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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Failure to exploit final modifier for pointer to member devirtualization"
href="https://llvm.org/bugs/show_bug.cgi?id=23195">23195</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Failure to exploit final modifier for pointer to member devirtualization
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>listmail@philipreames.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>We appear to be failing to devirtualize a pointer-to-member call when we know
that the type of the receiver is a final class.
(NOTE: This bug report is based on data collected on a ToT build in early
March. I have not confirmed that this still applies to ToT today.)
I originally noted this when writing a toy interpreter. Here's a simplified
bit of code pulled from that project:
struct Interpreter final {
typedef void (Interpreter::*BytecodeFuncType)(void *);
BytecodeFuncType BytecodeFuncs[NumBytecodes] = { ... };
void dispatch_next(void* stack){
BytecodeFuncType NextFunc = BytecodeFuncs[bytecode[current_index]];
current_index++;
(this->*NextFunc)(stack);
}
};
The important part is this line:
(this->*NextFunc)(stack);
This is calling a pointer-to-member function where the receiver type is
statically known. In particular, we know that "this" is a static type of
Interpreter and that, because of the final annotation, there are no other
possible dynamic types.
We emit LLVM IR that looks like this (after -O3):
%13 = load { i64, i64 }* %arrayidx3.i, align 8, !tbaa !10
%.fca.0.extract.i = extractvalue { i64, i64 } %13, 0
%.fca.1.extract.i = extractvalue { i64, i64 } %13, 1
%14 = bitcast %struct.Interpreter* %this to i8*
%15 = getelementptr inbounds i8* %14, i64 %.fca.1.extract.i
%this.adjusted.i = bitcast i8* %15 to %struct.Interpreter*
%16 = and i64 %.fca.0.extract.i, 1
%memptr.isvirtual.i = icmp eq i64 %16, 0
br i1 %memptr.isvirtual.i, label %memptr.nonvirtual.i, label
%memptr.virtual.i
memptr.virtual.i: ; preds =
%_ZNSt6vectorIlSaIlEE9push_backEOl.exit
%17 = bitcast i8* %15 to i8**
%vtable.i = load i8** %17, align 8, !tbaa !11
%18 = add i64 %.fca.0.extract.i, -1
%19 = getelementptr i8* %vtable.i, i64 %18
%20 = bitcast i8* %19 to void (%struct.Interpreter*, %"class.std::vector"*)**
%memptr.virtualfn.i = load void (%struct.Interpreter*,
%"class.std::vector"*)** %20, align 8
br label %_ZN11Interpreter13dispatch_nextERSt6vectorIlSaIlEE.exit
memptr.nonvirtual.i: ; preds =
%_ZNSt6vectorIlSaIlEE9push_backEOl.exit
%memptr.nonvirtualfn.i = inttoptr i64 %.fca.0.extract.i to void
(%struct.Interpreter*, %"class.std::vector"*)*
br label %_ZN11Interpreter13dispatch_nextERSt6vectorIlSaIlEE.exit
_ZN11Interpreter13dispatch_nextERSt6vectorIlSaIlEE.exit: ; preds =
%memptr.virtual.i, %memptr.nonvirtual.i
%21 = phi void (%struct.Interpreter*, %"class.std::vector"*)* [
%memptr.virtualfn.i, %memptr.virtual.i ], [ %memptr.nonvirtualfn.i,
%memptr.nonvirtual.i ]
tail call void %21(%struct.Interpreter* %this.adjusted.i,
%"class.std::vector"* dereferenceable(24) %stack)
ret void
The virtual dispatch path of this control flow is impossible. Similarly, all of
the this adjustment code is unnecessary.</pre>
</div>
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