On Tue, Sep 18, 2018 at 8:58 PM Jeff Law <l...@redhat.com> wrote: > > On 9/18/18 11:12 AM, Martin Sebor wrote: > > >> My bad. Sigh. CCP doesn't track copies, just constants, so there's not > >> going to be any data structure you can exploit. And I don't think > >> there's a value number you can use to determine the two objects are the > >> same. > >> > >> Hmm, let's back up a bit, what is does the relevant part of the IL look > >> like before CCP? Is the real problem here that we have unpropagated > >> copies lying around in the IL? Hmm, more likely the IL looksl ike: > >> > >> _8 = &pb_3(D)->a; > >> _9 = _8; > >> _1 = _9; > >> strncpy (MEM_REF (&pb_3(D)->a), ...); > >> MEM[(struct S *)_1].a[n_7] = 0; > > > > Yes, that is what the folder sees while the strncpy call is > > being transformed/folded by ccp. The MEM_REF is folded just > > after the strncpy call and that's when it's transformed into > > > > MEM[(struct S *)_8].a[n_7] = 0; > > > > (The assignments to _1 and _9 don't get removed until after > > the dom walk finishes). > > > >> > >> If we were to propagate the copies out we'd at best have: > >> > >> _8 = &pb_3(D)->a; > >> strncpy (MEM_REF (&pb_3(D)->a), ...); > >> MEM[(struct S *)_8].a[n_7] = 0; > >> > >> > >> Is that in a form you can handle? Or would we also need to forward > >> propagate the address computation into the use of _8? > > > > The above works as long as we look at the def_stmt of _8 in > > the MEM_REF (we currently don't). That's also what the last > > iteration of the loop does. In this case (with _8) it would > > be discovered in the first iteration, so the loop could be > > replaced by a simple if statement. > > > > But I'm not sure I understand the concern with the loop. Is > > it that we are looping at all, i.e., the cost? Or that ccp > > is doing something wrong or suboptimal? (Should have > > propagated the value of _8 earlier?) > I suspect it's more a concern that things like copies are typically > propagated away. So their existence in the IL (and consequently your > need to handle them) raises the question "has something else failed to > do its job earlier". > > During which of the CCP passes is this happening? Can we pull the > warning out of the folder (even if that means having a distinct warning > pass over the IL?)
Note CCP also propagates copies. Richard. > > Jeff
