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No longer let the GC replace dead keys with the LJ_TDEADKEY tag.

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Important: this changes the semantics of the write barrier!
Carefully read the big comment block in lj_obj.h
This helps HREFK key slot specialization and allows safely hoisting
HREF/HREFK across GC steps, too (fix for a barely reproducible bug).
Dead keys are only removed during a table resize (as before).
This commit is contained in:
Mike Pall
2010-04-21 01:45:58 +02:00
parent d8cb69ed07
commit ab45481199
13 changed files with 65 additions and 43 deletions
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41
src/lj_obj.h
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@@ -75,9 +75,37 @@ typedef struct GCRef {
** a barrier has been omitted are annotated with a NOBARRIER comment.
**
** The same logic applies for stores to table slots (array part or hash
** part). ALL uses of lj_tab_set* require a barrier for the stored *value*
** (if it's a GC object). The barrier for the *key* is already handled
** internally by lj_tab_newkey.
** part). ALL uses of lj_tab_set* require a barrier for the stored value
** *and* the stored key, based on the above rules. In practice this means
** a barrier is needed if *either* of the key or value are a GC object.
**
** It's ok to LEAVE OUT the write barrier in the following special cases:
** - The stored value is nil. The key doesn't matter because it's either
** not resurrected or lj_tab_newkey() will take care of the key barrier.
** - The key doesn't matter if the *previously* stored value is guaranteed
** to be non-nil (because the key is kept alive in the table).
** - The key doesn't matter if it's guaranteed not to be part of the table,
** since lj_tab_newkey() takes care of the key barrier. This applies
** trivially to new tables, but watch out for resurrected keys. Storing
** a nil value leaves the key in the table!
**
** In case of doubt use lj_gc_anybarriert() as it's rather cheap. It's used
** by the interpreter for all table stores.
**
** Note: In contrast to Lua's GC, LuaJIT's GC does *not* specially mark
** dead keys in tables. The reference is left in, but it's guaranteed to
** be never dereferenced as long as the value is nil. It's ok if the key is
** freed or if any object subsequently gets the same address.
**
** Not destroying dead keys helps to keep key hash slots stable. This avoids
** specialization back-off for HREFK when a value flips between nil and
** non-nil and the GC gets in the way. It also allows safely hoisting
** HREF/HREFK across GC steps. Dead keys are only removed if a table is
** resized (i.e. by NEWREF) and xREF must not be CSEd across a resize.
**
** The trade-off is that a write barrier for tables must take the key into
** account, too. Implicitly resurrecting the key by storing a non-nil value
** may invalidate the incremental GC invariant.
*/
/* -- Common type definitions --------------------------------------------- */
@@ -136,10 +164,7 @@ typedef const TValue cTValue;
/* More external and GCobj tags for internal objects. */
#define LAST_TT LUA_TTHREAD
#define LUA_TPROTO (LAST_TT+1)
#define LUA_TUPVAL (LAST_TT+2)
#define LUA_TDEADKEY (LAST_TT+3)
/* Internal object tags.
**
@@ -170,7 +195,7 @@ typedef const TValue cTValue;
#define LJ_TTHREAD (-7)
#define LJ_TPROTO (-8)
#define LJ_TFUNC (-9)
#define LJ_TDEADKEY (-10)
/* Unused (-10) */
#define LJ_TTAB (-11)
#define LJ_TUDATA (-12)
/* This is just the canonical number type used in some places. */
@@ -689,7 +714,7 @@ static LJ_AINLINE int32_t lj_num2bit(lua_Number n)
/* -- Miscellaneous object handling --------------------------------------- */
/* Names and maps for internal and external object tags. */
LJ_DATA const char *const lj_obj_typename[1+LUA_TUPVAL+1];
LJ_DATA const char *const lj_obj_typename[1+LUA_TPROTO+1];
LJ_DATA const char *const lj_obj_itypename[~LJ_TNUMX+1];
#define typename(o) (lj_obj_itypename[itypemap(o)])