diff --git a/library/common.h b/library/common.h
index bf18d725ccff..49e2c97ea049 100644
--- a/library/common.h
+++ b/library/common.h
@@ -350,4 +350,31 @@ static inline const unsigned char *mbedtls_buffer_offset_const(
 #define MBEDTLS_STATIC_ASSERT(expr, msg)
 #endif
 
+/* Suppress compiler warnings for unused functions and variables. */
+#if !defined(MBEDTLS_MAYBE_UNUSED) && defined(__has_attribute)
+#    if __has_attribute(unused)
+#        define MBEDTLS_MAYBE_UNUSED __attribute__((unused))
+#    endif
+#endif
+#if !defined(MBEDTLS_MAYBE_UNUSED) && defined(__GNUC__)
+#    define MBEDTLS_MAYBE_UNUSED __attribute__((unused))
+#endif
+#if !defined(MBEDTLS_MAYBE_UNUSED) && defined(__IAR_SYSTEMS_ICC__) && defined(__VER__)
+/* IAR does support __attribute__((unused)), but only if the -e flag (extended language support)
+ * is given; the pragma always works.
+ * Unfortunately the pragma affects the rest of the file where it is used, but this is harmless.
+ * Check for version 5.2 or later - this pragma may be supported by earlier versions, but I wasn't
+ * able to find documentation).
+ */
+#    if (__VER__ >= 5020000)
+#        define MBEDTLS_MAYBE_UNUSED _Pragma("diag_suppress=Pe177")
+#    endif
+#endif
+#if !defined(MBEDTLS_MAYBE_UNUSED) && defined(_MSC_VER)
+#    define MBEDTLS_MAYBE_UNUSED __pragma(warning(suppress:4189))
+#endif
+#if !defined(MBEDTLS_MAYBE_UNUSED)
+#    define MBEDTLS_MAYBE_UNUSED
+#endif
+
 #endif /* MBEDTLS_LIBRARY_COMMON_H */
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index aa70f22e2cae..df26cb1599a6 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -174,6 +174,10 @@ pre_initialize_variables () {
     if [ -z "${MAKEFLAGS+set}" ]; then
         export MAKEFLAGS="-j$(all_sh_nproc)"
     fi
+    # if CC is not set, use clang by default (if present) to improve build times
+    if [ -z "${CC+set}" ] && (type clang > /dev/null 2>&1); then
+        export CC="clang"
+    fi
 
     # Include more verbose output for failing tests run by CMake or make
     export CTEST_OUTPUT_ON_FAILURE=1
@@ -1657,7 +1661,7 @@ component_test_default_no_deprecated () {
     # configuration leaves something consistent.
     msg "build: make, default + MBEDTLS_DEPRECATED_REMOVED" # ~ 30s
     scripts/config.py set MBEDTLS_DEPRECATED_REMOVED
-    make CC=gcc CFLAGS='-O -Werror -Wall -Wextra'
+    make CFLAGS='-O -Werror -Wall -Wextra'
 
     msg "test: make, default + MBEDTLS_DEPRECATED_REMOVED" # ~ 5s
     make test
@@ -1666,7 +1670,7 @@ component_test_default_no_deprecated () {
 component_test_full_no_deprecated () {
     msg "build: make, full_no_deprecated config" # ~ 30s
     scripts/config.py full_no_deprecated
-    make CC=gcc CFLAGS='-O -Werror -Wall -Wextra'
+    make CFLAGS='-O -Werror -Wall -Wextra'
 
     msg "test: make, full_no_deprecated config" # ~ 5s
     make test
@@ -1680,7 +1684,7 @@ component_test_full_no_deprecated_deprecated_warning () {
     scripts/config.py full_no_deprecated
     scripts/config.py unset MBEDTLS_DEPRECATED_REMOVED
     scripts/config.py set MBEDTLS_DEPRECATED_WARNING
-    make CC=gcc CFLAGS='-O -Werror -Wall -Wextra'
+    make CFLAGS='-O -Werror -Wall -Wextra'
 
     msg "test: make, full_no_deprecated config, MBEDTLS_DEPRECATED_WARNING" # ~ 5s
     make test
@@ -1693,6 +1697,8 @@ component_test_full_deprecated_warning () {
     scripts/config.py full
     scripts/config.py set MBEDTLS_DEPRECATED_WARNING
     # Expect warnings from '#warning' directives in check_config.h.
+    # Note that gcc is required to allow the use of -Wno-error=cpp, which allows us to
+    # display #warning messages without them being treated as errors.
     make CC=gcc CFLAGS='-O -Werror -Wall -Wextra -Wno-error=cpp' lib programs
 
     msg "build: make tests, full config + MBEDTLS_DEPRECATED_WARNING, expect warnings" # ~ 30s
@@ -1862,7 +1868,7 @@ component_build_no_pk_rsa_alt_support () {
     scripts/config.py set MBEDTLS_X509_CRT_WRITE_C
 
