diff --git a/examples/natmod/deflate/deflate.c b/examples/natmod/deflate/deflate.c
index 9de7e101a7689..56ce8c8385e92 100644
--- a/examples/natmod/deflate/deflate.c
+++ b/examples/natmod/deflate/deflate.c
@@ -68,3 +68,20 @@ mp_obj_t mpy_init(mp_obj_fun_bc_t *self, size_t n_args, size_t n_kw, mp_obj_t *a
 
     MP_DYNRUNTIME_INIT_EXIT
 }
+
+// On x86 and x64 (at least) Clang brings in its own memset() implementation,
+// which will check at runtime which CPU features are available and then pick
+// the fastest implementation depending on the running environment.
+//
+// This unfortunately includes a series of dependencies that do have a
+// non-empty data section, which is currently not supported.  Therefore if we
+// detect Clang we provide our own naïve memset implementation.
+
+#if defined(__clang__) && (defined(__x86_64__) || defined(__i386__))
+__attribute__((weak)) void *memset(void *pointer, int character, size_t length) {
+    for (size_t index = 0; index < length; ++index) {
+	((char *)pointer)[index] = (char)character;
+    }
+    return pointer;
+}
+#endif
diff --git a/examples/natmod/framebuf/framebuf.c b/examples/natmod/framebuf/framebuf.c
index 5fd7c6be3a456..c1f414ead23d7 100644
--- a/examples/natmod/framebuf/framebuf.c
+++ b/examples/natmod/framebuf/framebuf.c
@@ -52,3 +52,20 @@ mp_obj_t mpy_init(mp_obj_fun_bc_t *self, size_t n_args, size_t n_kw, mp_obj_t *a
 
     MP_DYNRUNTIME_INIT_EXIT
 }
+
+// On x86 and x64 (at least) Clang brings in its own memset() implementation,
+// which will check at runtime which CPU features are available and then pick
+// the fastest implementation depending on the running environment.
+//
+// This unfortunately includes a series of dependencies that do have a
+// non-empty data section, which is currently not supported.  Therefore if we
+// detect Clang we provide our own naïve memset implementation.
+
+#if defined(__clang__) && (defined(__x86_64__) || defined(__i386__))
+__attribute__((weak)) void *memset(void *pointer, int character, size_t length) {
+    for (size_t index = 0; index < length; ++index) {
+       ((char *)pointer)[index] = (char)character;
+    }
+    return pointer;
+}
+#endif
diff --git a/examples/natmod/re/re.c b/examples/natmod/re/re.c
index c0279ee7e816d..f9b9fd1de382d 100644
--- a/examples/natmod/re/re.c
+++ b/examples/natmod/re/re.c
@@ -89,3 +89,20 @@ mp_obj_t mpy_init(mp_obj_fun_bc_t *self, size_t n_args, size_t n_kw, mp_obj_t *a
 
     MP_DYNRUNTIME_INIT_EXIT
 }
+
+// On x86 and x64 (at least) Clang brings in its own memset() implementation,
+// which will check at runtime which CPU features are available and then pick
+// the fastest implementation depending on the running environment.
+//
+// This unfortunately includes a series of dependencies that do have a
+// non-empty data section, which is currently not supported.  Therefore if we
+// detect Clang we provide our own naïve memset implementation.
+
+#if defined(__clang__) && (defined(__x86_64__) || defined(__i386__))
+__attribute__((weak)) void *memset(void *pointer, int character, size_t length) {
+    for (size_t index = 0; index < length; ++index) {
+       ((char *)pointer)[index] = (char)character;
+    }
+    return pointer;
+}
+#endif
diff --git a/py/dynruntime.mk b/py/dynruntime.mk
index 3902acbd0b32a..e544c7c0c8c40 100644
--- a/py/dynruntime.mk
+++ b/py/dynruntime.mk
@@ -2,6 +2,7 @@
 # MPY_DIR must be set to the top of the MicroPython source tree
 
 BUILD ?= build
+CC ?= gcc
 
 ECHO = @echo
 RM = /bin/rm
@@ -104,13 +105,15 @@ MICROPY_FLOAT_IMPL ?= float
 else ifeq ($(ARCH),rv32imc)
 
 # rv32imc
+ifeq ($(firstword $(shell $(CC) --version)),gcc)
 CROSS = riscv64-unknown-elf-
+endif
 CFLAGS_ARCH += -march=rv32imac -mabi=ilp32 -mno-relax
 # If Picolibc is available then select it explicitly.  Ubuntu 24.04 ships its
 # bare metal RISC-V toolchain with Picolibc rather than Newlib, and the default
 # is "nosys" so a value must be provided.  To avoid having per-distro
 # workarounds, always select Picolibc if available.
-PICOLIBC_SPECS := $(shell $(CROSS)gcc --print-file-name=picolibc.specs)
+PICOLIBC_SPECS := $(shell $(CROSS)$(CC) --print-file-name=picolibc.specs)
 ifneq ($(PICOLIBC_SPECS),picolibc.specs)
 CFLAGS_ARCH += -specs=$(PICOLIBC_SPECS)
 USE_PICOLIBC := 1
@@ -123,13 +126,15 @@ MICROPY_FLOAT_IMPL ?= none
 else ifeq ($(ARCH),rv64imc)
 
