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

[Michael Eager]

Hi Roland --

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Roland McGrath wrote:
> 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
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Michael Eager	 eager at eagercon.com
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