     # Only compile - this is primarily to test for compile issues
-    make CC=gcc CFLAGS='-Werror -Wall -Wextra -I../tests/include/alt-dummy'
+    make CFLAGS='-Werror -Wall -Wextra -I../tests/include/alt-dummy'
 }
 
 component_test_no_use_psa_crypto_full_cmake_asan() {
@@ -2619,7 +2625,7 @@ component_test_memory_buffer_allocator_backtrace () {
     scripts/config.py set MBEDTLS_PLATFORM_MEMORY
     scripts/config.py set MBEDTLS_MEMORY_BACKTRACE
     scripts/config.py set MBEDTLS_MEMORY_DEBUG
-    CC=gcc cmake -DCMAKE_BUILD_TYPE:String=Release .
+    cmake -DCMAKE_BUILD_TYPE:String=Release .
     make
 
     msg "test: MBEDTLS_MEMORY_BUFFER_ALLOC_C and MBEDTLS_MEMORY_BACKTRACE"
@@ -2630,7 +2636,7 @@ component_test_memory_buffer_allocator () {
     msg "build: default config with memory buffer allocator"
     scripts/config.py set MBEDTLS_MEMORY_BUFFER_ALLOC_C
     scripts/config.py set MBEDTLS_PLATFORM_MEMORY
-    CC=gcc cmake -DCMAKE_BUILD_TYPE:String=Release .
+    cmake -DCMAKE_BUILD_TYPE:String=Release .
     make
 
     msg "test: MBEDTLS_MEMORY_BUFFER_ALLOC_C"
@@ -2745,7 +2751,7 @@ component_test_ssl_alloc_buffer_and_mfl () {
     scripts/config.py set MBEDTLS_MEMORY_DEBUG
     scripts/config.py set MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
     scripts/config.py set MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH
-    CC=gcc cmake -DCMAKE_BUILD_TYPE:String=Release .
+    cmake -DCMAKE_BUILD_TYPE:String=Release .
     make
 
     msg "test: MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH, MBEDTLS_MEMORY_BUFFER_ALLOC_C, MBEDTLS_MEMORY_DEBUG and MBEDTLS_SSL_MAX_FRAGMENT_LENGTH"
@@ -2845,7 +2851,7 @@ component_test_malloc_0_null () {
 component_test_aes_fewer_tables () {
     msg "build: default config with AES_FEWER_TABLES enabled"
     scripts/config.py set MBEDTLS_AES_FEWER_TABLES
-    make CC=gcc CFLAGS='-Werror -Wall -Wextra'
+    make CFLAGS='-O2 -Werror -Wall -Wextra'
 
     msg "test: AES_FEWER_TABLES"
     make test
@@ -2854,7 +2860,7 @@ component_test_aes_fewer_tables () {
 component_test_aes_rom_tables () {
     msg "build: default config with AES_ROM_TABLES enabled"
     scripts/config.py set MBEDTLS_AES_ROM_TABLES
-    make CC=gcc CFLAGS='-Werror -Wall -Wextra'
+    make CFLAGS='-O2 -Werror -Wall -Wextra'
 
     msg "test: AES_ROM_TABLES"
     make test
@@ -2864,7 +2870,7 @@ component_test_aes_fewer_tables_and_rom_tables () {
     msg "build: default config with AES_ROM_TABLES and AES_FEWER_TABLES enabled"
     scripts/config.py set MBEDTLS_AES_FEWER_TABLES
     scripts/config.py set MBEDTLS_AES_ROM_TABLES
-    make CC=gcc CFLAGS='-Werror -Wall -Wextra'
+    make CFLAGS='-O2 -Werror -Wall -Wextra'
 
