Re: [Linaro-TCWG-CI] gcc-17-570-g79e29562907b: 1 regression on aarch64

On Tue, May 19, 2026 at 11:03 AM Avi Kivity <[email protected]> wrote:

>
>
> On Tue, May 19, 2026 at 2:22 AM <[email protected]> wrote:
>
>> Dear contributor,
>>
>> Our automatic CI has detected problems related to your patch(es). Please
>> find some details below.
>>
>> In  aarch64 native, after:
>>   | commit gcc-17-570-g79e29562907b
>>   | Author: Avi Kivity <[email protected]>
>>   | Date:   Thu Feb 26 19:59:41 2026 +0200
>>   |
>>   |     libstdc++: optimize std::uninitialized_move{,_n}() to memcpy when
>> possible [PR121789]
>>   |
>>   |     std::uninitialized_move{,_n} delegates to the corresponding
>>   |     std::uninitialized_copy() variant after wrapping with a move
>>   |     iterator, but the std::uninitialized_copy() doesn't unwrap the
>>   | ... 17 lines of the commit log omitted.
>>
>> Produces 1 regression:
>>   |
>>   | regressions.sum:
>>   | Running g++:g++.dg/torture/dg-torture.exp ...
>>   | FAIL: g++.dg/torture/pr118521.C -O1  (test for excess errors)
>>
>
> I'll take a look.
>

In file included from /mnt/gcc.bin/include/c++/17.0.0/vector:67,
                 from ./gcc/testsuite/g++.dg/torture/pr118521.C:5:
In function ‘_ForwardIterator std::uninitialized_fill_n(_ForwardIterator,
_Size, const _Tp&) [with _ForwardIterator = char*; _Size = long unsigned
int; _Tp = char]’,
    inlined from ‘constexpr _ForwardIterator
std::__uninitialized_fill_n_a(_ForwardIterator, _Size, const _Tp&,
allocator<_Tp>&) [with _ForwardIterator = char*; _Size = long unsigned int;
_Tp = char; _Tp2 = char]’ at
/mnt/gcc.bin/include/c++/17.0.0/bits/stl_uninitialized.h:778:39,
    inlined from ‘constexpr void std::vector<_Tp,
_Alloc>::_M_fill_append(size_type, const value_type&) [with _Tp = char;
_Alloc = std::allocator<char>]’ at
/mnt/gcc.bin/include/c++/17.0.0/bits/vector.tcc:776:36,
    inlined from ‘constexpr void std::vector<_Tp,
_Alloc>::_M_fill_insert(iterator, size_type, const value_type&) [with _Tp =
char; _Alloc = std::allocator<char>]’ at
/mnt/gcc.bin/include/c++/17.0.0/bits/vector.tcc:668:20,
    inlined from ‘constexpr std::vector<_Tp, _Alloc>::iterator
std::vector<_Tp, _Alloc>::insert(const_iterator, size_type, const
value_type&) [with _Tp = char; _Alloc = std::allocator<char>]’ at
/mnt/gcc.bin/include/c++/17.0.0/bits/stl_vector.h:1565:16,
    inlined from ‘void foo()’ at
./gcc/testsuite/g++.dg/torture/pr118521.C:12:11:
/mnt/gcc.bin/include/c++/17.0.0/bits/stl_uninitialized.h:577:39: warning:
‘void* __builtin_memset(void*, int, long unsigned int)’ writing 2 bytes
into a region of size 0 overflows the destination [-Wstringop-overflow=]
  577 |                       __builtin_memset(__dest, (unsigned char)__x,
__n);
      |
~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I think it's a victim of an existing (and very annoying) misdetection of
memory overwrites (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125187).
It's not directly related to my patch (which optimizes moving a range, not
filling it), but my patch changes the detection logic causing gcc to think
the region is of size 0 (where it should be 4).

I enhanced the test code to be a full program and verified it works with
asan.

I'm not sure how to proceed. I don't want to patch pr118521.C to disable
-Wstringop-overflow, since that's what it's testing.

Copying Richard, the original author of the test
(a2755339c6c9832467c573d956e91565943ecdc1).


>
>
>>
>> Used configuration :
>>  *CI config* tcwg_gcc_check aarch64-linux-gnu
>>  *configure and test flags:* none, autodetected on
>> aarch64-unknown-linux-gnu--disable-multilib --enable-fix-cortex-a53-835769
>> --enable-fix-cortex-a53-843419 --with-arch=armv8-a
>>
>> We track this bug report under
>> https://linaro.atlassian.net/browse/GNU-1906. Please let us know if you
>> have a fix.
>>
>> If you have any questions regarding this report, please ask on
>> [email protected] mailing list.
>>
>>
>> -----------------8<--------------------------8<--------------------------8<--------------------------
>>
>> The information below contains the details of the failures, and the ways
>> to reproduce a debug environment:
>>
>> You can find the failure logs in *.log.1.xz files in
>>  *
>> http://54.172.246.49:9090/jobs/tcwg_gcc_check--master-aarch64-build/builds/10197/archive/artifacts/00-sumfiles/
>> The full lists of regressions and improvements as well as configure and
>> make commands are in
>>  *
>> http://54.172.246.49:9090/jobs/tcwg_gcc_check--master-aarch64-build/builds/10197/archive/artifacts/notify/
>> The list of [ignored] baseline and flaky failures are in
>>  *
>> http://54.172.246.49:9090/jobs/tcwg_gcc_check--master-aarch64-build/builds/10197/archive/artifacts/sumfiles/xfails.xfail
>>
>> Current build   :
>> http://54.172.246.49:9090/jobs/tcwg_gcc_check--master-aarch64-build/builds/10197/archive/artifacts/
>> Reference build :
>> http://54.172.246.49:9090/jobs/tcwg_gcc_check--master-aarch64-build/builds/10196/archive/artifacts/
>>
>> Instruction to reproduce the build :
>> https://gitlab.com/LinaroLtd/tcwg/ci/interesting-commits/-/raw/master/gcc/sha1/79e29562907b454cdb867195b34cf63156d2d0cc/tcwg_gcc_check/master-aarch64/reproduction_instructions.txt
>>
>> Full commit : See in git+ssh://[email protected]/git/gcc.git
>
>