llvm dev - Nov 2012 - [LLVMdev] Bug Report -- Possible optimizer bug with thread_local variables

On Fri, Nov 9, 2012 at 11:23 AM, Hans Wennborg <hans at chromium.org>
wrote:
> Hi Tom,
>
> On Wed, Nov 7, 2012 at 11:43 PM, Tom Bergan <tbergan at
cs.washington.edu>
> wrote:
> > Hello,
> >
> > I apologize if this has already been fixed or reported.  I believe
there
> is
> > a bug in the way the optimizer deals with thread_local variables.  The
> > attached program, test.c, has a thread-local variable "int
Foo" and a
> global
> > variable "int *Ptr".  The program takes the following steps:
> >
> > 1) The main thread spawns a new thread and waits
> > 2) The new thread writes Foo = 50 and Ptr = &Foo, then signals the
main
> > thread and waits
> > 3) The main thread prints *Ptr, releases the new thread, and exits
> >
> > The crux of this example is that the main thread obtains a pointer to
the
> > new thread's TLS via "Ptr".  When I compile with gcc,
the program prints
> > "50" as expected.  When I compile with LLVM, the program
prints "0".
>  This
> > is confirmed in the following three versions of LLVM:
> > * the 2.9 release
> > * whatever version of LLVM is driving http://llvm.org/demo/index.cgi
> > * svn revision 167568 on trunk (this was the most-recent revision as
of a
> > few hours ago)
>
> I tried your test.c, compiled with "clang -O3", but couldn't
reproduce
> the error you're seeing. (I was using revison 167547).
>
> What flags did you use when you compiled locally?
>
> Thanks,
> Hans
>
Sorry for the incomplete report.  Originally, I actually compiled test.c
using LTO, via "opt -std-compile-opts -std-link-opts".  The bug
manifests
more simply if the variables "Ptr" and "Foo" are declared
static -- in this
case, the bug is demonstrated directly with "clang -O3".

Attached are three files:
* test.c, which is the same as the old test.c, but with "Ptr" and
"Foo"
declared static
* test.0.ll, which was built with "clang -emit-llvm -S -O0 test.c -o
test.0.ll"
* test.3.ll, which was built with "clang -emit-llvm -S -O3 test.c -o
test.3.ll"

To demonstrate the bug (verified with revision 167568):
$ clang -O3 -lpthread test.c -o test
$ ./test   # prints "Foo: 0"

It is also pretty clear that "test.3.ll" is an incorrect optimization
of
"test.0.ll"

Thanks for looking at this!
-Tom
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On Fri, Nov 9, 2012 at 4:54 PM, Tom Bergan <tbergan at
cs.washington.edu>
> The bug manifests
> more simply if the variables "Ptr" and "Foo" are
declared static -- in this
> case, the bug is demonstrated directly with "clang -O3".
>
> Attached are three files:
> * test.c, which is the same as the old test.c, but with "Ptr" and
"Foo"
> declared static
> * test.0.ll, which was built with "clang -emit-llvm -S -O0 test.c -o
> test.0.ll"
> * test.3.ll, which was built with "clang -emit-llvm -S -O3 test.c -o
> test.3.ll"
>
> To demonstrate the bug (verified with revision 167568):
> $ clang -O3 -lpthread test.c -o test
> $ ./test   # prints "Foo: 0"
>
> It is also pretty clear that "test.3.ll" is an incorrect
optimization of
> "test.0.ll"
This looks like a bug indeed. Can you please file it at http://llvm.org/bugs/ ?

Thanks,
Hans

Submitted as http://llvm.org/bugs/show_bug.cgi?id=14309

On Fri, Nov 9, 2012 at 5:34 PM, Hans Wennborg <hans at chromium.org>
wrote:
> On Fri, Nov 9, 2012 at 4:54 PM, Tom Bergan <tbergan at
cs.washington.edu>
> > The bug manifests
> > more simply if the variables "Ptr" and "Foo" are
declared static -- in
> this
> > case, the bug is demonstrated directly with "clang -O3".
> >
> > Attached are three files:
> > * test.c, which is the same as the old test.c, but with
"Ptr" and "Foo"
> > declared static
> > * test.0.ll, which was built with "clang -emit-llvm -S -O0 test.c
-o
> > test.0.ll"
> > * test.3.ll, which was built with "clang -emit-llvm -S -O3 test.c
-o
> > test.3.ll"
> >
> > To demonstrate the bug (verified with revision 167568):
> > $ clang -O3 -lpthread test.c -o test
> > $ ./test   # prints "Foo: 0"
> >
> > It is also pretty clear that "test.3.ll" is an incorrect
optimization of
> > "test.0.ll"
>
> This looks like a bug indeed. Can you please file it at
> http://llvm.org/bugs/ ?
>
> Thanks,
> Hans
>
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