[Bug c/48623] New: gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[Bug c/48623] New: gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

           Summary: gcc 4.6.0 generates no code for a function with
                    __attribute__((always_inline))
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: richard@nod.at


gcc 4.6.0 builds a non-functional User Mode Linux kernel.
It seems to optimize away the function sub_preempt_count() used in
kernel/softirq.c:__local_bh_enable().
Thus, the preempt counter gets out of balance and the kernel crashes.

A standalone test case is attached.
Just compile it with:
gcc testcase.c -Os -g -c -o testcase.o

Using objdump you can see that no code was generated for sub_preempt_count().
gcc 4.3, 4.4 and 4.5 generate code.
Without __attribute__((always_inline)) 4.6.0 produces code...

Thanks,
//richard

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

--- Comment #1 from Richard Weinberger <richard at nod dot at> 2011-04-15 14:16:22 UTC ---
Created attachment 23995
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23995
Testcase for gcc 4.6.0

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-15 14:46:32 UTC ---
Because current_thread_info() returns garbage (an address derived from an
address of a stack local).

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-15 14:52:04 UTC ---
Instead using

static inline struct thread_info *current_thread_info(void)
{
  struct thread_info *ti;
  void *p;
  asm volatile ("" : "=r" (p) : "0" (&ti));
  ti = (struct thread_info *) (((unsigned long) p) & ~mask);
  return ti;
}

might confuse GCC enough and is still architecture independent.

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

--- Comment #4 from Richard Weinberger <richard at nod dot at> 2011-04-15 17:34:33 UTC ---
Created attachment 24000
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24000
objdump of __local_bh_enable

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

Richard Weinberger <richard at nod dot at> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |

--- Comment #5 from Richard Weinberger <richard at nod dot at> 2011-04-15 17:37:23 UTC ---
(In reply to comment #3)

It's not that easy.
Your trick solves the problem only for the test case.

Within the kernel again no code has been produced.
I have the objdump of the __local_bh_enable function attached.
See line 86.

Sorry for not providing a standalone test.

Here you can see the source code of __local_bh_enable, it's a pretty simple
function.
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=kernel/softirq.c;h=174f976c2874a19f1d06fee972468e2c730bc7f9;hb=HEAD#l134

Thanks,
//richard

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

--- Comment #6 from Richard Weinberger <richard at nod dot at> 2011-04-15 17:38:55 UTC ---
Created attachment 24001
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24001
objdump of __local_bh_enable

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2011.04.15 20:11:31
     Ever Confirmed|0                           |1

--- Comment #7 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-15 20:11:31 UTC ---
Please provide preprocessed source for the translation unit that has the
broken function.

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

--- Comment #8 from Richard Weinberger <richard at nod dot at> 2011-04-15 20:26:03 UTC ---
Created attachment 24006
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24006
preprocessed __local_bh_enable function

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

--- Comment #9 from Richard Weinberger <richard at nod dot at> 2011-04-15 20:26:48 UTC ---
Created attachment 24007
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24007
preprocessed softirq.c

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[matz at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

Michael Matz <matz at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
                 CC|                            |matz at gcc dot gnu.org
         Resolution|                            |INVALID

--- Comment #10 from Michael Matz <matz at gcc dot gnu.org> 2011-04-15 22:03:05 UTC ---
You didn't change the current_thread_info carefully enough as per
comment #3.  It still reads:

static inline __attribute__((always_inline)) struct thread_info
*current_thread_info(void)
{
 struct thread_info *ti;
 void *p;
 unsigned long mask = ((1 << 0) * ((1UL) << 12)) - 1;
 asm volatile ("" : "=r" (p) : "0" (&ti));
 ti = (struct thread_info *) (((unsigned long) &ti) & ~mask);
 return ti;
}

You have to make use of 'p' of course.  Your return value still is based
on &ti.

[Bug c/48623] gcc 4.6.0 generates no code for a function with __attribute__((always_inline))

[richard at nod dot at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48623

--- Comment #11 from Richard Weinberger <richard at nod dot at> 2011-04-15 22:27:19 UTC ---
(In reply to comment #10)
> You have to make use of 'p' of course.  Your return value still is based
> on &ti.

Damn, you're right!

Now gcc produces a functional UML kernel. :-)

I fear current_thread_info() is not the only function in the kernel
which returns a local stack address.

Thanks,
//richard