diff --git a/mk/rules.mk b/mk/rules.mk
index 5db86e182cc..6fff48fa1bf 100644
--- a/mk/rules.mk
+++ b/mk/rules.mk
@@ -40,6 +40,7 @@ CFLAGS=\
 	-Wmissing-field-initializers \
 	-Wcast-function-type-mismatch \
 	-Wsemicolon-before-method-body \
+	-Wsign-compare \
 	-g \
 	-I.
 SWIFTFLAGS=-g -emit-library
diff --git a/runtime/trampolines-arm64.h b/runtime/trampolines-arm64.h
index 60e1fb6ea06..30d3046e173 100644
--- a/runtime/trampolines-arm64.h
+++ b/runtime/trampolines-arm64.h
@@ -57,8 +57,8 @@ struct XamarinCallState {
 
 struct ParamIterator {
 	struct XamarinCallState *state;
-	int ngrn; // Next General-purpose Register Number
-	int nsrn; // Next SIMD and Floating-point Register Number
+	unsigned int ngrn; // Next General-purpose Register Number
+	unsigned int nsrn; // Next SIMD and Floating-point Register Number
 	size_t ngrn_offset; // Offset into the current general purpose register number
 	uint8_t *nsaa; // Next stacked argument address.
 
diff --git a/runtime/trampolines-arm64.m b/runtime/trampolines-arm64.m
index 5969be4368e..6d94905aad6 100644
--- a/runtime/trampolines-arm64.m
+++ b/runtime/trampolines-arm64.m
@@ -211,11 +211,11 @@
 	bool is_hfa = is_struct && struct_length >= 2 && struct_length <= 4;
 	if (is_hfa) {
 		is_hfa = struct_name [0] == 'f' || struct_name [0] == 'd';
-		for (int i = 1; i < struct_length; i++)
+		for (size_t i = 1; i < struct_length; i++)
 			is_hfa &= struct_name [i] == struct_name [0];
 	}
 
-	int ngrn_registers_left = 8 - it->ngrn;
+	size_t ngrn_registers_left = 8 - it->ngrn;
 	bool read_register = ngrn_registers_left > 0;
 
 	if (size > 16 && !is_hfa) {
@@ -310,7 +310,7 @@
 			(size == 8 && !strncmp (struct_name, "d", 1))) {
 			LOGZ ("        marshalling as %i doubles (struct name: %s)\n", (int) size / 8, struct_name);
 			double* ptr = (double *) mono_object_unbox (value);
-			for (int i = 0; i < size / 8; i++) {
+			for (size_t i = 0; i < size / 8; i++) {
 				LOGZ ("        #%i: %f\n", i, ptr [i]);
 				it->q [i].q = ptr [i];
 			}
@@ -320,7 +320,7 @@
 				   (size == 4 && !strncmp (struct_name, "f", 1))) {
 			LOGZ ("        marshalling as %i floats (struct name: %s)\n", (int) size / 4, struct_name);
 			float* ptr = (float *) mono_object_unbox (value);
-			for (int i = 0; i < size / 4; i++) {
+			for (size_t i = 0; i < size / 4; i++) {
 				LOGZ ("        #%i: %f\n", i, ptr [i]);
 				it->q [i].f.f1 = ptr [i];
 			}
diff --git a/tests/test-libraries/libtest.m b/tests/test-libraries/libtest.m
index 6db61172d60..14ff6db7fcb 100644
--- a/tests/test-libraries/libtest.m
+++ b/tests/test-libraries/libtest.m
@@ -1513,7 +1513,7 @@ -(void) addObject: (WeakReferencedObject *) obj
 
 -(void) callDoSomething: (int*) nilObjectCount nonNilObjectCount: (int *) nonNilObjectCount gotExpectedResponse: (int *) gotExpectedResponse gotUnexpectedResponse: (int *) gotUnexpectedResponse gotFinalizedResponse: (int *) gotFinalizedResponse
 {
-	for (int i = 0; i < self.array.count; i++) {
+	for (unsigned int i = 0; i < self.array.count; i++) {
 		WeakReferencedObject *obj = (WeakReferencedObject *) [self.array pointerAtIndex: (NSUInteger) i];
 		if (obj) {
 			*nonNilObjectCount= *nonNilObjectCount + 1;