[Dwarf-discuss] PROPOSAL: Constant expressions in locations lists

[Roland McGrath]

This was moved to the private mailing list and it doesn't look like there
has been any traffic about it for a while.  So I don't know what its status is.
I think the formulation should be adjusted a little from what was proposed.

The block is often overkill, but I think adding a plethora of
constant_value[248] and so forth would be a waste of opcodes and needless
new repetition in decoding.  Instead, I suggest using a special suffix
operation DW_OP_implicit_value.  It can follow DW_OP_const*, DW_OP_lit*, etc.

Doing it this way also makes it a natural way to describe more cases.
(This is why I used DW_OP_implicit_value rather than DW_OP_constant_value.)
Imagine:

	for (int i = 0, j = 0; i < n; ++i, j += 4) {
		...
	}

j is optimized away and has no location, but its value is computable, e.g.:

    "i" location expr: DW_OP_reg0
    "j" location expr: DW_OP_breg0(0) DW_OP_lit4 DW_OP_mul DW_OP_implicit_value

(In fact, I think I might prefer DW_OP_computed_value to be more clear.)

Note that this makes DW_CFA_val_expression redundant with using
DW_CFA_expression and DW_OP_implicit_value.  If we had contemplated these
cases earlier, we might have used this instead of adding DW_CFA_val_expression.


Furthermore, imagine a compiler generating the very same code for:

	for (int i[2] = {0,0}; i[0] < n; ++i[0], i[1] += 4) {
		...
	}

Now i[0] has a usable location but i[1] does not.  So in the 070512.1
grammar's terms, this says that the DW_OP_implicit_value suffix in fact
must be a "simple_location_expression".  This permits i's location to be:

    DW_OP_reg0 DW_OP_piece(4) \
    DW_OP_breg0(0) DW_OP_lit4 DW_OP_mul DW_OP_implicit_value DW_OP_piece(4)


Incidentally, it should be noted that DW_AT_const_value is now redundant
with a trivial location expression using DW_OP_implicit_value.  Using it
should be encouraged for the cases it covers, both because of the existing
practice and because of the simpler and smaller encoding.


Thanks,
Roland