[llvm-dev] Emitting a local alias

[Eli Friedman via llvm-dev]

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<mailto:_ZNSt13bad_exceptionD2Ev.stub at PLT-.L_ZTVSt13bad_exception)-8>
        .long   (_ZNSt13bad_exceptionD0Ev.stub at PLT-.L_ZTVSt13bad_exception)-8<mailto:_ZNSt13bad_exceptionD0Ev.stub at PLT-.L_ZTVSt13bad_exception)-8>
        .long   (_ZNKSt13bad_exception4whatEv.stub at PLT-.L_ZTVSt13bad_exception)-8<mailto:_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<mailto:_ZNSt13bad_exceptionD2Ev.stub at PLT-.L_ZTVSt13bad_exception)-8>
        .long   (_ZNSt13bad_exceptionD0Ev.stub at PLT-.L_ZTVSt13bad_exception)-8<mailto:_ZNSt13bad_exceptionD0Ev.stub at PLT-.L_ZTVSt13bad_exception)-8>
        .long   (_ZNKSt13bad_exception4whatEv.stub at PLT-.L_ZTVSt13bad_exception)-8<mailto:_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<http://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/20200507/8661c1c5/attachment.html>