Hi Martin,
On Thu, 17 Jan 2019 at 02:51, Martin Sebor <mse...@gmail.com> wrote: > > On 1/16/19 6:14 PM, Jeff Law wrote: > > On 1/15/19 8:21 AM, Martin Sebor wrote: > >> On 1/15/19 4:07 AM, Richard Biener wrote: > >>> On Tue, Jan 15, 2019 at 1:08 AM Martin Sebor <mse...@gmail.com> wrote: > >>>> > >>>> The gimple_fold_builtin_memory_op() function folds calls to memcpy > >>>> and similar to MEM_REF when the size of the copy is a small power > >>>> of 2, but it does so without considering whether the copy might > >>>> write (or read) past the end of one of the objects. To detect > >>>> these kinds of errors (and help distinguish them from -Westrict) > >>>> the folder calls into the wrestrict pass and lets it diagnose them. > >>>> Unfortunately, that can lead to false positives for even some fairly > >>>> straightforward code that is ultimately found to be unreachable. > >>>> PR 88800 is a report of one such problem. > >>>> > >>>> To avoid these false positives the attached patch adjusts > >>>> the function to avoid issuing -Warray-bounds for out-of-bounds > >>>> calls to memcpy et al. Instead, the patch disables the folding > >>>> of such invalid calls (and only those). Those that are not > >>>> eliminated during DCE or other subsequent passes are eventually > >>>> diagnosed by the wrestrict pass. > >>>> > >>>> Since this change required removing the dependency of the detection > >>>> on the warning options (originally done as a micro-optimization to > >>>> avoid spending compile-time cycles on something that wasn't needed) > >>>> the patch also adds tests to verify that code generation is not > >>>> affected as a result of warnings being enabled or disabled. With > >>>> the patch as is, the invalid memcpy calls end up emitted (currently > >>>> they are folded into equally invalid MEM_REFs). At some point, > >>>> I'd like us to consider whether they should be replaced with traps > >>>> (possibly under the control of as has been proposed a number of > >>>> times in the past. If/when that's done, these tests will need to > >>>> be adjusted to look for traps instead. > >>>> > >>>> Tested on x86_64-linux. > >>> > >>> I've said in the past that I feel delaying of folding is wrong. > >>> > >>> To understand, the PR is about emitting a warning for out-of-bound > >>> accesses in a dead code region? > >> > >> Yes. I am keeping in my mind your preference of not delaying > >> the folding of valid code. > >> > >>> > >>> If we think delaying/disablign the folding is the way to go the > >>> patch looks OK. > >> > >> I do, at least for now. I'm taking this as your approval to commit > >> the patch (please let me know if you didn't mean it that way). > > Note we are in stage4, so we're supposed to be addressing regression > > bugfixes and documentation issues. > > > > So I think Richi needs to be explicit about whether or not he wants > > this in gcc-9 or if it should defer to gcc-10. > > > > I have no technical objections to the patch and would easily ack it in > > stage1 or stage3. > > The warning is a regression introduced in GCC 8. I was just about > to commit the fix so please let me know if I should hold off until > stage 1. > After your commit (r268037), I'm seeing excess errors on some arm targets: FAIL: c-c++-common/Wrestrict.c -Wc++-compat (test for excess errors) Excess errors: /gcc/testsuite/c-c++-common/Wrestrict.c:195:3: warning: 'memcpy' accessing 4 bytes at offsets [2, 3] and 0 overlaps between 1 and 2 bytes at offset [2, 3] [-Wrestrict] /gcc/testsuite/c-c++-common/Wrestrict.c:202:3: warning: 'memcpy' accessing 4 bytes at offsets [2, 3] and 0 overlaps between 1 and 2 bytes at offset [2, 3] [-Wrestrict] /gcc/testsuite/c-c++-common/Wrestrict.c:207:3: warning: 'memcpy' accessing 4 bytes at offsets [2, 3] and 0 overlaps between 1 and 2 bytes at offset [2, 3] [-Wrestrict] This is not true for all arm toolchains, so for instance if you want to reproduce it, you can build for target arm-eabi and keep default cpu/fpu/mode. Or force -march=armv5t when running the test. To give you an idea, you can look at http://people.linaro.org/~christophe.lyon/cross-validation/gcc/trunk/268043/report-build-info.html the red cells correspond to the regressions, you can deduce the configure flags. Christophe > Martin
