diff --git a/include/mbedtls/bn_mul.h b/include/mbedtls/bn_mul.h
index a0bc4d0..6414e54 100644
--- a/include/mbedtls/bn_mul.h
+++ b/include/mbedtls/bn_mul.h
@@ -677,6 +677,15 @@
 #if defined(__arm__) && !defined(MULADDC_CANNOT_USE_R7)
 
 #if defined(__thumb__) && !defined(__thumb2__)
+#if !defined(__ARMCC_VERSION) && !defined(__clang__) \
+    && !defined(__llvm__) && !defined(__INTEL_COMPILER)
+/*
+ * Thumb 1 ISA. This code path has only been tested successfully on gcc;
+ * it does not compile on clang or armclang.
+ *
+ * Other compilers which define __GNUC__ may not work. The above macro
+ * attempts to exclude these untested compilers.
+ */
 
 #define MULADDC_INIT                                    \
     asm(                                                \
@@ -731,6 +740,8 @@
            "r6", "r7", "r8", "r9", "cc"         \
          );
 
+#endif /* Compiler is gcc */
+
 #elif (__ARM_ARCH >= 6) && \
     defined (__ARM_FEATURE_DSP) && (__ARM_FEATURE_DSP == 1)