[PATCH] D75617: [WPD] Provide a way to prevent function(s) from being devirtualized

[Eugene Leviant via Phabricator via llvm-commits]

evgeny777 marked an inline comment as done.
evgeny777 added a comment.

> This won't work for the index-only WPD.

This can be addressed at the moment, because index based WPD supports only single-impl devirtualization. In this case we have function name in the index.

> Probably the best way to cover that case is to use the option in ModuleSummaryAnalysis.cpp to block functions from being added to the VTableFuncs in the summary in findFuncPointers.

I don't think this is enough. If we have call slot { foo, bar }, and want to prevent devirtualization of foo, then entire call slot can't be devirtualized. If `bar` is not used by any other call slot then it won't be devirtualized as well



================
Comment at: llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-check.ll:2
 ; RUN: opt -S -wholeprogramdevirt -whole-program-visibility -pass-remarks=wholeprogramdevirt %s 2>&1 | FileCheck %s
+; Skipping vf0i1 is identical to setting public LTO visibility. We don't devirtualize vf0i1 and all other
+; virtual call targets.
----------------
tejohnson wrote:
> Why does skipping this one in particular cause both vf0i1 and vf1i1 to be skipped?
> 
> Also the comment here says that we won't devirtualize vf0i1 "and all other virtual call targets" except that the checking still looks for devirtualizations of vf*i32.
We have 4 vtables:
```
vtbl1: { vf0i1, vf1i1, vf1i32 }
vtbl2: { vf1i1, vf0i1, vf2i32 }
vtbl3: { vf0i1, vf1i1, vf3i32 }
vtbl4: { vf1i1, vf0i1, vf4i32 }
```
and 3 call slots:
```
CS1: (ByteOffset == 16) : { vf1i32, vf2i32, vf3i32, vf4i32 }
CS2: (ByteOffset == 8) : { vf1i1, vf0i1 }
CS3: (ByteOffset == 0) : { vf0i1, vf1i1 }
```
If we want to prevent devirtualization of vf0i1 then both CS2 and CS3 can't be devirtualzied and final devirtualized set is { vf1i32, vf2i32, vf3i32, vf4i32 }
If we want to prevent devirtualization of  vf1i32 then CS1 can't be devirtualized and final devirtualized set is { vf0i1 vf1i1 }


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  https://reviews.llvm.org/D75617/new/

https://reviews.llvm.org/D75617