[clang] [llvm] [WPD]: Apply speculative WPD in non-lto mode. (PR #145031)

[Hassnaa Hamdi via cfe-commits]

hassnaaHamdi wrote:

> > Thanks for your patience - I needed to think about the best option to use for the new functionality, and what this should look like longer term. I found one used by gcc for this purpose, -fdevirtualize-speculatively, see info in comments below. I have some comments sprinkled throughout but here is a summary along with a suggestion for splitting the patch and how to structure.
> > Long-term, ideally we can always do speculative devirtualization (probably under the above option), for any public visibility vtable, and non-speculative devirt for any hidden ones (which might include those that were originally public under LTO and whole program visibility options that separately convert the visibility). The eventually state should probably be something like:
> > ```
> > +------------------------------------+------------------------------------+------------------------------------+
> > | LTO+WPV                            | -fdevirtualize-speculatively off   | -fdevirtualize-speculatively on    |
> > +------------------------------------+------------------------------------+------------------------------------+
> > | off                                | none                               | spec devirt for all vis (or just   |
> > |                                    |                                    | public and non-spec devirt for     |
> > |                                    |                                    | hidden vis?)                       |
> > +------------------------------------+------------------------------------+------------------------------------+
> > | on                                 | non-spec devirt for hidden vis     | non-spec devirt for hidden vis and |
> > |                                    |                                    | spec devirt for public vis         |
> > +------------------------------------+------------------------------------+------------------------------------+
> > ```
> > 
> > 
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> > (hopefully that renders ok)
> > I'd suggest staging the changes like this:
> > PR 1: LLVM changes only, using a cl::opt that generically enables speculative devirt (and TODOs noting that the code should eventually support spec devirt for any public visibility vtables longer term) PR 2: clang changes to add new option and LLVM pipeline changes to enable the new optimization (probably WPV+LTO overriding it for now with TODO noting longer term plan). PR 3 (longer term): conditionally doing spec devirt for public visibility vtables
> 
> PR1 is ready: #159048

PR2 is ready: https://github.com/llvm/llvm-project/pull/159685 

https://github.com/llvm/llvm-project/pull/145031