<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [LTO] -fwhole-program-vtables causes bad generated code in hybrid lto/non-lto build"
href="https://bugs.llvm.org/show_bug.cgi?id=45712">45712</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[LTO] -fwhole-program-vtables causes bad generated code in hybrid lto/non-lto build
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</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>Interprocedural Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sylvain.audi@ubisoft.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, peter@pcc.me.uk, rnk@google.com, tejohnson@google.com
</td>
</tr></table>
<p>
<div>
<pre>When trying to enable Thin-LTO on one of our games, we found invalid generated
code which caused an infinite loop when we enabled -the
`-fwhole-program-vtables`.
The bug also appeared when we tried LTO.
I mentionned this bug in comments of D55153
Note that we build the game against some libraries that were not built with
LTO/ThinLTO.
Here it is, reduced as a relatively small repro:
$cat main.cpp
#include "inc.h"
extern void UseExternal();
int main() {
UseExternal();
return 0;
}
$cat inc.h
class A {
public:
virtual void f() {
pNext->f();
}
A* pNext = nullptr;
};
class B : public A {
public:
virtual void f() {}
};
$cat external.cpp
#include "inc.h"
static A s_A;
static B s_B;
void UseExternal() {
s_A.pNext = &s_B;
(&s_A)->f();
}
$cat def.cpp
#include "inc.h"
A g_A;
$clang-cl main.cpp def.cpp -c /Z7 /O2 /GR- -flto -fwhole-program-vtables
$clang-cl extern.cpp -c /Z7 /O2 /GR-
$lld-link /NODEFAULTLIB /ENTRY:main /OUT:main.exe def.obj main.obj external.obj
In the generated code, we can see that A::f() is generated as an endless loop.
14000103f: cc int3
140001040: eb fe jmp 0x140001040 ; endless loop
140001042: cc int3
It looks like it was wrongly devirtualized:
virtual void f() {
pNext->f();
}
where pNext->f() becomes an endless recursion.
IMPORTANT: The bug appears only if def.obj is the first object in the lld-link
command.
This bug is pretty fragile to reproduce.
The repro above is with clang-cl / lld-link; I didn't manage to repro it under
clang/ld.lld.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>