Disable /OPT:ICF on windows, it merges identical functions breaking i…

[vargaz]

…call symbol lookup.

[lewing]

fixes #49588

[lewing]

runtime (Libraries Test Run release mono Linux x64 Debug) failure is #49583

[jaykrell]

1. What is the size cost?
2. How about you make the afflicted functions not identical? For example, give them some static data that they use in a pointless way.
3. Can it be made to work despite the functions being identical?

gcc has an option or attribute to consider address-taken functions not unique, and avoid folding them.
This would fix that. But Visual C++ doesn't have that feature. Alas.

I think SQL Server has this very same problem as Mono, and also disables ICF to workaround it. Alas.

[jaykrell]

https://developercommunity.visualstudio.com/t/link-opticf-is-too-aggressive/1369631

[lewing]

runtime (Libraries Test Run release mono windows x64 Debug) is #49569

[lewing]

1. What is the size cost?
2. How about you make the afflicted functions not identical? For example, give them some static data that they use in a pointless way.
3. Can it be made to work despite the functions being identical?

gcc has an option or attribute to consider address-taken functions not unique, and avoid folding them.
This would fix that. But Visual C++ doesn't have that feature. Alas.

I think SQL Server has this very same problem as Mono, and also disables ICF to workaround it. Alas.

It is a correctness fix with schedule impact so I'm landing it now but I opened an issue to reexamine the optimization before release.

[jaykrell]

Understood and I expect no relief upstream but couldn't ignore it, sorry.

[AntonLapounov]

Note that in VM we use the FCUnique macro to prevent some otherwise identical functions from folding. See #39810 for an example.