On Mon, 21 Aug 2017, Martin Sebor wrote: > On 08/21/2017 01:51 AM, Richard Biener wrote: > > On Sat, 19 Aug 2017, Andreas Schwab wrote: > > > > > On Aug 17 2017, Richard Biener <rguent...@suse.de> wrote: > > > > > > > I was notifed I broke proper handling of undefined overflow in > > > > multiplicative ops handling. The following resurrects previous > > > > behavior (and adds a testcase). > > > > > > > > Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk. > > > > > > This breaks gfortran.dg/alloc_comp_auto_array_2.f90 on aarch64 with > > > -mabi=ilp32 (only for -O3): > > > > > > FAIL: gfortran.dg/alloc_comp_auto_array_2.f90 -O3 -g (test for excess > > > errors) > > > Excess errors: > > > /opt/gcc/gcc-20170818/gcc/testsuite/gfortran.dg/alloc_comp_auto_array_2.f90:33:0: > > > Warning: '__builtin_memcpy' specified size between 2147483648 and > > > 4294967295 exceeds maximum object size 2147483647 [-Wstringop-overflow=] > > > > I believe this is an issue that went latent when I broke VRP earlier. > > > > I have opened PR81908, will amend with some initial analysis. > > FWIW, the core of the problem is that the warning tends to either > expose limitations in optimizations that were not written to make > use of range information, or indicate additional optimization > opportunities due to range information. In this case, since > the only valid value in the range the memcpy argument is in (i.e., > ~[0, INT_MAX]) is zero, the call could be eliminated. But this > isn't noticed by any pass until the expander checks the call for > validity. > > It seems to me that this could be handled by enhancing gimple-fold > in two ways: 1) fold arguments whose range contains only one valid > value into constants, and 2) transform calls with one or more > arguments entirely in invalid ranges into __builtin_unreachable. > > I have been thinking prototyping this solution for a while but > haven't gotten around to it yet so I can't say what problems it > might run into.
Well, there will always be missed optimizations so the correct fix for this is to not warn about memcpy with size == 0. Sure, folding can be improved but the get_size_range code is broken and has to be fixed. Richard.
