diff --git a/stdlib/public/stubs/LibcShims.cpp b/stdlib/public/stubs/LibcShims.cpp
index 0140772ee147a..39c2d3643a61f 100644
--- a/stdlib/public/stubs/LibcShims.cpp
+++ b/stdlib/public/stubs/LibcShims.cpp
@@ -30,10 +30,12 @@ using namespace swift;
 static_assert(std::is_same<ssize_t, swift::__swift_ssize_t>::value,
               "__swift_ssize_t must be defined as equivalent to ssize_t");
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 void swift::_swift_stdlib_free(void *ptr) {
   free(ptr);
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 int swift::_swift_stdlib_putchar_unlocked(int c) {
 #if defined(_MSC_VER)
   return _putc_nolock(c, stdout);
@@ -42,21 +44,25 @@ int swift::_swift_stdlib_putchar_unlocked(int c) {
 #endif
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 __swift_size_t swift::_swift_stdlib_fwrite_stdout(const void *ptr,
                                                   __swift_size_t size,
                                                   __swift_size_t nitems) {
   return fwrite(ptr, size, nitems, stdout);
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 __swift_size_t swift::_swift_stdlib_strlen(const char *s) {
   return strlen(s);
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 int swift::_swift_stdlib_memcmp(const void *s1, const void *s2,
                                 __swift_size_t n) {
   return memcmp(s1, s2, n);
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 __swift_ssize_t
 swift::_swift_stdlib_read(int fd, void *buf, __swift_size_t nbyte) {
 #if defined(_MSC_VER)
@@ -66,6 +72,7 @@ swift::_swift_stdlib_read(int fd, void *buf, __swift_size_t nbyte) {
 #endif
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 __swift_ssize_t
 swift::_swift_stdlib_write(int fd, const void *buf, __swift_size_t nbyte) {
 #if defined(_MSC_VER)
@@ -75,6 +82,7 @@ swift::_swift_stdlib_write(int fd, const void *buf, __swift_size_t nbyte) {
 #endif
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 int swift::_swift_stdlib_close(int fd) {
 #if defined(_MSC_VER)
   return _close(fd);
@@ -85,21 +93,25 @@ int swift::_swift_stdlib_close(int fd) {
 
 #if defined(__APPLE__)
 #include <malloc/malloc.h>
+SWIFT_RUNTIME_STDLIB_INTERFACE
 size_t swift::_swift_stdlib_malloc_size(const void *ptr) {
   return malloc_size(ptr);
 }
 #elif defined(__GNU_LIBRARY__) || defined(__CYGWIN__) || defined(__ANDROID__)
 #include <malloc.h>
+SWIFT_RUNTIME_STDLIB_INTERFACE
 size_t swift::_swift_stdlib_malloc_size(const void *ptr) {
   return malloc_usable_size(const_cast<void *>(ptr));
 }
 #elif defined(_MSC_VER)
 #include <malloc.h>
+SWIFT_RUNTIME_STDLIB_INTERFACE
 size_t swift::_swift_stdlib_malloc_size(const void *ptr) {
   return _msize(const_cast<void *>(ptr));
 }
 #elif defined(__FreeBSD__)
 #include <malloc_np.h>
+SWIFT_RUNTIME_STDLIB_INTERFACE
 size_t swift::_swift_stdlib_malloc_size(const void *ptr) {
   return malloc_usable_size(const_cast<void *>(ptr));
 }
@@ -113,10 +125,12 @@ static std::mt19937 &getGlobalMT19937() {
   return theGlobalMT19937.get();
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 __swift_uint32_t swift::_swift_stdlib_cxx11_mt19937() {
   return getGlobalMT19937()();
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 __swift_uint32_t
 swift::_swift_stdlib_cxx11_mt19937_uniform(__swift_uint32_t upper_bound) {
   if (upper_bound > 0)
@@ -125,24 +139,30 @@ swift::_swift_stdlib_cxx11_mt19937_uniform(__swift_uint32_t upper_bound) {
   return RandomUniform(getGlobalMT19937());
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 float swift::_swift_stdlib_remainderf(float dividend, float divisor) {
   return std::remainder(dividend, divisor);
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 float swift::_swift_stdlib_squareRootf(float x) { return std::sqrt(x); }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 double swift::_swift_stdlib_remainder(double dividend, double divisor) {
   return std::remainder(dividend, divisor);
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 double swift::_swift_stdlib_squareRoot(double x) { return std::sqrt(x); }
 
 #if (defined __i386__ || defined __x86_64__) && !defined _MSC_VER
+SWIFT_RUNTIME_STDLIB_INTERFACE
 void swift::_swift_stdlib_remainderl(void *_self, const void *_other) {
   *(long double *)_self = std::remainder(*(long double *)_self,
                                          *(const long double *)_other);
 }
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 void swift::_swift_stdlib_squareRootl(void *_self) {
   *(long double *)_self = std::sqrt(*(long double *)_self);
 }