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<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Eli<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org>
<b>On Behalf Of </b>Leonard Chan via llvm-dev<br>
<b>Sent:</b> Wednesday, May 6, 2020 5:22 PM<br>
<b>To:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [EXT] [llvm-dev] Emitting a local alias<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Hi,<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">I wanted to see if there was a way in IR to emit a local alias such that I would get:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">```<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> .type _ZTVSt13bad_exception,@object # @_ZTVSt13bad_exception<br>
.globl _ZTVSt13bad_exception<br>
_ZTVSt13bad_exception:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><b>.L_ZTVSt13bad_exception:</b><br>
.long 0 # 0x0<br>
.long (_ZTISt13bad_exception.rtti_proxy-.L_ZTVSt13bad_exception)-8<br>
.long (<a href="mailto:_ZNSt13bad_exceptionD2Ev.stub@PLT-.L_ZTVSt13bad_exception)-8">_ZNSt13bad_exceptionD2Ev.stub@PLT-.L_ZTVSt13bad_exception)-8</a><br>
.long (<a href="mailto:_ZNSt13bad_exceptionD0Ev.stub@PLT-.L_ZTVSt13bad_exception)-8">_ZNSt13bad_exceptionD0Ev.stub@PLT-.L_ZTVSt13bad_exception)-8</a><br>
.long (<a href="mailto:_ZNKSt13bad_exception4whatEv.stub@PLT-.L_ZTVSt13bad_exception)-8">_ZNKSt13bad_exception4whatEv.stub@PLT-.L_ZTVSt13bad_exception)-8</a><br>
```<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">rather than:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">```<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> .type _ZTVSt13bad_exception,@object # @_ZTVSt13bad_exception<br>
.globl _ZTVSt13bad_exception<br>
_ZTVSt13bad_exception:<br>
.long 0 # 0x0<br>
.long (_ZTISt13bad_exception.rtti_proxy-.L_ZTVSt13bad_exception)-8<br>
.long (<a href="mailto:_ZNSt13bad_exceptionD2Ev.stub@PLT-.L_ZTVSt13bad_exception)-8">_ZNSt13bad_exceptionD2Ev.stub@PLT-.L_ZTVSt13bad_exception)-8</a><br>
.long (<a href="mailto:_ZNSt13bad_exceptionD0Ev.stub@PLT-.L_ZTVSt13bad_exception)-8">_ZNSt13bad_exceptionD0Ev.stub@PLT-.L_ZTVSt13bad_exception)-8</a><br>
.long (<a href="mailto:_ZNKSt13bad_exception4whatEv.stub@PLT-.L_ZTVSt13bad_exception)-8">_ZNKSt13bad_exception4whatEv.stub@PLT-.L_ZTVSt13bad_exception)-8</a><br>
<br>
<b>.set .L_ZTVSt13bad_exception, _ZTVSt13bad_exception</b><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">```<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">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 .<a href="http://data.rel.ro">data.rel.ro</a>
which requires that the object does not need a relocation.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">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".<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">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"?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">- Leonard<o:p></o:p></p>
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