diff --git a/CMakeLists.txt b/CMakeLists.txt
index cec5009de..1877ff8f1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -145,6 +145,7 @@ INCLUDE(CheckIncludeFile)
 INCLUDE(CheckTypeSize)
 INCLUDE(GNUInstallDirs) # for CMAKE_INSTALL_LIBDIR
 
+CHECK_INCLUDE_FILE(assert.h     HAVE_ASSERT_H)
 CHECK_INCLUDE_FILE(dirent.h     HAVE_DIRENT_H)
 CHECK_INCLUDE_FILE(sys/stat.h   HAVE_SYS_STAT_H)
 CHECK_INCLUDE_FILE(sys/types.h  HAVE_SYS_TYPES_H)
@@ -169,12 +170,22 @@ if(NOT MSVC AND NOT CMAKE_C_COMPILER_ID STREQUAL "XL")
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror")
 endif()
 
+CHECK_C_SOURCE_COMPILES(
+  "int main(void) { __builtin_expect(1+1, 2); return 0; }"
+  HAVE_BUILTIN_EXPECT
+)
+
 CHECK_C_SOURCE_COMPILES(
   "#include <stddef.h>
    int main(void) { int a,b; size_t m; __builtin_mul_overflow(a,b,&m); return 0; }"
   HAVE_BUILTIN_MUL_OVERFLOW
 )
 
+CHECK_C_SOURCE_COMPILES(
+  "int main(void) { if (0) __builtin_unreachable(); return 0; }"
+  HAVE_BUILTIN_UNREACHABLE
+)
+
 CHECK_C_SOURCE_COMPILES(
   "int main(void) { char buf[128] __attribute__((uninitialized)); (void)buf; return 0; }"
   HAVE_ATTRIBUTE_UNINITIALIZED
diff --git a/config-cmake.h.in b/config-cmake.h.in
index 6539d7735..f0091b225 100644
--- a/config-cmake.h.in
+++ b/config-cmake.h.in
@@ -1,6 +1,9 @@
 /* config.h for CMake builds */
 
+#cmakedefine HAVE_ASSERT_H 1
+#cmakedefine HAVE_BUILTIN_EXPECT 1
 #cmakedefine HAVE_BUILTIN_MUL_OVERFLOW 1
+#cmakedefine HAVE_BUILTIN_UNREACHABLE 1
 #cmakedefine HAVE_ATTRIBUTE_UNINITIALIZED 1
 #cmakedefine HAVE_DIRENT_H 1
 #cmakedefine HAVE_SYS_STAT_H 1
diff --git a/configure.ac b/configure.ac
index 5646c5a2b..74aa7de44 100644
--- a/configure.ac
+++ b/configure.ac
@@ -73,6 +73,20 @@ AC_SYS_LARGEFILE
 
 PCRE2_VISIBILITY
 
+# Check for the expect() builtin
+
+AC_MSG_CHECKING([for __builtin_expect()])
+AC_LANG_PUSH([C])
+AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[__builtin_expect(1+1, 2)]])],
+  [pcre2_cc_cv_builtin_expect=yes],
+  [pcre2_cc_cv_builtin_expect=no])
+AC_MSG_RESULT([$pcre2_cc_cv_builtin_expect])
+if test "$pcre2_cc_cv_builtin_expect" = yes; then
+  AC_DEFINE([HAVE_BUILTIN_EXPECT], 1,
+    [Define this if your compiler provides __builtin_expect()])
+fi
+AC_LANG_POP([C])
+
 # Check for the mul_overflow() builtin
 
 AC_MSG_CHECKING([for __builtin_mul_overflow()])
@@ -95,6 +109,20 @@ if test "$pcre2_cc_cv_builtin_mul_overflow" = yes; then
 fi
 AC_LANG_POP([C])
 
