Thought on inlining indirect function calls

[FRÉDÉRIC RECOULES]

> I think it is because during early opts we do not know if f is not 
> modified by atoi call and at IPA time (where we already know that from 
> resolution info) we do not have jump functions to track global variables.

I confirm that if I remove the calls to atoi, the function is correctly inlined.
Note however that, if I declare f as static, such as it can theoretically not be
impacted by atoi, the compiler still doesn't inline the function.

Yet, if STEP 1 was an issue, and that your answers has provided some clarification,
what is about the STEP 2? As far as I know, it is not supported, it is?

Frédéric Recoules


----- Mail original -----
De: "Jan Hubicka" <hubicka@ucw.cz>
À: "Richard Biener" <richard.guenther@gmail.com>
Cc: "gcc" <gcc@gcc.gnu.org>, "FRÉDÉRIC RECOULES" <frederic.recoules@univ-grenoble-alpes.fr>, mjambor@suse.cz
Envoyé: Samedi 14 Mars 2020 14:05:07
Objet: Re: Thought on inlining indirect function calls

> >I was pretty disappointed to see that even if the compiler knows we are 
> >calling f_add, it doesn't inline the call (it ends up with "call 
> >f_add"). 
> 
> It's probably because we know it's only called once and thus not performance relevant. Try put it into a loop. 

I think it is because during early opts we do not know if f is not 
modified by atoi call and at IPA time (where we already know that from 
resolution info) we do not have jump functions to track global variables. 

Honza 
> 
> Richard. 
> 
> >I can but only suppose it is because its address is taken and from a 
> >blind black box user perspective, it doesn't sound too difficult to 
> >completely inline it. 
> > 
> >STEP 2: statically known as being among a pool of less than 
> > (arbitrarily fixed = 2) N functions 
> > 
> >#include <stdlib.h> 
> >#include <stdio.h> 
> >#include <string.h> 
> > 
> >int main (int argc, char *argv[]) 
> >{ 
> > int x, y, z; 
> > enum f_e e; 
> > if (argc < 4) return -1; 
> > if (strcmp(argv[1], "add") == 0) 
> > e = ADD; 
> > else if (strcmp(argv[1], "sub") == 0) 
> > e = SUB; 
> > else return -1; 
> > f_init(e); 
> > x = atoi(argv[2]); 
> > y = atoi(argv[3]); 
> > z = f(x, y); 
> > printf("%d\n", z); 
> > return 0; 
> >} 
> > 
> >Here the compiler can't know at compile time the function that will be 
> >called but I suppose that it knows that it will be either f_add or 
> >f_sub. 
> >A simple work around would be for the compiler to test at the call site 
> >the value of f and inline the call thereafter: 
> > 
> > if (f == &f_add) 
> > z = f_add(x, y); 
> > else if (f == &f_sub) 
> > z = f_sub(x, y); 
> > else __builtin_unreachable(); /* or z = f(x, y) to be conservative */ 
> > 
> >Once again, this transformation don't sound too complicated to 
> >implement. 
> >Still, easy to say-so without diving into the compiler's code. 
> > 
> > 
> >I hope it will assist you in your reflections, 
> >Have a nice day, 
> >Frédéric Recoules 
>