[llvm-dev] Status of the function merging pass?

[Davide Italiano via llvm-dev]

On Thu, Jan 31, 2019 at 12:24 PM Shoaib Meenai via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> (Disclaimer: I don’t know anything about MergeFunctions; I’m just assuming functionality from the name)
>
>
>
> How does MergeFunctions compare to performing identical code folding (ICF) in the linker?
>
>

The linker has access to a narrower set of informations, so it can
only merge byte-by-byte identical functions, while, in theory, you can
imagine that mergefunc could discover equivalences (through VN), and
understand that, e.g.
```
func pat(i32 %blah) {
  %0 = sub %blah, 1
   ret %0
}
```
and
```
func tinky(i32 %blah) {
  %0 = add %blah, 0
  ret %0
}
```

are equivalent. Some things to keep in mind:
1) My example is relatively silly, but some slightly more elaborate
patterns might show up in real code
2) I think llvm does enough canonicalization that we should be able to
catch these anyways
3) I don't think the pass as-is in tree does any equivalence finding,
but that's a potential advantage of running this analysis as IR pass.

Thanks,

--
Davide