Re: get_ref_base_and_extent() semantics

["Richard Guenther" <richard dot guenther at gmail dot com>]

On Fri, Dec 12, 2008 at 12:29 AM, Martin Jambor <mjambor@suse.cz> wrote:
> Hi,
>
> today  I have  encountered  an unpleasant  problem  with the  function
> get_ref_base_and_extent() when it claimed  a known and constant offset
> for the expression  insn_4(D)->u.fld[arg.82_3].rt_rtvec.  (arg being a
> default_def parameter of the function,  insn is an rtx).  Moreover, it
> also  returned constant  size  and  max_size, all  three  equal to  64
> (bits).
>
> This  is certainly  wrong  (I  believe the  function  got confused  by
> unions) but after looking at the function and what it did in debugger,
> I grew unsure  what the expected behavior is.   The two alternatives I
> consider  possibly  correct  are  returned  offset  equal  to  -1  or,
> alternatively, offset  equal to the offset  of the array  (+ offset of
> rt_rtvec which is  zero) and max_size equal either to  the size of the
> array or -1 if it is unknown.
>
> At  the moment, the  function never  returns offset  equal to  -1, the
> comment above it does not  even mention such possibility and, from the
> limited research  I did, its callers  do not expect and  check for it.
> However, at  the same time,  the special handling of  non-constants in
> array  indices  after  the  label  "done" does  not  trigger  in  this
> particular case (here I suspect the  unions come to play because it is
> the last part of the conjunction that evaluates to false).
>
> Which (or what  else) is the correct semantics  of the function?  Both
> make sense to me (I would  prefer the former but I suspect other users
> rely on  the latter).   What would  be the correct  fix, when  a union
> field is  itself a  record ending with  a variable-length  array?  How
> much would I pessimize things if I just returned -1 max_size if both a
> non-constant index and a union was encountered?
>
> Or did I miss something else?

The function is supposed to return -1 for max_size in case the access
accesses a variable array index of an array of unknown size.

insn_4(D)->u.fld[arg.82_3].rt_rtvec accesses struct rtvec_def -- in which
case the access _does_ have known size (an int plus one rtx pointer).
But maybe I am missing context here?

Richard.

> Thanks a lot in advance,
>
> Martin
>