https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127999
Bug ID: 127999
Summary: [12/13/14/15/16 Regression] -fprofile-use coverage
mismatch when a no_profile_instrument_function callee
is early-inlined
Product: gcc
Version: 16.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ipa
Assignee: unassigned at gcc dot gnu.org
Reporter: harmen at stoppels dot ch
Target Milestone: ---
A function marked no_profile_instrument_function that is called from an
instrumented function is not early-inlined with -fprofile-generate, but it is
early-inlined with -fprofile-use. The caller's CFG at instrumentation time then
differs from the one at profile-read time, so an otherwise identical
-fprofile-use build fails with -Werror=coverage-mismatch.
```
$ cat t.c
__attribute__((no_profile_instrument_function))
static inline int helper(int x) { return x & 1 ? x * 3 : x / 2; }
__attribute__((noinline)) int f(int n) {
int s = 0;
for (int i = 0; i < n; ++i)
s += helper(i);
return s;
}
int main(int argc, char **argv) { return f(argc * 1000) & 0x7f; }
$ gcc -O2 -fprofile-generate=$PWD/prof -c t.c -o t.o
$ gcc -fprofile-generate=$PWD/prof t.o -o t && ./t
$ gcc -O2 -fprofile-use=$PWD/prof -c t.c -o t.o
t.c: In function ‘f’:
t.c:4:31: error: number of counters in profile data for function ‘f’ does not
match its profile data (counter ‘arcs’, expected 2 and have 3)
[-Werror=coverage-mismatch]
4 | __attribute__((noinline)) int f(int n) {
| ^
t.c:4:31: error: the control flow of function ‘f’ does not match its profile
data (counter ‘time_profiler’) [-Werror=coverage-mismatch]
cc1: some warnings being treated as errors
```
In case it's helpful, the LLM I'm using points to the following:
> This appears to come from the no_profile_instrument_function check in
> can_early_inline_edge_p (PR gcov-profile/80223, moved there by the fix for
> PR ipa/103636), which is only active with -fprofile-generate
> (profile_arc_flag).
> For the profile to match, the early inliner has to make the same decision in
> both modes, e.g. by also applying the check with -fprofile-use.
