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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 --- - LLD fails to devirtualize the code that Gold plugin does"
href="https://llvm.org/bugs/show_bug.cgi?id=28554">28554</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>LLD fails to devirtualize the code that Gold plugin does
</td>
</tr>
<tr>
<th>Product</th>
<td>lld
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<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>ELF
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>krasin@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=16742" name="attach_16742" title="devirt.cc">attachment 16742</a> <a href="attachment.cgi?id=16742&action=edit" title="devirt.cc">[details]</a></span>
devirt.cc
Consider the attached program, devirt.cc. With Gold and LLVM Gold plugin we
get:
$ clang++ -O2 -fuse-ld=gold -o lala -fvisibility=hidden devirt.cc -std=c++11
-flto -fwhole-program-vtables -Wl,-plugin-opt,-pass-remarks=wholeprogramdevirt
-Wl,-plugin-opt,O1 -Wl,-plugin-opt,-function-sections
-Wl,-plugin-opt,save-temps
LLVM gold plugin: <unknown>:0:0: devirtualized call
LLVM gold plugin: <unknown>:0:0: devirtualized call
LLVM gold plugin: <unknown>:0:0: devirtualized call
$
In the bitcode (lala.opt.bc) we see the call was devirtualized:
%vfn = getelementptr inbounds i1 (%class.Base*, i32)*, i1 (%class.Base*,
i32)** %vtable, i64 2
%10 = load i1 (%class.Base*, i32)*, i1 (%class.Base*, i32)** %vfn, align 8
%11 = icmp ne i8* %9, getelementptr (i8, i8* bitcast ([6 x i8*]* @_ZTV1A to
i8*), i64 16)
%conv = zext i1 %11 to i32
%call19 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x
i8], [4 x i8]* @.str.1, i64 0, i64 0), i32 %conv)
If we link with lld, nothing happens:
$ clang++ -fuse-ld=lld -o lala devirt.cc -std=c++11 -flto -fvisibility=hidden
-fwhole-program-vtables -Wl,-mllvm,-pass-remarks=wholeprogramdevirt
-Wl,--lto-O1 -Wl,-save-temps
$
And the bitcode shows that we still have a virtual call:
%vfn = getelementptr inbounds i1 (%class.Base*, i32)*, i1 (%class.Base*,
i32)** %vtable, i64 2
%19 = load i1 (%class.Base*, i32)*, i1 (%class.Base*, i32)** %vfn, align 8
%call17 = invoke zeroext i1 %19(%class.Base* %call15, i32 1)
to label %invoke.cont16 unwind label %lpad
invoke.cont16: ; preds = %sw.epilog
%conv = zext i1 %call17 to i32
%call19 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8],
[4 x i8]* @.str.1, i32 0, i32 0), i32 %conv)
This prevents lld from being deployed in Chromium as the default linker on
Linux.</pre>
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