     msg "test: AES_FEWER_TABLES + AES_ROM_TABLES"
     make test
@@ -3099,7 +3105,7 @@ support_test_m32_everest () {
 component_test_mx32 () {
     msg "build: 64-bit ILP32, make, gcc" # ~ 30s
     scripts/config.py full
-    make CC=gcc CFLAGS='-Werror -Wall -Wextra -mx32' LDFLAGS='-mx32'
+    make CC=gcc CFLAGS='-O2 -Werror -Wall -Wextra -mx32' LDFLAGS='-mx32'
 
     msg "test: 64-bit ILP32, make, gcc"
     make test
@@ -3126,7 +3132,7 @@ component_test_have_int32 () {
     scripts/config.py unset MBEDTLS_HAVE_ASM
     scripts/config.py unset MBEDTLS_AESNI_C
     scripts/config.py unset MBEDTLS_PADLOCK_C
-    make CC=gcc CFLAGS='-Werror -Wall -Wextra -DMBEDTLS_HAVE_INT32'
+    make CC=gcc CFLAGS='-O2 -Werror -Wall -Wextra -DMBEDTLS_HAVE_INT32'
 
     msg "test: gcc, force 32-bit bignum limbs"
     make test
@@ -3137,7 +3143,7 @@ component_test_have_int64 () {
     scripts/config.py unset MBEDTLS_HAVE_ASM
     scripts/config.py unset MBEDTLS_AESNI_C
     scripts/config.py unset MBEDTLS_PADLOCK_C
-    make CC=gcc CFLAGS='-Werror -Wall -Wextra -DMBEDTLS_HAVE_INT64'
+    make CC=gcc CFLAGS='-O2 -Werror -Wall -Wextra -DMBEDTLS_HAVE_INT64'
 
     msg "test: gcc, force 64-bit bignum limbs"
     make test
@@ -3545,7 +3551,7 @@ component_build_zeroize_checks () {
     scripts/config.py full
 
     # Only compile - we're looking for sizeof-pointer-memaccess warnings
-    make CC=gcc CFLAGS="'-DMBEDTLS_USER_CONFIG_FILE=\"../tests/configs/user-config-zeroize-memset.h\"' -DMBEDTLS_TEST_DEFINES_ZEROIZE -Werror -Wsizeof-pointer-memaccess"
+    make CFLAGS="'-DMBEDTLS_USER_CONFIG_FILE=\"../tests/configs/user-config-zeroize-memset.h\"' -DMBEDTLS_TEST_DEFINES_ZEROIZE -Werror -Wsizeof-pointer-memaccess"
 }
 
 
@@ -3578,12 +3584,13 @@ component_test_zeroize () {
 }
 
 component_test_psa_compliance () {
+    # The arch tests build with gcc, so require use of gcc here to link properly
     msg "build: make, default config + CMAC, libmbedcrypto.a only"
     scripts/config.py set MBEDTLS_CMAC_C
-    make -C library libmbedcrypto.a
+    CC=gcc make -C library libmbedcrypto.a
 
     msg "unit test: test_psa_compliance.py"
-    ./tests/scripts/test_psa_compliance.py
+    CC=gcc ./tests/scripts/test_psa_compliance.py
 }
 
 support_test_psa_compliance () {
diff --git a/tests/scripts/depends.py b/tests/scripts/depends.py
index 6c2e6d6e3b9d..c6a438ee362f 100755
--- a/tests/scripts/depends.py
+++ b/tests/scripts/depends.py
@@ -392,7 +392,7 @@ def config_symbols_matching(self, regexp):
 
     def __init__(self, options, conf):
         """Gather data about the library and establish a list of domains to test."""
-        build_command = [options.make_command, 'CFLAGS=-Werror']
+        build_command = [options.make_command, 'CFLAGS=-Werror -O2']
         build_and_test = [build_command, [options.make_command, 'test']]
         self.all_config_symbols = set(conf.settings.keys())
         # Find hash modules by name.
diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index b1340614567b..443bc0861bfe 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -19,6 +19,7 @@
     mbedtls_ecp_point_init(x);
 
 /* Auxiliary function to compare two mbedtls_ecp_group objects. */
+MBEDTLS_MAYBE_UNUSED
 inline static int mbedtls_ecp_group_cmp(mbedtls_ecp_group *grp1,
                                         mbedtls_ecp_group *grp2)
 {