ __builtin_expect_with_probability (! ! (cond), 0, (prob))
+#elif defined(MHD_HAVE___BUILTIN_EXPECT)
+# define mhd_COND_PRED_TRUE_P(cond,prob) __builtin_expect (! ! (cond), ! 0)
+# define mhd_COND_PRED_FALSE_P(cond,prob) __builtin_expect (! ! (cond), 0)
+#else
+# define mhd_COND_PRED_TRUE_P(cond,prob) (! ! (cond))
+# define mhd_COND_PRED_FALSE_P(cond,prob) (! ! (cond))
+#endif
+
+/*
+ * Implementation notes:
+ * - The double negation (!!) normalises any non-zero to 1 and keeps 0 as 0,
+ * ensuring the 'expected' value matches the intrinsic's contract.
+ * - The literal '!0' expresses boolean true without implying an integer
+ * width.
+ */
+
+
+/*
+ * Readable convenience wrappers
+ */
+
+/**
+ * Hint: @p cond is true ~99.999% of the time.
+ *
+ * Useful for conditions whose failure indicates exceptional or defensive
+ * paths that almost never triggered in normal operation.
+ */
+#define mhd_COND_VIRTUALLY_ALWAYS(cond) mhd_COND_PRED_TRUE_P ((cond), 0.99999)
+/**
+ * Hint: @p cond is true ~99% of the time.
+ */
+#define mhd_COND_ALMOST_ALWAYS(cond) mhd_COND_PRED_TRUE_P ((cond), 0.99)
+/**
+ * Hint: @p cond is true ~95% of the time.
+ */
+#define mhd_COND_PREDOMINANTLY(cond) mhd_COND_PRED_TRUE_P ((cond), 0.95)
+/**
+ * Hint: @p cond is true ~90% of the time.
+ */
+#define mhd_COND_USUALLY(cond) mhd_COND_PRED_TRUE_P ((cond), 0.9)
+
+/**
+ * Hint: @p cond is false ~90% of the time (i.e., the true branch is rare).
+ */
+#define mhd_COND_RARELY(cond) mhd_COND_PRED_FALSE_P ((cond), 0.9)
+/**
+ * Hint: @p cond is false ~95% of the time (i.e., the true branch is very rare).
+ */
+#define mhd_COND_VERY_RARELY(cond) mhd_COND_PRED_FALSE_P ((cond), 0.95)
+/**
+ * Hint: @p cond is false ~99% of the time (i.e., the true branch almost never
+ * triggers).
+ */
+#define mhd_COND_ALMOST_NEVER(cond) mhd_COND_PRED_FALSE_P ((cond), 0.99)
+/**
+ * Hint: @p cond is false ~99.999% of the time.
+ *
+ * Useful for conditions whose truth indicates exceptional or defensive
+ * paths that almost never triggered in normal operation.
+ */
+#define mhd_COND_HARDLY_EVER(cond) mhd_COND_PRED_FALSE_P ((cond), 0.99999)
+
+/** @} */ /* end of mhd2_predict group */
+
+#endif /* ! MHD_PREDICT_H */
