Added SDL_DelayPrecise()

SDL_DelayNS() now passes through to the high precision OS delay function, and SDL_DelayPrecise() tries to busy wait to get as close as possible to the desired wait time.

Fixes https://github.com/libsdl-org/SDL/issues/11141

include/SDL3/SDL_timer.h

@@ -115,8 +115,8 @@ extern SDL_DECLSPEC void SDLCALL SDL_Delay(Uint32 ms);
  * Wait a specified number of nanoseconds before returning.
  *
  * This function waits a specified number of nanoseconds before returning. It
- * will attempt to wait as close to the requested time as possible, busy
- * waiting if necessary, but could return later due to OS scheduling.
+ * waits at least the specified time, but possibly longer due to OS
+ * scheduling.
  *
  * \param ns the number of nanoseconds to delay.
  *
@@ -124,6 +124,19 @@ extern SDL_DECLSPEC void SDLCALL SDL_Delay(Uint32 ms);
  */
 extern SDL_DECLSPEC void SDLCALL SDL_DelayNS(Uint64 ns);
 
+/**
+ * Wait a specified number of nanoseconds before returning.
+ *
+ * This function waits a specified number of nanoseconds before returning. It
+ * will attempt to wait as close to the requested time as possible, busy
+ * waiting if necessary, but could return later due to OS scheduling.
+ *
+ * \param ns the number of nanoseconds to delay.
+ *
+ * \since This function is available since SDL 3.1.5.
+ */
+extern SDL_DECLSPEC void SDLCALL SDL_DelayPrecise(Uint64 ns);
+
 /**
  * Definition of the timer ID type.
  *

src/dynapi/SDL_dynapi.sym

@@ -1177,6 +1177,7 @@ SDL3_0.0.0 {
     SDL_wcsstr;
     SDL_wcstol;
     SDL_StepBackUTF8;
+    SDL_DelayPrecise;
     # extra symbols go here (don't modify this line)
   local: *;
 };

src/dynapi/SDL_dynapi_overrides.h

@@ -1202,3 +1202,4 @@
 #define SDL_wcsstr SDL_wcsstr_REAL
 #define SDL_wcstol SDL_wcstol_REAL
 #define SDL_StepBackUTF8 SDL_StepBackUTF8_REAL
+#define SDL_DelayPrecise SDL_DelayPrecise_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -1208,3 +1208,4 @@ SDL_DYNAPI_PROC(wchar_t*,SDL_wcsnstr,(const wchar_t *a, const wchar_t *b, size_t
 SDL_DYNAPI_PROC(wchar_t*,SDL_wcsstr,(const wchar_t *a, const wchar_t *b),(a,b),return)
 SDL_DYNAPI_PROC(long,SDL_wcstol,(const wchar_t *a, wchar_t **b, int c),(a,b,c),return)
 SDL_DYNAPI_PROC(Uint32,SDL_StepBackUTF8,(const char *a, const char **b),(a,b),return)
+SDL_DYNAPI_PROC(void,SDL_DelayPrecise,(Uint64 a),(a),)

src/main/generic/SDL_sysmain_callbacks.c

@@ -65,7 +65,7 @@ int SDL_EnterAppMainCallbacks(int argc, char* argv[], SDL_AppInit_func appinit,
             } else {
                 const Uint64 now = SDL_GetTicksNS();
                 if (next_iteration > now) { // Running faster than the limit, sleep a little.
-                    SDL_DelayNS(next_iteration - now);
+                    SDL_DelayPrecise(next_iteration - now);
                 } else {
                     next_iteration = now; // running behind (or just lost the window)...reset the timer.
                 }

src/render/SDL_render.c

@@ -4948,7 +4948,7 @@ static void SDL_SimulateRenderVSync(SDL_Renderer *renderer)
     elapsed = (now - renderer->last_present);
     if (elapsed < interval) {
         Uint64 duration = (interval - elapsed);
-        SDL_DelayNS(duration);
+        SDL_DelayPrecise(duration);
         now = SDL_GetTicksNS();
     }
 

src/timer/SDL_timer.c

@@ -657,6 +657,11 @@ void SDL_Delay(Uint32 ms)
 }
 
 void SDL_DelayNS(Uint64 ns)
+{
+    SDL_SYS_DelayNS(ns);
+}
+
+void SDL_DelayPrecise(Uint64 ns)
 {
     Uint64 current_value = SDL_GetTicksNS();
     Uint64 target_value = current_value + ns;

test/testtimer.c

@@ -178,6 +178,34 @@ int main(int argc, char *argv[])
     /* Wait for the results to be seen */
     SDL_Delay(1 * 1000);
 
+    /* Check accuracy of nanosecond delay */
+    {
+        Uint64 desired_delay = SDL_NS_PER_SECOND / 60;
+        Uint64 actual_delay;
+        Uint64 total_overslept = 0;
+
+        start = SDL_GetTicksNS();
+        SDL_DelayNS(1);
+        now = SDL_GetTicksNS();
+        actual_delay = (now - start);
+        SDL_Log("Minimum nanosecond delay: %" SDL_PRIu64 " ns\n", actual_delay);
+
+        SDL_Log("Timing 100 frames at 60 FPS\n");
+        for (i = 0; i < 100; ++i) {
+            start = SDL_GetTicksNS();
+            SDL_DelayNS(desired_delay);
+            now = SDL_GetTicksNS();
+            actual_delay = (now - start);
+            if (actual_delay > desired_delay) {
+                total_overslept += (actual_delay - desired_delay);
+            }
+        }
+        SDL_Log("Overslept %.2f ms\n", (double)total_overslept / SDL_NS_PER_MS);
+    }
+
+    /* Wait for the results to be seen */
+    SDL_Delay(1 * 1000);
+
     /* Check accuracy of precise delay */
     {
         Uint64 desired_delay = SDL_NS_PER_SECOND / 60;