Adding SDL_Assume macro to give hint to the optimiser

with a given condition generating less instructions.

Author: devnexen

include/SDL_assert.h

@@ -69,6 +69,18 @@ assert can have unique static variables associated with it.
     #define SDL_TriggerBreakpoint()
 #endif
 
+#if defined(_MSC_VER) && (_MSC_VER > 1400)
+    #define SDL_Assume(cond) __assume(cond)
+#elif _SDL_HAS_BUILTIN(__builtin_assume)
+    #define SDL_Assume(cond) __builtin_assume(cond)
+#elif defined(__clang__) || defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 4))
+    #define SDL_Assume(cond) do {						\
+        (__builtin_expect(!(cond), 0) ? __builtin_unreachable() : (void)0);	\
+    } while (0)
+#else
+    #define SDL_Assume(cond)
+#endif
+
 #if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) /* C99 supports __func__ as a standard. */
 #   define SDL_FUNCTION __func__
 #elif ((defined(__GNUC__) && (__GNUC__ >= 2)) || defined(_MSC_VER) || defined (__WATCOMC__))

test/testplatform.c

@@ -11,6 +11,7 @@
 */
 
 #include <stdio.h>
+#include <time.h>
 
 #include "SDL.h"
 
@@ -442,6 +443,19 @@ TestAssertions(SDL_bool verbose)
     return (0);
 }
 
+int
+TestAssume(SDL_bool verbose)
+{
+    int max, count, i;
+
+    max = 16;
+    srand(time(0));
+    count = rand() % max;
+    SDL_Assume(count <= max);
+    for (i = 0; i < count; i ++);
+    return (0);
+}
+
 int
 main(int argc, char *argv[])
 {
@@ -463,6 +477,7 @@ main(int argc, char *argv[])
     status += Test64Bit(verbose);
     status += TestCPUInfo(verbose);
     status += TestAssertions(verbose);
+    status += TestAssume(verbose);
 
     return status;
 }