<comments inline>
On 11/7/25 6:58 AM, Mark Wielaard wrote:
Hi Ben,
On Thu, 2025-11-06 at 11:34 -0800, Ben Woodard via Dwarf-discuss wrote:
On 11/6/25 9:46 AM, Mark Wielaard via Dwarf-discuss wrote:
Given what I now know and the problem that you pointed out with CFI.
I think that I would suggest:
Rename DW_OP_mod to DW_OP_rem in DWARF6 and then specify that it uses
truncated division so that it follows the semantics of C99's % operator
for signed and unsigned types. This would minimize the implementation
difficulty for most consumers. This should be backward compatible with
existing usage because of the undocumented limitation on the domain of
DW_OP_mod. If people feel there is a need, it can also be extended to
support floating point types.
Then if the committee feels that it is useful to have a true modulo function,
they could also add a DW_OP_modulo which uses floored division from Knuth.
I think renaming is really confusing. And I think extending to
supporting floating point types should be a separate issue that would
also look at the other operators.
Maybe a compromise would be to keep DW_OP_mod (and make DW_OP_rem an
alias?)
I would do it the other way around make DW_OP_mod be a legacy alias and
call the same operation DW_OP_rem.
We are already renaming a bunch of operators to give them more
appropriate names and keeping the old names as aliases (and we are
already going to be renaming a bunch more to make them more accurately
reflect what they do) and so there is precedent for this. e.g.
DW_OP_push_object_address became the more appropriately named
DW_OP_push_object_location
which is only defined for use with (unsigned) generic type
values.
With Jakub's example, I think that we have a compelling reason to expand
the domain of DW_OP_rem (the former DW_OP_mod) to include signed
integral types as well as unsigned integral types. His example seems to
require the semantics of C99's % operator (truncated division).
If we do this, then it will be backward compatible. The only thing that
we would be changing is the domain over which the current DW_OP_mod
operates. We are not changing any of the semantics.
I'm ambivalent about expanding the domain to floating point values. If
someone has a reason for having it work on floating point types, then
sure why not. It is a bit of extra code in every consumer but whatever.
What I really care about is that when we update the description of
DW_OP_rem (the operation formerly known as DW_OP_mod) we specify both
the domain of the operator as well as the algorithm used.
Then introduce new DW_OP_modulo (defined using floored
division)
Again I'm personally ambivalent about the need for this. I don't think
that it is going to be used very often and I think if we do define it we
should consider pushing it into the new DW_OP_extended operation
encoding space. This will make its encoding a two byte operation but it
will reserve more of the one byte encodings for more frequently used
operations.
and DW_OP_remainder (defined using truncated division)
operators that are only to be used with typed DWARF stack values?
I really do believe that a better approach is to rename and expand the
domain of the current DW_OP_mod rather than adding another new special
purpose operator.
-ben
Cheers,
Mark
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