Re: Question about function splitting

[Hanke Zhang <hkzhang455@gmail.com>]

Martin Jambor <mjambor@suse.cz> 于2023年10月3日周二 00:34写道：
>
> Hello,
>
> On Mon, Oct 02 2023, Hanke Zhang via Gcc wrote:
> > Hi, I have some questions about the strategy and behavior of function
> > splitting in gcc, like the following code:
> >
> > int glob;
> > void f() {
> >   if (glob) {
> >     printf("short path\n");
> >     return;
> >   }
> >   // do lots of expensive things
> >   // ...
> > }
> >
> > I hope it can be broken down like below, so that the whole function
> > can perhaps be inlined, which is more efficient.
> >
> > int glob;
> > void f() {
> >   if (glob) {
> >     printf("short path\n");
> >     return;
> >   }
> >   f_part();
> > }
> >
> > void f_part() {
> >   // do lots of expensive things
> >   // ...
> > }
> >
> >
> > But on the contrary, gcc splits it like these, which not only does not
> > bring any benefits, but may increase the time consumption, because the
> > function call itself is a more resource-intensive thing.
> >
> > int glob;
> > void f() {
> >   if (glob) {
> >     f_part();
> >     return;
> >   }
> >   // do lots of expensive things
> >   // ...
> > }
> >
> > void f_part() {
> >   printf("short path\n"); // just do this????
> > }
> >
> > Are there any options I can offer to gcc to change this behavior? Or
> > do I need to make some changes in ipa-split.cc?
>
> I'd suggest you file a bug to Bugzilla with a specific example that is
> mis-handled, then we can have a look and discuss what and why happens
> and what can be done about it.
>
> Thanks,
>
> Martin

Hi, thanks for your reply.

I'm trying to create an account right now. And I put a copy of the
example code here in case someone is interested.

And I'm using gcc 12.3.0. When you complie the code below via 'gcc
test.c -O3 -flto -fdump-tree-fnsplit', you will find a phenomenon that
is consistent with what I described above in the gimple which is
dumped from fnsplit.

#include <stdio.h>
#include <stdlib.h>

int opstatus;
unsigned char *objcode = 0;
unsigned long position = 0;
char *globalfile;

int test_split_write(char *file) {
  FILE *fhd;

  if (!opstatus) {
    // short path here
    printf("Object code generation not active! Forgot to call "
           "quantum_objcode_start?\n");
    return 1;
  }

  if (!file)
    file = globalfile;

  fhd = fopen(file, "w");

  if (fhd == 0)
    return -1;

  fwrite(objcode, position, 1, fhd);

  fclose(fhd);

  int *arr = malloc(1000);
  for (int i = 0; i < 1000; i++) {
    arr[i] = rand();
  }

  return 0;
}

// to avoid `test_split_write` inlining into main
void __attribute__((noinline)) call() { test_split_write("./txt"); }

int main() {
  opstatus = rand();
  objcode = malloc(100);
  position = 0;
  call();
  return 0;
}