jit: document non-recordable snapshots

Author: Frityet

src/lj_crecord.c

@@ -2384,10 +2384,11 @@ static int crec_call(jit_State *J, RecordFFData *rd, GCcdata *cd)
     IRType t;
 
 #if !defined(LJ_FFI_CALLXS_TEST_ACTIVATE)
-    /* Default safety boundary: first-time callbacks still need ACTIVE ->
-    ** SUSPENDED publication, and boxed results need pre-rooted storage before
-    ** arbitrary generic calls can record by default. The test activation uses
-    ** this exact production seam for the already non-allocating return class. */
+    /* Default recorder safety boundary: first-time callbacks still need
+    ** ACTIVE -> SUSPENDED publication, and boxed results need pre-rooted
+    ** storage before arbitrary generic calls can record by default. The test
+    ** activation uses this exact production seam for the nonallocating
+    ** result-handoff classes. */
     lj_trace_err(J, LJ_TRERR_BLACKL);
 #else
     /* Until continuation/pcall/tail-return handoff has its own exact snapshot

src/lj_ffrecord.c

@@ -662,7 +662,7 @@ static void LJ_FASTCALL recff_next(jit_State *J, RecordFFData *rd)
 
 /* -- Math library fast functions ----------------------------------------- */
 
-/* Materialize the current caller state for a future generic native call.
+/* Materialize the current caller state for a generic native call.
 ** Keep the snapshot number out of the IR: LOOP substitution copies and
 ** reindexes snapshots, while backward assembly already tracks the matching
 ** snapshot for the copied marker.

src/lj_jit.h

@@ -221,7 +221,7 @@ typedef struct SnapShot {
   uint8_t count;	/* Count of taken exits for this snapshot. */
 } SnapShot;
 
-#define SNAPCOUNT_DONE	255	/* Already compiled and linked a side trace. */
+#define SNAPCOUNT_DONE	255	/* Side trace linked, or exit is non-recordable. */
 
 /* Compressed snapshot entry. */
 typedef uint32_t SnapEntry;