Re: [PATCH] c++: Retry the aliasing of base/complete cdtor optimization at import_export_decl time [PR113208]

[Jan Hubicka <hubicka@ucw.cz>]

> 
> Ah, you're right.
> If I compile (the one line modified) pr113208_0.C with
> -O -fno-early-inlining -fdisable-ipa-inline -std=c++20
> it does have just _ZN6vectorI12QualityValueEC2ERKS1_ in _ZN6vectorI12QualityValueEC2ERKS1_
> comdat and no _ZN6vectorI12QualityValueEC1ERKS1_
> and pr113208_1.C with -O -fno-early-inlining -fdisable-ipa-inline -std=c++20 -fno-omit-frame-pointer
> and link that together with the above mentioned third *.C file, I see
> 000000000040112a <_ZN6vectorI12QualityValueEC2ERKS1_>:
>   40112a:       53                      push   %rbx
>   40112b:       48 89 fb                mov    %rdi,%rbx
>   40112e:       48 89 f7                mov    %rsi,%rdi
>   401131:       e8 9c 00 00 00          call   4011d2 <_ZNK12_Vector_baseI12QualityValueE1gEv>
>   401136:       89 c2                   mov    %eax,%edx
>   401138:       be 01 00 00 00          mov    $0x1,%esi
>   40113d:       48 89 df                mov    %rbx,%rdi
>   401140:       e8 7b 00 00 00          call   4011c0 <_ZN12_Vector_baseI12QualityValueEC1Eii>
>   401145:       5b                      pop    %rbx
>   401146:       c3                      ret    
> i.e. the C2 prevailing from pr113208_0.s where it is the only symbol, and
> 0000000000401196 <_ZN6vectorI12QualityValueEC1ERKS1_>:
>   401196:       55                      push   %rbp
>   401197:       48 89 e5                mov    %rsp,%rbp
>   40119a:       53                      push   %rbx
>   40119b:       48 83 ec 08             sub    $0x8,%rsp
>   40119f:       48 89 fb                mov    %rdi,%rbx
>   4011a2:       48 89 f7                mov    %rsi,%rdi
>   4011a5:       e8 28 00 00 00          call   4011d2 <_ZNK12_Vector_baseI12QualityValueE1gEv>
>   4011aa:       89 c2                   mov    %eax,%edx
>   4011ac:       be 01 00 00 00          mov    $0x1,%esi
>   4011b1:       48 89 df                mov    %rbx,%rdi
>   4011b4:       e8 07 00 00 00          call   4011c0 <_ZN12_Vector_baseI12QualityValueEC1Eii>
>   4011b9:       48 8b 5d f8             mov    -0x8(%rbp),%rbx
>   4011bd:       c9                      leave  
>   4011be:       c3                      ret    
> which is the C1 alias originally aliased to C2 in C5 comdat.
> So, that would match linker behavior where it sees C1 -> C2 alias prevails,
> but a different version of C2 prevails, so let's either make C1 a non-alias
> or alias to a non-exported symbol or something like that.

Yep, I think the linker simply handles the C2 symbol as weak symbol and
after it decides to keep both comdat section (C2 and C5) the C5's weak
symbol is prevailed by C2.

I wrote patch doing the same with LTO - we need to handle alias
references specially and do not update them according to the resolution
info and then look for prevailed symbols which have aliases and turn
them to static symbols.  

I think for most scenarios this is OK, but I am not sure about
incremental linking (both LTO and non-LTO). What seems wrong is that we
produce C5 comdat that is not exporting what it should and thus breaking
the invariant that in valid code all comdats of same name are
semantically equivalent.  Perhaps it makes no difference since this
scenario is pretty special and we know that the functions are
semantically equivalent and their addresses are never compared for
equality (at least I failed to produce some useful testcase).

Honza