Re: PR79715: Special case strcpy/strncpy for dse

[Jeff Law <law@redhat.com>]

On 02/28/2017 05:59 AM, Prathamesh Kulkarni wrote:
> On 28 February 2017 at 15:40, Jakub Jelinek <jakub@redhat.com> wrote:
>> On Tue, Feb 28, 2017 at 03:33:11PM +0530, Prathamesh Kulkarni wrote:
>>> Hi,
>>> The attached patch adds special-casing for strcpy/strncpy to dse pass.
>>> Bootstrapped+tested on x86_64-unknown-linux-gnu.
>>> OK for GCC 8 ?
>>
>> What is special on strcpy/strncpy?  Unlike memcpy/memmove/memset, you don't
>> know the length they store (at least not in general), you don't know the
>> value, all you know is that they are for the first argument plain store
>> without remembering the pointer or anything based on it anywhere except in
>> the return value.
>> I believe stpcpy, stpncpy, strcat, strncat at least have the same behavior.
>> On the other side, without knowing the length of the store, you can't treat
>> it as killing something (ok, some calls like strcpy or stpcpy store at least
>> the first byte).
> Well, I assumed a store to dest by strcpy (and friends), which gets
> immediately freed would count
> as a dead store since free would kill the whole memory block pointed
> to by dest ?
Yes.  But does it happen often in practice?  I wouldn't mind exploring 
this for gcc-8, but I'd like to see real-world code where this happens.

jeff