On Fri, Dec 12, 2008 at 12:29 AM, Martin Jambor <mjam...@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 >
