[Dwarf-discuss] register name operators
David B. Anderson
Wed Nov 15 17:54:47 GMT 2006
Marcel Mettes writes:
|David B. Anderson wrote:
|>Marcel Mettes writes:
|I think I understand why DW_OP_piece is not an operator. Given that a
|expression must be used for both reading and writing, one cannot
|implement DW_OP_piece as a mask+shift operation. That would not
|work when writing. (not something I thought of the first time I studied
|However, in the example in the DWARF3 document, 2.6.5, there is also an
|in the last example:
| DW_OP_reg0 DW_OP_piece 4 DW_OP_piece 4 DW_OP_fbreg -12 DW_OP_piece 4
|DW_OP_fbreg is a regular operator. Is that also valid given the register
|name operator restriction?
Yes. In effect
are 3 distinct dwarf expressions.
Each dwarf expresion supplying a part of the result 'independently'.
So the restriction on DW_OP_reg<n> applies independently in
each of the 3 distinct dwarf expressions.
|I must confess that I do not really understand why the DW_OP_regx and
|have been restricted in the DWARF 3, except for perhaps making it work
|(older?) expression interpreters?
No. It's because nothing else makes sense.
DW_OP_reg0 says the variable is *in* the register.
All other dwarf-expressions compute an address and the variable
is at that computed address.
There is no way to combine DW_OP_reg3 (for example) with any other
operator (DW_OP_piece and DW_OP_bit_piece being the exceptions).
It makes no sense to do so. DW_OP_reg<n> do NOT say to dereference the register,
they just say 'register number <n>'. Since (for most architectures)
a register has no address the normal dwarf expression result
of 'address of the variable' makes no sense. DW_OP_reg<n>
provides a way to deal with the special case.
Don't know if I'm explaining this very well.
|In the 8th example, two DW_OP_reg's appear in one expression
| DW_OP_reg3 DW_OP_piece 4 DW_OP_reg10 DW_OP_piece 2
|So I wonder how to interprete this limitation as described for
|DW_OP_regx/DW_OP_reg3 in 2.6.1 .
These are *independent* as DW_OP_piece (DW_OP_bit_piece) delineate
the end of a simple expression (well sort of) and
with each simple expresson these are unique.
The problem is that there's no language in the document (that I recognize) that
really distinquishes the two categories of simple expressions:
simple simple expression
simple expression of a set of simple expressions terminated
Difficult to state.
operators..... (left as exercise)
simple-expression0 DW_OP_piece <n>
simple-expression0 DW_OP_bit_piece <n>
where the restriction applies to simple-expression0
Hmm. Then the <empty> needs to be accounted for.
And then the DW_OP_reg<n> restriction applies to simple-expression0
(not to simple-expression-n).
Hope this makes sense.
More information about the Dwarf-discuss