Re: ToT build error with ToT GCC on Aarch64

[Jeff Law <law at redhat dot com>]

On 07/23/2018 02:31 PM, Jeff Law wrote:
> On 07/23/2018 12:51 PM, Martin Sebor wrote:
>> On 07/23/2018 12:29 PM, Martin Sebor wrote:
>>> On 07/23/2018 11:26 AM, Florian Weimer wrote:
>>>> On 07/23/2018 06:19 PM, Jeff Law wrote:
>>>>> On 07/23/2018 09:51 AM, Steve Ellcey wrote:
>>>>>> I have run into a problem when building the ToT glibc with the ToT
>>>>>> GCC on
>>>>>> Aarch64.  I haven't dug into this enough to know if this is a GCC
>>>>>> problem,
>>>>>> a glibc problem, or just a message that needs to be ignored but I
>>>>>> wanted to
>>>>>> send out an email in case this is something that needs to be addressed
>>>>>> before GCC 2.28 is released.
>>>>
>>>>> Martin S. and I are already looking at this and I've already asked
>>>>> Martin to bring in Carlos and Florian.
>>>>
>>>> Would it be possible to print the full inlining stack for these GCC
>>>> warnings?  I don't see a %K format specifier (or whatever is used for
>>>> this).
>>>>
>>>> I think it would make it a bit easier to track these down.
>>>
>>> It's one of my own pet peeves with the -Warray-bounds warnings
>>> that they don't do that.  I've quickly hacked GCC to print
>>> the inlining stack.  It doesn't look to me like it helps that
>>> much but I'll let you be the judge:
>>>
>>> In file included from fnmatch.c:244:
>>> In function ‘findidxwc’,
>>>     inlined from ‘internal_fnwmatch’ at fnmatch_loop.c:404:10:
>>> ../locale/weightwc.h:124:28: warning: array subscript 1 is outside array
>>> bounds of ‘wint_t[1]’ {aka ‘unsigned int[1]’} [-Warray-bounds]
>>
>> Here's a bit more context showing the variable being referenced
>> (for simplicity, GCC takes singleton objects as arrays of one
>> element):
>>
>> In file included from fnmatch.c:244:
>> In function ‘findidxwc’,
>>     inlined from ‘internal_fnwmatch’ at fnmatch_loop.c:404:10:
>> ../locale/weightwc.h:124:28: error: array subscript 1 is outside array
>> bounds of ‘wint_t[1]’ {aka ‘unsigned int[1]’} [-Werror=array-bounds]
>>     if (cp[nhere - 1] > usrc[nhere -1])
>>                         ~~~~^~~~~~~~~~
>> In file included from fnmatch.c:315:
>> fnmatch_loop.c: In function ‘internal_fnwmatch’:
>> fnmatch_loop.c:342:13: note: while referencing ‘str’
>>        UCHAR str;
>>              ^~~
>>
>> There's an interesting comment above the declaration:
>>
>>     /* It's important that STR be a scalar variable rather
>>        than a one-element array, because GCC (at least 4.9.2
>>        -O2 on x86-64) can be confused by the array and
>>        diagnose a "used initialized" in a dead branch in the
>>        findidx function.  */
>>     UCHAR str;
> And it gets even more interesting if you dig into the history of this code.
> 
> https://sourceware.org/ml/libc-alpha/2014-11/msg00459.html
> 
> The implication is this is dead code in the case we're looking at.
> 
> I wonder if the solution here is to disable the oob warning along with
> the uninit warning.

A key follow-up from Joseph (easy to get lost in the thread...)

https://sourceware.org/ml/libc-alpha/2014-11/msg00616.html
https://sourceware.org/ml/libc-alpha/2014-11/msg00635.html

Leading to...

https://sourceware.org/ml/libc-alpha/2014-11/msg00639.html

Jeff