Make Callable::call_argv_fast public (#7315)

* Make Callable::call_argv_fast public

* Add rough specification of the calling convention

* Fix a typo

src/Callable.h

@@ -281,7 +281,6 @@ class Callable {
 
     // Note that the first entry in argv must always be a JITUserContext*.
     int call_argv_checked(size_t argc, const void *const *argv, const QuickCallCheckInfo *actual_cci) const;
-    int call_argv_fast(size_t argc, const void *const *argv) const;
 
     using FailureFn = std::function<int(JITUserContext *)>;
 
@@ -380,6 +379,24 @@ class Callable {
             };
         }
     }
+
+    /** Unsafe low-overhead way of invoking the Callable.
+     *
+     * This function relies on the same calling convention as the argv-based
+     * functions generated for ahead-of-time compiled Halide pilelines.
+     *
+     * Very rough specifications of the calling convention (but check the source
+     * code to be sure):
+     *
+     *   * Arguments are passed in the same order as they appear in the C
+     *     function argument list.
+     *   * The first entry in argv must always be a JITUserContext*. Please,
+     *     note that this means that argv[0] actually contains JITUserContext**.
+     *   * All scalar arguments are passed by pointer, not by value, regardless of size.
+     *   * All buffer arguments (input or output) are passed as halide_buffer_t*.
+     *
+     */
+    int call_argv_fast(size_t argc, const void *const *argv) const;
 };
 
 }  // namespace Halide