Re: [PATCH] Fixing PR107411

[Qing Zhao <qing.zhao@oracle.com>]

> On Feb 20, 2023, at 10:17 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> 
> On Mon, Feb 20, 2023 at 03:04:51PM +0000, Qing Zhao via Gcc-patches wrote:
>> 
>> 
>>> On Feb 17, 2023, at 5:35 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>>> 
>>> On Fri, Feb 17, 2023 at 10:26:03PM +0000, Qing Zhao via Gcc-patches wrote:
>>>> +	      else if (!DECL_NAME (lhs_var))
>>>> +		{
>>>> +		  char *lhs_var_name_str
>>>> +		    = xasprintf ("D.%u", DECL_UID (lhs_var));
>>> 
>>> Why xasprintf?
>> 
>> Just emulated the code in “gimple_add_init_for_auto_var” without thinking too much. -:)
>>> D.%u can be sprintfed into a fixed size automatic buffer,
>>> say 3 + (HOST_BITS_PER_INT + 2) / 3 would be a good upper bound for the size
>>> of the buffer.  Then you don't need to free it...
>> 
>> xasprintf is "like a sprintf but provided a pointer to malloc’d storage (without fail)”. If free the pointer properly, then it should be okay, right?
>> In addition to “no need to free”, what other benefit to use sprintf other than xasprintf?
> 
> xasprintf+free being significantly slower, exactly because it needs to
> malloc and free later, where both are fairly expensive functions.
> The glibc asprintf for short strings like the above uses a ~ 200 byte
> static buffer, stores in there, later mallocs the needed amount of memory
> and copies it there (so again, another waste because the string needs to be
> copied around), while for longer it can do perhaps many allocations and
> realloc at the end to the right size.
> The libiberty function actually performs the printing twice, once without
> writing result anywhere to compute size, then malloc, then again into the
> malloced buffer.

Okay, thanks a lot for the info.
I will replace xasprintf with sprintf for this patch.

Qing
> 
> 	Jakub
>