Hi everyone,
I'm a compiler developer working on detecting missed optimization
in real-world applications. Recently, we found that LLVM missed a
dead store elimination optimization in the PostgreSQL code
(https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/ruleutils.c#L3794)
in the master branch.
For the example below:
```
int dst[128];
memset(dst, 0, 128);
*unrelated = some_value;
dst[1] = 0;
dst[2] = 0;
```
LLVM cannot eliminate the useless stores after memset since the store to "unrelated" clobbers. But if we put the stores to "dst" ahead of the store to "unrelated", we could prevent confusing the compiler. See also the compiler explorer link: https://godbolt.org/z/P9jnKod3v and the issue of LLVM: https://github.com/llvm/llvm-project/issues/88632
To improve the codegen quality, I think it's also possible to
modify the source code, changing the order of initialization of
the member, to get better optimization. But I don't know whether
this can be considered as a bug, thus post the issue here.
If anyone could confirm this problem or post a patch for it, let
me know please. Thanks!
Best regards. Hongyu.
