https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752
--- Comment #46 from rguenther at suse dot de <rguenther at suse dot de> --- On Mon, 16 Nov 2015, jeehoon.kang at sf dot snu.ac.kr wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752 > > --- Comment #45 from Jeehoon Kang <jeehoon.kang at sf dot snu.ac.kr> --- > > I think this is not true. For example with MatLab (might be sth else, > > if I don't remember correctly) you are required to pass pointers to > > arrays in two halves in double(!) values (I believe the only function > > argument type they support). GCC happily makes points-to analysis work > > through those. > > Thank you for giving me an example. Yet, I think it is a little bit > unfortunate for MatLab (or sth else) to pass pointers by packing two into a > double, at least due to the readability problem. I think it is beyond the > intended usage of the C/C++ language, but I understand that GCC is the > time-honored compiler for various software and systems. > > > > > The other (unfortunate) thing is that in GCC pointer subtraction > > is always performed on integers, thus for the C source code > > > > int idx = ptr1 - ptr2; > > > > we internally have sth like > > > > int idx = ((long)ptr1 - (long)ptr2) / 4; > > > > so you can't really treat pointers as "escaped" here without loss. > > Thank you for giving me the information. I don't know the GCC internals, so I > would like to ask how much it would cost to introduce the syntax for pointer > subtractions. I hope it is not that huge, but I really don't have any idea. It would be quite some (mechanical) work but otherwise not too difficult. There is the choice whether to embed the division implicitely here or not. > > Note that we've had this (kind of) in the past and it tried to go > > without making pointers escaped at these points but only consider > > casts from integers to pointers as pointing to anything. But > > that's wrong correctness wise (not then treating the cast to integer > > as escape point). > > Treating the cast to integer as escape point is proven-correct by a > machine-checked proof (in Coq) for various standard optimization examples, > such > as CP, DCE, dead allocation elimination, etc. For more detail, please see the > paper above-mentioned. Yes, I agree that making this an escape point is enough.