+# Check for the unreachable() builtin
+
+AC_MSG_CHECKING([for __builtin_unreachable()])
+AC_LANG_PUSH([C])
+AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[if (0) __builtin_unreachable()]])],
+  [pcre2_cc_cv_builtin_unreachable=yes],
+  [pcre2_cc_cv_builtin_unreachable=no])
+AC_MSG_RESULT([$pcre2_cc_cv_builtin_unreachable])
+if test "$pcre2_cc_cv_builtin_unreachable" = yes; then
+  AC_DEFINE([HAVE_BUILTIN_UNREACHABLE], 1,
+    [Define this if your compiler provides __builtin_unreachable()])
+fi
+AC_LANG_POP([C])
+
 # Check for Clang __attribute__((uninitialized)) feature
 
 AC_MSG_CHECKING([for __attribute__((uninitialized))])
@@ -521,7 +549,7 @@ HAVE_BCOPY is defined. If your system has neither bcopy() nor memmove(), make
 sure both macros are undefined; an emulation function will then be used. */])
 
 # Checks for header files.
-AC_CHECK_HEADERS(limits.h sys/types.h sys/stat.h dirent.h)
+AC_CHECK_HEADERS(assert.h limits.h sys/types.h sys/stat.h dirent.h)
 AC_CHECK_HEADERS([windows.h], [HAVE_WINDOWS_H=1])
 AC_CHECK_HEADERS([sys/wait.h], [HAVE_SYS_WAIT_H=1])
 
diff --git a/src/config.h.generic b/src/config.h.generic
index 009294813..089e1f5ff 100644
--- a/src/config.h.generic
+++ b/src/config.h.generic
@@ -52,15 +52,24 @@ sure both macros are undefined; an emulation function will then be used. */
    LF does in an ASCII/Unicode environment. */
 /* #undef EBCDIC_NL25 */
 
+/* Define to 1 if you have the <assert.h> header file. */
+/* #undef HAVE_ASSERT_H */
+
 /* Define this if your compiler supports __attribute__((uninitialized)) */
 /* #undef HAVE_ATTRIBUTE_UNINITIALIZED */
 
 /* Define to 1 if you have the 'bcopy' function. */
 /* #undef HAVE_BCOPY */
 
+/* Define this if your compiler provides __builtin_expect() */
+/* #undef HAVE_BUILTIN_EXPECT */
+
 /* Define this if your compiler provides __builtin_mul_overflow() */
 /* #undef HAVE_BUILTIN_MUL_OVERFLOW */
 
+/* Define this if your compiler provides __builtin_unreachable() */
+/* #undef HAVE_BUILTIN_UNREACHABLE */
+
 /* Define to 1 if you have the <bzlib.h> header file. */
 /* #undef HAVE_BZLIB_H */
 
diff --git a/src/config.h.in b/src/config.h.in
index 3bb01c83d..7642d65c7 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -52,15 +52,24 @@ sure both macros are undefined; an emulation function will then be used. */
    LF does in an ASCII/Unicode environment. */
 #undef EBCDIC_NL25
 
+/* Define to 1 if you have the <assert.h> header file. */
+#undef HAVE_ASSERT_H
+
 /* Define this if your compiler supports __attribute__((uninitialized)) */
 #undef HAVE_ATTRIBUTE_UNINITIALIZED
 
 /* Define to 1 if you have the 'bcopy' function. */
 #undef HAVE_BCOPY
 
+/* Define this if your compiler provides __builtin_expect() */
+#undef HAVE_BUILTIN_EXPECT
+
 /* Define this if your compiler provides __builtin_mul_overflow() */
 #undef HAVE_BUILTIN_MUL_OVERFLOW
 
+/* Define this if your compiler provides __builtin_unreachable() */
+#undef HAVE_BUILTIN_UNREACHABLE
+
 /* Define to 1 if you have the <bzlib.h> header file. */
 #undef HAVE_BZLIB_H
 
diff --git a/src/pcre2.h.generic b/src/pcre2.h.generic
index 4ef0793a8..baa709512 100644
--- a/src/pcre2.h.generic
+++ b/src/pcre2.h.generic
@@ -1002,6 +1002,29 @@ PCRE2_SUFFIX a no-op. Otherwise, generate an error. */
 #endif
 #endif  /* PCRE2_CODE_UNIT_WIDTH is defined */
 
