diff --git a/CMakeLists.txt b/CMakeLists.txt
index 22bfeb4cd..5eeea4db5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,9 +19,6 @@ set(DEFAULT_BUILD_TYPE "RelWithDebInfo")
 # Generate Xcode schemas (but not for tests).
 set(CMAKE_XCODE_GENERATE_SCHEME 1)
 
-# Use the default flags (#6296) but undefine NDEBUG so that asserts remain enabled.
-add_definitions(-UNDEBUG)
-
 if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
     message(STATUS "Setting build type to default '${DEFAULT_BUILD_TYPE}'")
     set(CMAKE_BUILD_TYPE "${DEFAULT_BUILD_TYPE}")
diff --git a/src/common.cpp b/src/common.cpp
index 82390c9d7..391aa795b 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -1780,7 +1780,7 @@ void restore_term_foreground_process_group_for_exit() {
 bool is_main_thread() { return thread_id() == 1; }
 
 void assert_is_main_thread(const char *who) {
-    if (!is_main_thread() && !thread_asserts_cfg_for_testing) {
+    if (!likely(is_main_thread()) && !unlikely(thread_asserts_cfg_for_testing)) {
         FLOGF(error, L"%s called off of main thread.", who);
         FLOGF(error, L"Break on debug_thread_error to debug.");
         debug_thread_error();
@@ -1788,7 +1788,7 @@ void assert_is_main_thread(const char *who) {
 }
 
 void assert_is_not_forked_child(const char *who) {
-    if (is_forked_child()) {
+    if (unlikely(is_forked_child())) {
         FLOGF(error, L"%s called in a forked child.", who);
         FLOG(error, L"Break on debug_thread_error to debug.");
         debug_thread_error();
@@ -1796,7 +1796,7 @@ void assert_is_not_forked_child(const char *who) {
 }
 
 void assert_is_background_thread(const char *who) {
-    if (is_main_thread() && !thread_asserts_cfg_for_testing) {
+    if (unlikely(is_main_thread()) && !unlikely(thread_asserts_cfg_for_testing)) {
         FLOGF(error, L"%s called on the main thread (may block!).", who);
         FLOG(error, L"Break on debug_thread_error to debug.");
         debug_thread_error();
@@ -1806,7 +1806,7 @@ void assert_is_background_thread(const char *who) {
 void assert_is_locked(std::mutex &mutex, const char *who, const char *caller) {
     // Note that std::mutex.try_lock() is allowed to return false when the mutex isn't
     // actually locked; fortunately we are checking the opposite so we're safe.
-    if (mutex.try_lock()) {
+    if (unlikely(mutex.try_lock())) {
         FLOGF(error, L"%s is not locked when it should be in '%s'", who, caller);
         FLOG(error, L"Break on debug_thread_error to debug.");
         debug_thread_error();
@@ -1843,7 +1843,7 @@ void redirect_tty_output() {
 
 /// Display a failed assertion message, dump a stack trace if possible, then die.
 [[noreturn]] void __fish_assert(const char *msg, const char *file, size_t line, int error) {
-    if (error) {
+    if (unlikely(error)) {
         FLOGF(error, L"%s:%zu: failed assertion: %s: errno %d (%s)", file, line, msg, error,
               std::strerror(error));
     } else {
diff --git a/src/common.h b/src/common.h
index 28567d2e8..500be8780 100644
--- a/src/common.h
+++ b/src/common.h
@@ -222,7 +222,7 @@ extern const wcstring g_empty_string;
 #define DIE_ON_FAILURE(e)                                  \
     do {                                                   \
         int status = e;                                    \
-        if (status != 0) {                                 \
+        if (unlikely(status != 0)) {                                 \
             __fish_assert(#e, __FILE__, __LINE__, status); \
         }                                                  \
     } while (0)
@@ -308,6 +308,15 @@ void wcs2string_appending(const wchar_t *in, size_t len, std::string *receiver);
 #define TESTS_PROGRAM_NAME L"(ignore)"
 bool should_suppress_stderr_for_tests();
 
+/// Branch prediction hints. Idea borrowed from Linux kernel. Just used for asserts.
+#if __has_builtin(__builtin_expect)
+#define likely(x) __builtin_expect(!!(x), 1)
+#define unlikely(x) __builtin_expect(!!(x), 0)
+#else
+#define likely(x) (x)
+#define unlikely(x) (x)
+#endif
+
 void assert_is_main_thread(const char *who);
 #define ASSERT_IS_MAIN_THREAD_TRAMPOLINE(x) assert_is_main_thread(x)
 #define ASSERT_IS_MAIN_THREAD() ASSERT_IS_MAIN_THREAD_TRAMPOLINE(__FUNCTION__)