diff --git a/CMakeLists.txt b/CMakeLists.txt
index 301a713..baa8890 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -87,6 +87,7 @@
 # 2020-03-16 PH renamed dftables as pcre2_dftables (as elsewhere)
 # 2020-03-24 PH changed CMAKE_MODULE_PATH definition to add, not replace
 # 2020-04-08 Carlo added function check for secure_getenv, fixed strerror
+# 2020-04-16 enh added check for __attribute__((uninitialized))
 
 PROJECT(PCRE2 C)
 
@@ -113,8 +114,9 @@ FIND_PACKAGE( Editline )
 
 # Configuration checks
 
-INCLUDE(CheckIncludeFile)
+INCLUDE(CheckCSourceCompiles)
 INCLUDE(CheckFunctionExists)
+INCLUDE(CheckIncludeFile)
 INCLUDE(CheckTypeSize)
 
 CHECK_INCLUDE_FILE(dirent.h     HAVE_DIRENT_H)
@@ -130,6 +132,14 @@ CHECK_FUNCTION_EXISTS(memmove       HAVE_MEMMOVE)
 CHECK_FUNCTION_EXISTS(strerror      HAVE_STRERROR)
 CHECK_FUNCTION_EXISTS(secure_getenv HAVE_SECURE_GETENV)
 
+set(ORIG_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror")
+CHECK_C_SOURCE_COMPILES(
+  "int main() { char buf[128] __attribute__((uninitialized)); (void)buf; return 0; }"
+  HAVE_ATTRIBUTE_UNINITIALIZED
+)
+set(CMAKE_REQUIRED_FLAGS ${ORIG_CMAKE_REQUIRED_FLAGS})
+
 # User-configurable options
 #
 # Note: CMakeSetup displays these in alphabetical order, regardless of
diff --git a/ChangeLog b/ChangeLog
index f9663b2..546e740 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -104,6 +104,12 @@ LIST(APPEND...) to allow a setting from the command line to be included.
 25. Avoid using [-1] as a suffix in pcre2test because it can provoke a compiler 
 warning.
 
+26. Added tests for __attribute__((uninitialized)) to both the configure and 
+CMake build files, and then applied this attribute to the variable called 
+stack_frames_vector[] in pcre2_match(). When implemented, this disables 
+automatic initialization (a facility in clang), which can take time on big
+variables.
+
 
 Version 10.34 21-November-2019
 ------------------------------
diff --git a/config-cmake.h.in b/config-cmake.h.in
index 108f86c..3c5e0d3 100644
--- a/config-cmake.h.in
+++ b/config-cmake.h.in
@@ -1,5 +1,6 @@
 /* config.h for CMake builds */
 
+#cmakedefine HAVE_ATTRIBUTE_UNINITIALIZED 1
 #cmakedefine HAVE_DIRENT_H 1
 #cmakedefine HAVE_INTTYPES_H 1    
 #cmakedefine HAVE_STDINT_H 1                                                   
diff --git a/configure.ac b/configure.ac
index 4553fcd..0213d8e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -10,7 +10,7 @@ dnl be defined as -RC2, for example. For real releases, it should be empty.
 
 m4_define(pcre2_major, [10])
 m4_define(pcre2_minor, [35])
-m4_define(pcre2_prerelease, [-RC1])
+m4_define(pcre2_prerelease, [-RC2])
 m4_define(pcre2_date, [2020-04-15])
 
 # NOTE: The CMakeLists.txt file searches for the above variables in the first
@@ -72,6 +72,24 @@ AC_PROG_LN_S
 
 PCRE2_VISIBILITY
 
+# Check for Clang __attribute__((uninitialized)) feature
+
+AC_MSG_CHECKING([for __attribute__((uninitialized))])
+AC_LANG_PUSH([C])
+tmp_CFLAGS=$CFLAGS
+CFLAGS="$CFLAGS -Werror"
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM(,
+                   [[char buf[128] __attribute__((uninitialized));(void)buf]])],
+                   [pcre2_cc_cv_attribute_uninitialized=yes],
+                   [pcre2_cc_cv_attribute_uninitialized=no])
+AC_MSG_RESULT([$pcre2_cc_cv_attribute_uninitialized])
+if test "$pcre2_cc_cv_attribute_uninitialized" = yes; then
+  AC_DEFINE([HAVE_ATTRIBUTE_UNINITIALIZED], 1, [Define this if your compiler
+             supports __attribute__((uninitialized))])
+fi
+CFLAGS=$tmp_CFLAGS
+AC_LANG_POP([C])
+
 # Versioning
 
 PCRE2_MAJOR="pcre2_major"
diff --git a/src/config.h.in b/src/config.h.in
index 6b8eb7e..5406da0 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -52,6 +52,9 @@ sure both macros are undefined; an emulation function will then be used. */
    LF does in an ASCII/Unicode environment. */
 #undef EBCDIC_NL25
 
+/* Define this if your compiler supports __attribute__((uninitialized)) */
+#undef HAVE_ATTRIBUTE_UNINITIALIZED
+
 /* Define to 1 if you have the `bcopy' function. */
 #undef HAVE_BCOPY
 
diff --git a/src/pcre2_internal.h b/src/pcre2_internal.h
index cb81199..d8fad1e 100644
--- a/src/pcre2_internal.h
+++ b/src/pcre2_internal.h
@@ -76,6 +76,17 @@ typedef int BOOL;
 #include <valgrind/memcheck.h>
 #endif
 
+/* -ftrivial-auto-var-init support supports initializing all local variables
+to avoid some classes of bug, but this can cause an unacceptable slowdown
+for large on-stack arrays in hot functions. This macro lets us annotate
+such arrays. */
+
+#ifdef HAVE_ATTRIBUTE_UNINITIALIZED
+#define PCRE2_KEEP_UNINITIALIZED __attribute__((uninitialized))
+#else
+#define PCRE2_KEEP_UNINITIALIZED
+#endif
+
 /* Older versions of MSVC lack snprintf(). This define allows for
 warning/error-free compilation and testing with MSVC compilers back to at least
 MSVC 10/2010. Except for VC6 (which is missing some fundamentals and fails). */
diff --git a/src/pcre2_match.c b/src/pcre2_match.c
index 5115b53..11289d5 100644
--- a/src/pcre2_match.c
+++ b/src/pcre2_match.c
@@ -6164,7 +6164,8 @@ proves to be too small, it is replaced by a larger one on the heap. To get a
 vector of the size required that is aligned for pointers, allocate it as a
 vector of pointers. */
 
-PCRE2_SPTR stack_frames_vector[START_FRAMES_SIZE/sizeof(PCRE2_SPTR)];
+PCRE2_SPTR stack_frames_vector[START_FRAMES_SIZE/sizeof(PCRE2_SPTR)]
+    PCRE2_KEEP_UNINITIALIZED;
 mb->stack_frames = (heapframe *)stack_frames_vector;
 
 /* A length equal to PCRE2_ZERO_TERMINATED implies a zero-terminated