What does LJ_TNUMX+2 mean? about raptorjit

[fogfx]

This appears to be another genlibbc.lua thing; this check is located within the BC_ISTYPE and BC_ISNUM cases. This is checking the contents of those instructions' C operand.

So if we look at the code in that file, we see that operand C is being assigned a value located in name2itype, a table that maps short type names to values. The type name is derived from the name of the CHECK functions in the embedded Lua definitions (eg. CHECK_tab). We see that CHECK_int and CHECK_num would, according to this table, store the values of 14 and 15, respectively, into operand C.

LJ_TNUMX is defined as (~13u), so the result of this expression is 15, which corresponds to the num entry. So it appears this branch corresponds to CHECK_num (which appears to always generate BC_ISNUM instead of BC_ISTYPE like the others). So this looks like BC_ISNUM is sort-of-kind-of like a cut-down tonumber instruction.

[lukego]

Thanks for the explanation @fogfx. I am reading genlibbc.lua for the first time.

Does the value ~LJ_TNUMX+2 make sense even so? How come +2 specifically?

There is a comment:

/* These coercions need to correspond with lj_meta_istype(). */

but the code in lj_meta_istype() seems to use +1 instead of +2.

[fogfx]

but the code in lj_meta_istype() seems to use +1 instead of +2

It does, but you can see that it decrements tp first (the string checking branch is also different because of this). Why it does it that way instead of mirroring the lj_record.c code... who knows.