Refactor memory management (#2954)

This PR is a general cleanup for garbage collection and memory
allocation code paths.

Changes:
  * Removed an unnecesary local variable from 'ecma_gc_mark'.
  * Refactored 'ecma_gc_run' to have an implicit list head during
    iteration, which results in one less condition in the loops,
    and changed the loops to use compressed pointers to reduce the
    overall amount of compression/decompression.
  * Renamed 'jmem_free_unused_memory_severity_t' to 'jmem_pressure_t',
    and added additional values.
  * Removed 'jmem_free_unused_memory_callback', instead
    'ecma_free_unused_memory' is now called directly.
  * Reworked 'ecma_free_unused_memory' to handle all code paths related
    to 'jmem_pressure_t', and moved all relevant code paths into this
    function. This simplifies the code paths in other places.
  * Reworked 'jmem_heap_gc_and_alloc_block' to be more streamlined.
  * Changed mem-stats to not report unused pool chunks as allocated
    memory.
  * Created an allocator internal API for allocating/freeing memory blocks
    that are not reported as used memory in mem-stats.
  * Removed iteration statistics for the jerry allocator from mem-stats,
    as they don't provide any actually useful information.

Co-authored-by: Marko Fabo <mfabo@inf.u-szeged.hu>
JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

Author: dbatyai

docs/02.API-REFERENCE.md

@@ -87,10 +87,10 @@ Option bits for [jerry_parse](#jerry_parse) and
 
 Set garbage collection operational mode
 
- - JERRY_GC_SEVERITY_LOW - free unused objects
- - JERRY_GC_SEVERITY_HIGH - free as much memory as possible
+ - JERRY_GC_PRESSURE_LOW - free unused objects
+ - JERRY_GC_PRESSURE_HIGH - free as much memory as possible
 
-The difference between `JERRY_GC_SEVERITY_LOW` and `JERRY_GC_SEVERITY_HIGH`
+The difference between `JERRY_GC_PRESSURE_LOW` and `JERRY_GC_PRESSURE_HIGH`
 is that the former keeps memory allocated for performance improvements such
 as property hash tables for large objects. The latter frees all possible
 memory blocks but the performance may drop after the garbage collection.
@@ -844,7 +844,7 @@ main (void)
   jerry_value_t object_value = jerry_create_object ();
   jerry_release_value (object_value);
 
-  jerry_gc (JERRY_GC_SEVERITY_LOW);
+  jerry_gc (JERRY_GC_PRESSURE_LOW);
 
   jerry_cleanup ();
 }

docs/10.EXT-REFERENCE-HANDLER.md

@@ -69,7 +69,7 @@ An alias to `jerryx_handler_assert_fatal`.
 **Summary**
 
 Expose garbage collector to scripts. If the first argument of the function
-is logical true, it performs a high severity gc. Otherwise a low severity
+is logical true, it performs a high pressure gc. Otherwise a low pressure
 gc is performed, which is also the default if no parameters passed.
 
 **Prototype**

docs/14.EXT-REFERENCE-HANDLE-SCOPE.md

@@ -41,7 +41,7 @@ main (void)
   jerry_init (JERRY_INIT_EMPTY);
 
   test_handle_scope_val ();
-  jerry_gc (JERRY_GC_SEVERITY_LOW);
+  jerry_gc (JERRY_GC_PRESSURE_LOW);
 
   jerry_cleanup ();
 } /* main */
@@ -93,7 +93,7 @@ main (void)
   jerry_init (JERRY_INIT_EMPTY);
 
   test_handle_scope_val ();
-  jerry_gc (JERRY_GC_SEVERITY_LOW);
+  jerry_gc (JERRY_GC_PRESSURE_LOW);
 
   jerry_cleanup ();
 } /* main */

jerry-core/api/jerry.c

@@ -292,8 +292,14 @@ jerry_gc (jerry_gc_mode_t mode) /**< operational mode */
 {
   jerry_assert_api_available ();
 
-  ecma_gc_run (mode == JERRY_GC_SEVERITY_LOW ? JMEM_FREE_UNUSED_MEMORY_SEVERITY_LOW
-                                             : JMEM_FREE_UNUSED_MEMORY_SEVERITY_HIGH);
+  if (mode == JERRY_GC_PRESSURE_LOW)
+  {
+    /* Call GC directly, because 'ecma_free_unused_memory' might decide it's not yet worth it. */
+    ecma_gc_run ();
+    return;
+  }
+
+  ecma_free_unused_memory (JMEM_PRESSURE_HIGH);
 } /* jerry_gc */
 
 /**