| Issue |
52853
|
| Summary |
clang-13 performs inlining after mcount instrumentation
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tmiasko
|
In clang-13 inlining might follow the mcount instrumentation used by -pg.
The mcount relies on frame pointers to recover a call graph, so inlining after
inserting calls to mcount produces misleading end results. In the example
below, g which is called only once is recorded as being called 1025 times:
```c
int f(int x) {
return (x & (x << 1u)) == 0;
}
```
```c
#include <stdio.h>
int f(int);
int __attribute__ ((noinline)) g() {
int c = 0;
for (int i = 0; i < (1 << 10); ++i)
c += f(i);
return c;
}
int main() {
printf("%d\n", g());
}
```
```console
$ clang-13 -g -pg -flto -O2 -c a.c
$ clang-13 -g -pg -flto -O2 -c b.c
$ clang-13 -g -pg -flto -O2 a.o b.o
$ ./a.out > /dev/null && gprof -b | grep -A2 calls
time seconds seconds calls Ts/call Ts/call name
0.00 0.00 0.00 1025 0.00 0.00 g
```
```console
$ clang-12 -g -pg -flto -O2 -c a.c
$ clang-12 -g -pg -flto -O2 -c b.c
$ clang-12 -g -pg -flto -O2 a.o b.o
$ ./a.out > /dev/null && gprof -b | grep -A2 calls
time seconds seconds calls Ts/call Ts/call name
0.00 0.00 0.00 1 0.00 0.00 g
```
This seems to be result of changes from https://reviews.llvm.org/D97608,
which moved PostInlineEntryExitInstrumentationPass out of TargetPassConfig::addIRPasses.
cc @aeubanks
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