diff --git a/malloc.c b/malloc.c
index 74e402f..def5225 100644
--- a/malloc.c
+++ b/malloc.c
@@ -30,7 +30,10 @@ static_assert(sizeof(void *) == 8, "64-bit only");
 
 static void *memory_map(size_t size) {
     void *p = mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
-    if (p == MAP_FAILED) {
+    if (unlikely(p == MAP_FAILED)) {
+        if (errno != ENOMEM) {
+            fatal_error("non-ENOMEM mmap failure");
+        }
         return NULL;
     }
     return p;
@@ -38,7 +41,7 @@ static void *memory_map(size_t size) {
 
 static int memory_unmap(void *ptr, size_t size) {
     int ret = munmap(ptr, size);
-    if (ret && errno != ENOMEM) {
+    if (unlikely(ret) && errno != ENOMEM) {
         fatal_error("non-ENOMEM munmap failure");
     }
     return ret;
@@ -46,7 +49,7 @@ static int memory_unmap(void *ptr, size_t size) {
 
 static int memory_protect(void *ptr, size_t size, int prot) {
     int ret = mprotect(ptr, size, prot);
-    if (ret && errno != ENOMEM) {
+    if (unlikely(ret) && errno != ENOMEM) {
         fatal_error("non-ENOMEM mprotect failure");
     }
     return ret;