 # rv64imc
+ifeq ($(firstword $(shell $(CC) --version)),gcc)
 CROSS = riscv64-unknown-elf-
+endif
 CFLAGS_ARCH += -march=rv64imac -mabi=lp64 -mno-relax
 # If Picolibc is available then select it explicitly.  Ubuntu 24.04 ships its
 # bare metal RISC-V toolchain with Picolibc rather than Newlib, and the default
 # is "nosys" so a value must be provided.  To avoid having per-distro
 # workarounds, always select Picolibc if available.
-PICOLIBC_SPECS := $(shell $(CROSS)gcc --print-file-name=picolibc.specs)
+PICOLIBC_SPECS := $(shell $(CROSS)$(CC) --print-file-name=picolibc.specs)
 ifneq ($(PICOLIBC_SPECS),picolibc.specs)
 CFLAGS_ARCH += -specs=$(PICOLIBC_SPECS)
 USE_PICOLIBC := 1
@@ -143,7 +148,11 @@ else
 $(error architecture '$(ARCH)' not supported)
 endif
 
-ifneq ($(findstring -musl,$(shell $(CROSS)gcc -dumpmachine)),)
+ifeq ($(findstring clang,$(shell $(CC) --version)),clang)
+CROSS =
+endif
+
+ifneq ($(findstring -musl,$(shell $(CROSS)$(CC) -dumpmachine)),)
 USE_MUSL := 1
 endif
 
@@ -175,8 +184,8 @@ LIBM_NAME := libc.a
 else
 LIBM_NAME := libm.a
 endif
-LIBGCC_PATH := $(realpath $(shell $(CROSS)gcc $(CFLAGS) --print-libgcc-file-name))
-LIBM_PATH := $(realpath $(shell $(CROSS)gcc $(CFLAGS) --print-file-name=$(LIBM_NAME)))
+LIBGCC_PATH := $(realpath $(shell $(CROSS)$(CC) $(CFLAGS) --print-libgcc-file-name))
+LIBM_PATH := $(realpath $(shell $(CROSS)$(CC) $(CFLAGS) --print-file-name=$(LIBM_NAME)))
 ifeq ($(USE_PICOLIBC),1)
 ifeq ($(LIBM_PATH),)
 # The CROSS toolchain prefix usually ends with a dash, but that may not be
@@ -224,12 +233,12 @@ $(CONFIG_H): $(SRC)
 # Build .o from .c source files
 $(BUILD)/%.o: %.c $(CONFIG_H) Makefile
 	$(ECHO) "CC $<"
-	$(Q)$(CROSS)gcc $(CFLAGS) -o $@ -c $<
+	$(Q)$(CROSS)$(CC) $(CFLAGS) -o $@ -c $<
 
 # Build .o from .S source files
 $(BUILD)/%.o: %.S $(CONFIG_H) Makefile
 	$(ECHO) "AS $<"
-	$(Q)$(CROSS)gcc $(CFLAGS) -o $@ -c $<
+	$(Q)$(CROSS)$(CC) $(CFLAGS) -o $@ -c $<
 
 # Build .mpy from .py source files
 $(BUILD)/%.mpy: %.py
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 1574243e8ea99..277ab9d7a4a2b 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -192,14 +192,18 @@
 #endif
 
 #if MP_INT_TYPE == MP_INT_TYPE_INTPTR
+#if !defined(MP_INT_TYPES_DEFINED)
 typedef intptr_t mp_int_t;
 typedef uintptr_t mp_uint_t;
+#endif
 #define MP_INT_MAX INTPTR_MAX
 #define MP_INT_MIN INTPTR_MIN
 #define MP_UINT_MAX INTPTR_UMAX
 #elif MP_INT_TYPE == MP_INT_TYPE_INT64
+#if !defined(MP_INT_TYPES_DEFINED)
 typedef int64_t mp_int_t;
 typedef uint64_t mp_uint_t;
+#endif
 #define MP_INT_MAX INT64_MAX
 #define MP_INT_MIN INT64_MIN
 #define MP_UINT_MAX INT64_UMAX
diff --git a/tools/mpy_ld.py b/tools/mpy_ld.py
index 20d665ff8849f..7cbe521aba67c 100755
--- a/tools/mpy_ld.py
+++ b/tools/mpy_ld.py
@@ -1509,7 +1509,8 @@ def do_preprocess(args):
             "#include <stdint.h>\n"
             "typedef uintptr_t mp_uint_t;\n"
             "typedef intptr_t mp_int_t;\n"
-            "typedef uintptr_t mp_off_t;",
+            "typedef uintptr_t mp_off_t;\n"
+            "#define MP_INT_TYPES_DEFINED",
             file=f,
         )
         for i, q in enumerate(static_qstrs):