diff --git a/benchmarks/pthread_benchmark.cpp b/benchmarks/pthread_benchmark.cpp
index 621fcb624..c010dd2c0 100644
--- a/benchmarks/pthread_benchmark.cpp
+++ b/benchmarks/pthread_benchmark.cpp
@@ -18,11 +18,14 @@
 
 #include <pthread.h>
 
+// Stop GCC optimizing out our pure function.
+/* Must not be static! */ pthread_t (*pthread_self_fp)() = pthread_self;
+
 static void BM_pthread_self(int iters) {
   StartBenchmarkTiming();
 
   for (int i = 0; i < iters; ++i) {
-    pthread_self();
+    pthread_self_fp();
   }
 
   StopBenchmarkTiming();
diff --git a/benchmarks/unistd_benchmark.cpp b/benchmarks/unistd_benchmark.cpp
index f2c9d73cf..c35e7c369 100644
--- a/benchmarks/unistd_benchmark.cpp
+++ b/benchmarks/unistd_benchmark.cpp
@@ -30,11 +30,14 @@ static void BM_unistd_getpid(int iters) {
 }
 BENCHMARK(BM_unistd_getpid);
 
+// Stop GCC optimizing out our pure function.
+/* Must not be static! */ pid_t (*gettid_fp)() = gettid;
+
 static void BM_unistd_gettid(int iters) {
   StartBenchmarkTiming();
 
   for (int i = 0; i < iters; ++i) {
-    gettid();
+    gettid_fp();
   }
 
   StopBenchmarkTiming();
diff --git a/libc/include/errno.h b/libc/include/errno.h
index 2e5ce5f67..1a36b7aad 100644
--- a/libc/include/errno.h
+++ b/libc/include/errno.h
@@ -41,7 +41,7 @@ __BEGIN_DECLS
 #endif
 
 /* internal function returning the address of the thread-specific errno */
-extern volatile int*   __errno(void);
+extern volatile int* __errno(void) __pure2;
 
 /* a macro expanding to the errno l-value */
 #define  errno   (*__errno())
diff --git a/libc/include/pthread.h b/libc/include/pthread.h
index 5c9b6263e..29caafc6d 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -226,7 +226,7 @@ int pthread_rwlock_trywrlock(pthread_rwlock_t*) __nonnull((1));
 int pthread_rwlock_unlock(pthread_rwlock_t *rwlock) __nonnull((1));
 int pthread_rwlock_wrlock(pthread_rwlock_t*) __nonnull((1));
 
-pthread_t pthread_self(void);
+pthread_t pthread_self(void) __pure2;
 
 int pthread_setname_np(pthread_t, const char*) __nonnull((2));
 
diff --git a/libc/include/stdlib.h b/libc/include/stdlib.h
index 483aaf027..0f862c379 100644
--- a/libc/include/stdlib.h
+++ b/libc/include/stdlib.h
@@ -138,21 +138,21 @@ typedef struct {
     int  rem;
 } div_t;
 
-extern div_t   div(int, int);
+extern div_t   div(int, int) __pure2;
 
 typedef struct {
     long int  quot;
     long int  rem;
 } ldiv_t;
 
-extern ldiv_t   ldiv(long, long);
+extern ldiv_t   ldiv(long, long) __pure2;
 
 typedef struct {
     long long int  quot;
     long long int  rem;
 } lldiv_t;
 
-extern lldiv_t   lldiv(long long, long long);
+extern lldiv_t   lldiv(long long, long long) __pure2;
 
 /* BSD compatibility. */
 extern const char* getprogname(void);
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index b4982cb0d..ee6d7b12e 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -54,7 +54,7 @@ extern __noreturn void _exit(int);
 extern pid_t  fork(void);
 extern pid_t  vfork(void);
 extern pid_t  getpid(void);
-extern pid_t  gettid(void);
+extern pid_t  gettid(void) __pure2;
 extern pid_t  getpgid(pid_t);
 extern int    setpgid(pid_t, pid_t);
 extern pid_t  getppid(void);