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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - NRVO (elide-constructors) code generation is running in the C compiler"
href="https://bugs.llvm.org/show_bug.cgi?id=39048">39048</a>
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<th>Summary</th>
<td>NRVO (elide-constructors) code generation is running in the C compiler
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>7.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Frontend
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>nat-llvmbugs@mulle-kybernetik.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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</tr></table>
<p>
<div>
<pre>I had to fix a really peculiar bug when migrating mulle-clang from clang 6 to
clang 7. Maybe this affects others maybe not.
mulle-clang is an Objective-C compiler, based on C and its own runtime. It
can't do C++.
1)
In CompilerInvocation:
<a href="https://github.com/llvm-mirror/clang/blob/master/lib/Frontend/CompilerInvocation.cpp#L2457">https://github.com/llvm-mirror/clang/blob/master/lib/Frontend/CompilerInvocation.cpp#L2457</a>
Opts.ElideConstructors = !Args.hasArg(OPT_fno_elide_constructors);
is turned on by default. Also the driver says:
<a href="https://github.com/llvm-mirror/clang/blob/master/lib/Driver/ToolChains/Clang.cpp#L4281">https://github.com/llvm-mirror/clang/blob/master/lib/Driver/ToolChains/Clang.cpp#L4281</a>
// -felide-constructors is the default.
It's my opinion, that if the language is set to C, C++ only features should
not even be queried in the CompilerInvocation and be set to off. Then you would
never have to think about a possible need to query two different
LanguageOptions.
2)
During CodeGenFunction::EmitAutoVarAlloca
<a href="https://github.com/llvm-mirror/clang/blob/master/lib/CodeGen/CGDecl.cpp#L1143">https://github.com/llvm-mirror/clang/blob/master/lib/CodeGen/CGDecl.cpp#L1143</a>
a different path is taken, if the local variable is deemed to be NRVO
compatible:
```
if (Ty->isConstantSizeType()) {
bool NRVO = getLangOpts().ElideConstructors &&
D.isNRVOVariable();
```
The following NRVO code, which I didn't scrutinize, then triggers my type
assert.
3)
My variable declaration got set as isNRVOVariable during
Scope::mergeNRVOIntoParent(). This is run unconditionally during
Sema::ActOnPopScope.
I think it would be a fix and a general improvement for C anyway, to do this
only conditionally:
<a href="https://github.com/llvm-mirror/clang/blob/master/lib/Sema/SemaDecl.cpp#L1802">https://github.com/llvm-mirror/clang/blob/master/lib/Sema/SemaDecl.cpp#L1802</a>
```
if( getLangOpts().CPlusPlus)
S->mergeNRVOIntoParent();
```</pre>
</div>
</p>
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