| Issue |
110232
|
| Summary |
[LTO] Dynamic initializer incorrectly optimized out
|
| Labels |
new issue
|
| Assignees |
rnk
|
| Reporter |
mstorsjo
|
Since 601645c3b70e2a17d18779a3a51b8bc9ecdc9aa6, CC @rnk, the following testcase fails:
main.cpp:
```c++
int DoIt() { return 0; }
inline int Unused0 = DoIt();
int GetUnused();
int main(int, char **) { return GetUnused(); }
```
lib.cpp:
```c++
int DoIt();
inline int Unused0 = DoIt();
int GetUnused() { return Unused0; }
```
stub.c:
```c
void __cxa_guard_acquire(void) {}
void __cxa_guard_release(void) {}
void __gxx_personality_seh0(void) {}
void __cxa_guard_abort(void) {}
void _Unwind_Resume(void) {}
void __main(void) {}
```
To reproduce:
```console
$ clang -target x86_64-w64-mingw32 -c stub.c
$ clang -target x86_64-w64-mingw32 -c main.cpp -mguard=cf -flto=thin
$ clang -target x86_64-w64-mingw32 -c lib.cpp -mguard=cf -flto=thin
$ lld-link lib.o main.o stub.o -out:out.exe -entry:main
lld-link: error: undefined symbol: __cxx_global_var_init
>>> referenced by out.exe.lto.lib.obj
```
The reason for this is visible, if we inspect the output of the LTO compile:
```
$ lld-link lib.o main.o stub.o -out:out.exe -entry:main -lldemit:asm
$ cat out.exe.lto.main.s | grep -A1 -B1 __cxx_global_var_init
.section .gfids$y,"dr"
.symidx __cxx_global_var_init
.section .giats$y,"dr"
--
.addrsig
.addrsig_sym __cxx_global_var_init
.addrsig_sym Unused0
```
So due to the `-mguard=cf`, we end up keeping a reference to `__cxx_global_var_init` in the `.gfids$y` section - but the LTO optimization itself has decided to optimize out the `__cxx_global_var_init` function entirely.
It seems like the exact ordering of object files is crucial here; if I swap the order of `lib.o` and `main.o` in this example, then `out.exe.lto.main.s` does retain the `__cxx_global_var_init` function, but `out.exe.lto.lto.s` has lost the function instead.
This was originally reported downstream at https://github.com/mstorsjo/llvm-mingw/issues/457.
Debugging what really happens during the LTO compilation is somewhat finicky; if I grab the LTO IR with `-lldemit:llvm -out:lto.bc` and try to compile that with `llc`, I get different results as well. (In that case, I end up with an undefined `GetUnused()` function.)
As the referenced commit 601645c3b70e2a17d18779a3a51b8bc9ecdc9aa6 was NFCI, no functional changes intended, but we do have functional changes here, I think it would be reasonable to revert the commit.
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