Re: [PATCH] PR 78534 Change character length from int to size_t

[Janne Blomqvist <blomqvist dot janne at gmail dot com>]

On Wed, Dec 21, 2016 at 1:50 PM, Andre Vehreschild <vehre@gmx.de> wrote:
>> Here p is the character variable, and _p is the charlen. My guess is
>> that the problem is that with -O1 it sees that the second memmove
>> would overflow p, but it doesn't realize that branch is never taken.
>> Cranking up the optimization level to -O2 and beyond makes it realize
>> it, and thus the warning disappears.
>>
>> Perhaps one could rewrite that to something like
>>
>> __builtin_memmove ((void *) *p, (void *) &"12345679"[1]{lb: 1 sz: 1},
>> MIN_EXPR<(unsigned long) D.3598,8>);
>> if ((unsigned long) D.3598 > 8)
>>   {
>>   __builtin_memset ((void*) *p + 8, 32, D.3599);
>>   }
>
> That looks interesting. It assumes though, that D.3598 will *never* be
> negative. Because when it is negative 8 characters (cast to unsigned makes the
> negative number huge) will be copied, while in the former code memmove will
> reject the coping of a negative number of bytes. Therefore I propose to omit
> the cast in the MIN_EXPR and make the constant 8 signed, too. That should
> comply and mimick the former behavior more closely. What do you think? Who's
> going to try?

Now when I think about this some more, I have a vague recollection
that a long time ago it used to be something like that.  The problem
is that MIN_EXPR<CONSTANT, NON-CONSTANT> will of course be
NON-CONSTANT, so the memcpy call can't be inlined. Hence it was
changed to two separate __builtin_memmove() calls to have better
opportunity to inline. So probably a no-go to change it back. :(

-- 
Janne Blomqvist