[llvm-dev] Emitting a local alias

[Leonard Chan via llvm-dev]

Ah, you're right. This accomplishes what I need. Thanks!

On Wed, May 6, 2020 at 5:43 PM Eli Friedman <efriedma at quicinc.com> wrote:

> You’re creating the alias backwards: _ZTVSt13bad_exception needs to be an
> externally visible alias pointing to the internal linkage
> L_ZTVSt13bad_exception. Then you can refer to L_ZTVSt13bad_exception, which
> is always the local one.
>
>
>
> -Eli
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Leonard
> Chan via llvm-dev
> *Sent:* Wednesday, May 6, 2020 5:22 PM
> *To:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* [EXT] [llvm-dev] Emitting a local alias
>
>
>
> Hi,
>
>
>
> I wanted to see if there was a way in IR to emit a local alias such that I
> would get:
>
>
>
> ```
>
>         .type   _ZTVSt13bad_exception, at object # @_ZTVSt13bad_exception
>         .globl  _ZTVSt13bad_exception
> _ZTVSt13bad_exception:
>
> *.L_ZTVSt13bad_exception:*
>         .long   0                       # 0x0
>         .long
> (_ZTISt13bad_exception.rtti_proxy-.L_ZTVSt13bad_exception)-8
>         .long   (
> _ZNSt13bad_exceptionD2Ev.stub at PLT-.L_ZTVSt13bad_exception)-8
>         .long   (
> _ZNSt13bad_exceptionD0Ev.stub at PLT-.L_ZTVSt13bad_exception)-8
>         .long   (
> _ZNKSt13bad_exception4whatEv.stub at PLT-.L_ZTVSt13bad_exception)-8
> ```
>
>
>
> rather than:
>
>
>
> ```
>
>         .type   _ZTVSt13bad_exception, at object # @_ZTVSt13bad_exception
>         .globl  _ZTVSt13bad_exception
> _ZTVSt13bad_exception:
>         .long   0                       # 0x0
>         .long
> (_ZTISt13bad_exception.rtti_proxy-.L_ZTVSt13bad_exception)-8
>         .long   (
> _ZNSt13bad_exceptionD2Ev.stub at PLT-.L_ZTVSt13bad_exception)-8
>         .long   (
> _ZNSt13bad_exceptionD0Ev.stub at PLT-.L_ZTVSt13bad_exception)-8
>         .long   (
> _ZNKSt13bad_exception4whatEv.stub at PLT-.L_ZTVSt13bad_exception)-8
>
> *.set .L_ZTVSt13bad_exception, _ZTVSt13bad_exception*
>
> ```
>
>
>
> The latter is what's generated when using a GlobalAlias with private
> linkage. I'm asking because in either case, I want to be able to place
> _ZTVSt13bad_exception in .rodata instead of .data.rel.ro which requires
> that the object does not need a relocation.
>
>
>
> According to `Constant::needsRelocation()`, a reloc isn't needed if the
> offset I'm taking (.long symbol_A - symbol_B) contains symbols that are
> both dso_local. I can guarantee that `symbol_A` will be, but `symbol_B`
> needs to be a global with default visibility. My solution around this is to
> emit a local alias, but the issue is that with optimizations (-O3), the
> latter snippet seems to "resolve" the aliases and instead replaces them
> with my aliasee (_ZTVSt13bad_exception) which is not dso_local. An
> alternative solution to my problem would also be finding a way to somehow
> prevent the aliases from being "optimized out".
>
>
>
> Any ideas on either (1) IR that can codegen into my first snippet or (2)
> prevent my alias in the second snippet from being "optimized out"?
>
>
>
> - Leonard
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200507/859a7d64/attachment-0001.html>