But when I change the code 'opstatus = rand()' to 'opstatus = rand() %2', the probability of opstatus being 0 should be 50%, but the result remains the same, i.e. still split at that point.
And the specific information can be found in Bugzilla, the link is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111672 Richard Biener <richard.guent...@gmail.com> 于2023年10月4日周三 16:20写道： > > On Mon, Oct 2, 2023 at 7:15 PM Hanke Zhang via Gcc <gcc@gcc.gnu.org> wrote: > > > > Martin Jambor <mjam...@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. > > I think fnsplit currently splits out _cold_ code, I suppose !opstatus > is predicted to be false most of the time. > > It looks like your intent is to inline this very early check as > > if (!opstatus) { test_split_write_1 (..); } else { test_split_write_2 (..); > } > > to possibly elide that test? I would guess that IPA-CP is supposed to > do this but eventually refuses to create a clone for this case since > it would be large. > > Unfortunately function splitting doesn't run during IPA transforms, > but maybe IPA-CP can be teached how to avoid the expensive clone > by performing what IPA split does in the case a check in the entry > block which splits control flow can be optimized? > > Richard. > > > #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; > > }
