diff --git a/src/lj_ir.h b/src/lj_ir.h
index 8b2fe638..3de57046 100644
--- a/src/lj_ir.h
+++ b/src/lj_ir.h
@@ -294,7 +294,9 @@ LJ_DATA const uint8_t lj_ir_mode[IR__MAX+1];
 
 /* -- IR instruction types ------------------------------------------------ */
 
-/* Map of itypes to non-negative numbers. ORDER LJ_T.
+#define IRTSIZE_PGC		(LJ_GC64 ? 8 : 4)
+
+/* Map of itypes to non-negative numbers and their sizes. ORDER LJ_T.
 ** LJ_TUPVAL/LJ_TTRACE never appear in a TValue. Use these itypes for
 ** IRT_P32 and IRT_P64, which never escape the IR.
 ** The various integers are only used in the IR and can only escape to
@@ -302,12 +304,13 @@ LJ_DATA const uint8_t lj_ir_mode[IR__MAX+1];
 ** contiguous and next to IRT_NUM (see the typerange macros below).
 */
 #define IRTDEF(_) \
-  _(NIL, 4) _(FALSE, 4) _(TRUE, 4) _(LIGHTUD, LJ_64 ? 8 : 4) _(STR, 4) \
-  _(P32, 4) _(THREAD, 4) _(PROTO, 4) _(FUNC, 4) _(P64, 8) _(CDATA, 4) \
-  _(TAB, 4) _(UDATA, 4) \
+  _(NIL, 4) _(FALSE, 4) _(TRUE, 4) _(LIGHTUD, LJ_64 ? 8 : 4) \
+  _(STR, IRTSIZE_PGC) _(P32, 4) _(THREAD, IRTSIZE_PGC) _(PROTO, IRTSIZE_PGC) \
+  _(FUNC, IRTSIZE_PGC) _(P64, 8) _(CDATA, IRTSIZE_PGC) _(TAB, IRTSIZE_PGC) \
+  _(UDATA, IRTSIZE_PGC) \
   _(FLOAT, 4) _(NUM, 8) _(I8, 1) _(U8, 1) _(I16, 2) _(U16, 2) \
   _(INT, 4) _(U32, 4) _(I64, 8) _(U64, 8) \
-  _(SOFTFP, 4)  /* There is room for 9 more types. */
+  _(SOFTFP, 4)  /* There is room for 8 more types. */
 
 /* IR result type and flags (8 bit). */
 typedef enum {