Re: [PATCH] Fixing PR107411

[Jakub Jelinek <jakub@redhat.com>]

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.

	Jakub