opslabs: be cautious casting psize values

I could have assert()ed in the macro, but that would have duplicated
the rounding expression.

embed.fnc

@@ -4706,6 +4706,7 @@ RST	|bool	|scalar_mod_type|NULLOK const OP *o			\
 				|I32 type
 RS	|OP *	|search_const	|NN OP *o
 S	|void	|simplify_sort	|NN OP *o
+Ti	|U16	|size_to_psize	|size_t size
 RS	|OP *	|too_few_arguments_pv					\
 				|NN OP *o				\
 				|NN const char *name			\

embed.h

@@ -1425,6 +1425,7 @@
 #     define scalarkids(a)                      S_scalarkids(aTHX_ a)
 #     define search_const(a)                    S_search_const(aTHX_ a)
 #     define simplify_sort(a)                   S_simplify_sort(aTHX_ a)
+#     define size_to_psize                      S_size_to_psize
 #     define too_few_arguments_pv(a,b,c)        S_too_few_arguments_pv(aTHX_ a,b,c)
 #     define too_many_arguments_pv(a,b,c)       S_too_many_arguments_pv(aTHX_ a,b,c)
 #     define voidnonfinal(a)                    S_voidnonfinal(aTHX_ a)

op.c

@@ -207,7 +207,13 @@ Perl_op_prune_chain_head(OP** op_p)
 #endif
 
 /* rounds up to nearest pointer */
-#define SIZE_TO_PSIZE(x)	((U16)(((x) + sizeof(I32 *) - 1)/sizeof(I32 *)))
+PERL_STATIC_INLINE U16
+S_size_to_psize(size_t sz) {
+    size_t psize = (sz + sizeof(I32 *) - 1) / sizeof(I32 *);
+    assert(psize < U16_MAX);
+
+    return (U16)psize;
+}
 
 #define DIFF(o,p)	\
     (assert((size_t)((char *)(p) - (char *)(o)) % sizeof(I32**) == 0),  \
@@ -221,7 +227,7 @@ Perl_op_prune_chain_head(OP** op_p)
     )
 
 /* opslot_size includes the size of the slot header, and an op can't be smaller than BASEOP */
-#define OPSLOT_SIZE_BASE (SIZE_TO_PSIZE(sizeof(OPSLOT)))
+#define OPSLOT_SIZE_BASE (size_to_psize(sizeof(OPSLOT)))
 
 /* the number of bytes to allocate for a slab with sz * sizeof(I32 **) space for op */
 #define OpSLABSizeBytes(sz) \
@@ -350,7 +356,7 @@ Perl_Slab_Alloc(pTHX_ size_t sz)
     else ++(head_slab = (OPSLAB *)CvSTART(PL_compcv))->opslab_refcnt;
 
     /* size in pointer units, including the OPSLOT header */
-    U16 sz_in_p = SIZE_TO_PSIZE(sz + OPSLOT_HEADER);
+    U16 sz_in_p = size_to_psize(sz + OPSLOT_HEADER);
 
     /* The head slab for each CV maintains a free list of OPs. In particular, constant folding
        will free up OPs, so it makes sense to re-use them where possible. A
@@ -9501,7 +9507,7 @@ Perl_newFOROP(pTHX_ I32 flags, OP *sv, OP *expr, OP *block, OP *cont)
      * keep it in-place if there's space */
     if (loop->op_slabbed
         &&    OpSLOT(loop)->opslot_size
-            < SIZE_TO_PSIZE(sizeof(LOOP) + OPSLOT_HEADER))
+            < size_to_psize(sizeof(LOOP) + OPSLOT_HEADER))
     {
         /* no space; allocate new op */
         LOOP *tmp;