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

jerry-core/jmem/jmem.h

@@ -70,19 +70,21 @@ typedef uint16_t jmem_cpointer_t;
 #endif /* ENABLED (JERRY_CPOINTER_32_BIT) */
 
 /**
- * Severity of a 'try give memory back' request
+ * Memory usage pressure for reclaiming unused memory.
  *
- * The request are posted sequentially beginning from
- * low to high until enough memory is freed.
+ * Each failed allocation will try to reclaim memory with increasing pressure,
+ * until enough memory is freed to fulfill the allocation request.
  *
- * If not enough memory is freed upon a high request
+ * If not enough memory is freed and JMEM_PRESSURE_FULL is reached,
  * then the engine is shut down with ERR_OUT_OF_MEMORY.
  */
 typedef enum
 {
-  JMEM_FREE_UNUSED_MEMORY_SEVERITY_LOW,  /**< 'low' severity */
-  JMEM_FREE_UNUSED_MEMORY_SEVERITY_HIGH, /**< 'high' severity */
-} jmem_free_unused_memory_severity_t;
+  JMEM_PRESSURE_NONE, /**< no memory pressure */
+  JMEM_PRESSURE_LOW,  /**< low memory pressure */
+  JMEM_PRESSURE_HIGH, /**< high memory pressure */
+  JMEM_PRESSURE_FULL, /**< memory full */
+} jmem_pressure_t;
 
 /**
  * Node for free chunk list
@@ -133,15 +135,6 @@ typedef struct
 
   size_t property_bytes; /**< allocated memory for properties */
   size_t peak_property_bytes; /**< peak allocated memory for properties */
-
-  size_t skip_count; /**< Number of skip-aheads during insertion of free block */
-  size_t nonskip_count; /**< Number of times we could not skip ahead during
-                         *   free block insertion */
-
-  size_t alloc_count; /**< number of memory allocations */
-  size_t free_count; /**< number of memory frees */
-  size_t alloc_iter_count; /**< Number of iterations required for allocations */
-  size_t free_iter_count; /**< Number of iterations required for inserting free blocks */
 } jmem_heap_stats_t;
 
 void jmem_stats_allocate_byte_code_bytes (size_t property_size);
@@ -154,21 +147,13 @@ void jmem_stats_allocate_property_bytes (size_t property_size);
 void jmem_stats_free_property_bytes (size_t property_size);
 
 void jmem_heap_get_stats (jmem_heap_stats_t *);
+void jmem_heap_stats_reset_peak (void);
+void jmem_heap_stats_print (void);
 #endif /* ENABLED (JERRY_MEM_STATS) */
 
 jmem_cpointer_t JERRY_ATTR_PURE jmem_compress_pointer (const void *pointer_p);
 void * JERRY_ATTR_PURE jmem_decompress_pointer (uintptr_t compressed_pointer);
 
-/**
- * A free memory callback routine type.
- */
-typedef void (*jmem_free_unused_memory_callback_t) (jmem_free_unused_memory_severity_t);
-
-void jmem_register_free_unused_memory_callback (jmem_free_unused_memory_callback_t callback);
-void jmem_unregister_free_unused_memory_callback (jmem_free_unused_memory_callback_t callback);
-
-void jmem_run_free_unused_memory_callbacks (jmem_free_unused_memory_severity_t severity);
-
 /**
  * Define a local array variable and allocate memory for the array on the heap.
  *
@@ -245,6 +230,7 @@ void jmem_run_free_unused_memory_callbacks (jmem_free_unused_memory_severity_t s
 
 void *jmem_pools_alloc (size_t size);
 void jmem_pools_free (void *chunk_p, size_t size);
+void jmem_pools_collect_empty (void);
 
 /**
  * @}