This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [RFC/RFT] Improving SMS by data dependence export


Hi. Sorry for the previous empty reply.

also, i see
+ /* We do not use operand_equal_p for ORIG_EXPRs because we need to
+ distinguish memory references at different points of the loop (which
+ would have different indices in SSA form, like a[i_1] and a[i_2], but
+ were later rewritten to same a[i]). */
+ && (p->orig_expr == q->orig_expr));


This doesn't do enough to distinguish memory references at different
points of the loop, while also eliminating from consideration that
*are* the same.

What if they are regular old VAR_DECL?
This will still return true, but they may be different accesses at
different points in the loop.

Sorry, I don't really follow. The comment is somewhat badly worded indeed. The purpose of making handling of MEM_ORIG_EXPRs (introduced by this patch) different from MEM_EXPRs in ignoring operand_equal'ity of trees pointed to by this field is enforcing that MEMs corresponding to accesses to objects of the same type but with (potentially) different addresses will not share MEM_ATTRS structure. So, if both are VAR_DECLs, returning true is OK, since different accesses still correspond to the same memory location.


The first sentence also implies that potentially different accesses could be merged here, but I don't see any reason for that except for NULL MEM_ORIG_EXPRs. Could you please elaborate on this?

In any case, this doesn't belong in mem_attrs_htab_eq, because if they
are operand_equal_p, for purposes of memory attributes, they *are*
equal.  They may still be different accesses, which is something you
have to discover later on.

I don't follow this either. Since I add a new field to MEM_ATTRS struct, which in some cases allows better disambiguation, why should I enforce MEM_EXPR's rules on it? If I, similarly to MEM_EXPRs, apply operand_equal_p also to MEM_ORIG_EXPRs, this will give me incorrect results, since different MEMs will be annotated with same MEM_ORIG_EXPR, which is wrong, since the latter is flow-sensitive, and operand_equal_p will discard that (since trees will look the same after out-of-SSA). I do not see a better way to provide flow-sensitive annotations for MEMs.


DDR will mark them as data refs

Come again? :)


Thanks.
--
Alexander Monakov


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]