https://bugs.llvm.org/show_bug.cgi?id=45712
Bug ID: 45712
Summary: [LTO] -fwhole-program-vtables causes bad generated
code in hybrid lto/non-lto build
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: Interprocedural Optimizations
Assignee: unassignedb...@nondot.org
Reporter: sylvain.a...@ubisoft.com
CC: llvm-bugs@lists.llvm.org, pe...@pcc.me.uk,
r...@google.com, tejohn...@google.com
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.
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