+/* Assertion macros */
+
+#ifdef HAVE_BUILTIN_UNREACHABLE
+#define PCRE2_UNREACHABLE() __builtin_unreachable()
+#else
+#define PCRE2_UNREACHABLE() do {} while(0)
+#endif
+
+#ifdef PCRE2_DEBUG
+#if defined(HAVE_BUILTIN_EXPECT) && defined(HAVE_BUILTIN_UNREACHABLE)
+#define PCRE2_ASSERT(x) do { if (__builtin_expect(!(x), 0)) __builtin_unreachable(); } while (0)
+#elif defined(HAVE_ASSERT_H)
+#include <assert.h>
+#define PCRE2_ASSERT(x) assert(x)
+#elif defined(HAVE_STDLIB_H) && defined(HAVE_STDIO_H)
+#define PCRE2_ASSERT(x) do { if (!(x)) { fprintf(stderr, "Assertion failed at " __FILE__ ":%d\n", __LINE__); abort(); }} while(0)
+#else
+#define PCRE2_ASSERT(x) do {} while(0)
+#endif
+#else
+#define PCRE2_ASSERT(x) do {} while(0)
+#endif
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif
diff --git a/src/pcre2.h.in b/src/pcre2.h.in
index a13362daf..2f6b4e866 100644
--- a/src/pcre2.h.in
+++ b/src/pcre2.h.in
@@ -1003,6 +1003,29 @@ PCRE2_SUFFIX a no-op. Otherwise, generate an error. */
 #endif
 #endif  /* PCRE2_CODE_UNIT_WIDTH is defined */
 
+/* Assertion macros */
+
+#ifdef HAVE_BUILTIN_UNREACHABLE
+#define PCRE2_UNREACHABLE() __builtin_unreachable()
+#else
+#define PCRE2_UNREACHABLE() do {} while(0)
+#endif
+
+#ifdef PCRE2_DEBUG
+#if defined(HAVE_BUILTIN_EXPECT) && defined(HAVE_BUILTIN_UNREACHABLE)
+#define PCRE2_ASSERT(x) do { if (__builtin_expect(!(x), 0)) __builtin_unreachable(); } while (0)
+#elif defined(HAVE_ASSERT_H)
+#include <assert.h>
+#define PCRE2_ASSERT(x) assert(x)
+#elif defined(HAVE_STDLIB_H) && defined(HAVE_STDIO_H)
+#define PCRE2_ASSERT(x) do { if (!(x)) { fprintf(stderr, "Assertion failed at " __FILE__ ":%d\n", __LINE__); abort(); }} while(0)
+#else
+#define PCRE2_ASSERT(x) do {} while(0)
+#endif
+#else
+#define PCRE2_ASSERT(x) do {} while(0)
+#endif
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif
diff --git a/vms/configure.com b/vms/configure.com
index b50365d4f..c6024e87c 100644
--- a/vms/configure.com
+++ b/vms/configure.com
@@ -496,12 +496,20 @@ sure both macros are undefined; an emulation function will then be used. */
 /* Define this if your compiler supports __attribute__((uninitialized)) */
 #undef HAVE_ATTRIBUTE_UNINITIALIZED
 
+/* Define to 1 if you have the <assert.h> header file. */
+#define HAVE_ASSERT_H 1
+
 /* Define to 1 if you have the 'bcopy' function. */
 #define HAVE_BCOPY 1
 
+/* Define this if your compiler provides __builtin_expect() */
+#undef HAVE_BUILTIN_EXPECT
+
 /* Define this if your compiler provides __builtin_mul_overflow() */
 #undef HAVE_BUILTIN_MUL_OVERFLOW
 
+/* Define this if your compiler provides __builtin_unreachable() */
+#undef HAVE_BUILTIN_UNREACHABLE
 
 /* Define to 1 if you have the <dirent.h> header file. */
 #define HAVE_DIRENT_H 1