On Tue, Jul 07, 2020 at 09:26:58AM +0000, Tomar, Sourabh Singh wrote:
> Thanks for clarifying this. I went through literature and other stuff
> available, it describes operation and semantics. Unfortunately I wasn't
> able to get to the actual DWARF Issue/proposal when it was proposed.
>
> One small doubt still remains. It seems(to me) that DW_OP_stack_value and
> DW_OP_implicit_value both can be used to describe most optimized
> location/value of a variable. Then representing/choosing appropriate
> operation involves matter like space/compactness(just like GCC does) or
> maybe other factors too ?
>
> I'm unable to come-up with an motivating example where
> DW_OP_implicit_value is indispensable. Would you please, if possible,
> share an example(test case or any specific situation) covering this.
I wasn't involved in DWARF standard when DW_OP_implicit_value has been
added, but it was added in DWARF4, where the DWARF stack could only hold
integers (and only up to the size of address). So, DW_OP_implicit_value
was the only way to hold floating point constants, or integers larger than
address, or other non-integral values. It is like using DW_AT_const_value,
except DW_AT_const_value, except that DW_AT_const_value is for the case
when a variable etc. has the same value at all spots, while
DW_OP_implicit_value can be used in .debug_loc, you can say the object
has this constant value at certain locations, and another one at others.
DW_OP_stack_value is for the cases where the value is computable using
the DWARF stack operations.
Jakub
