Martin Jambor <[email protected]> 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;
}
