Subject: [PATCH 00/11] Add support for GCC's __builtin_unreachable() and use it in BUG (v2).

From: David Daney <[email-masked]>
Date: Mon, 14 Sep 2009 14:50:58 -0700

When I sent the first version, I had not realized that Roland McGrath
had only a day or two earlier submitted a very similar patch (although
one that only fixed up the x86 case).

I have been working on this quite a while now, starting with adding
the required support to GCC, so with an eye towards finishing it up I
have this new version.

 From the announcement of the first version:

Starting with version 4.5, GCC has a new built-in function called
__builtin_unreachable().  The function tells the compiler that control
flow will never reach that point.  Currently we trick the compiler by
putting in for(;;); but this has the disadvantage that extra code is
emitted for an endless loop.  For an i386 kernel using
__builtin_unreachable() results in an defaultconfig that is nearly 4000
bytes smaller.

This patch set adds support to compiler.h creating a
new macro usable in the kernel called unreachable().  If the compiler
lacks __builtin_unreachable(), it just expands to for(;;).

The x86 and MIPS patches I actually tested with a GCC-4.5 snapshot.
Lacking the ability to test the rest of the architectures, I just did
what seemed right without even trying to compile the kernel.

For version 2:

I fixed a couple of checkpatch issues, and simplified the
unreachable() macro for the pre-GCC-4.5 case (as suggested by Richard
Henderson).  Also several Acked-by: were added.

New in this version (as suggested by Ingo Molnar) I added 11/11 which
uses unreachable() in asm-generic/bug.h for !CONFIG_BUG case.  This
one may be a little controversial as it will end up making code
slightly larger when !CONFIG_BUG and you are using a pre-GCC-4.5
compiler.

I will reply with the 11 patches.

David Daney (11):
   Add support for GCC-4.5's __builtin_unreachable() to compiler.h (v2)
   x86: Convert BUG() to use unreachable()
   MIPS: Convert BUG() to use unreachable()
   s390: Convert BUG() to use unreachable()
   mn10300: Convert BUG() to use unreachable()
   parisc: Convert BUG() to use unreachable()
   powerpc: Convert BUG() to use unreachable()
   alpha: Convert BUG() to use unreachable()
   avr32: Convert BUG() to use unreachable()
   blackfin: Convert BUG() to use unreachable()
   Use unreachable() in asm-generic/bug.h for !CONFIG_BUG case.

  arch/alpha/include/asm/bug.h    |    3 ++-
  arch/avr32/include/asm/bug.h    |    2 +-
  arch/blackfin/include/asm/bug.h |    2 +-
  arch/mips/include/asm/bug.h     |    4 +---
  arch/mn10300/include/asm/bug.h  |    3 ++-
  arch/parisc/include/asm/bug.h   |    4 ++--
  arch/powerpc/include/asm/bug.h  |    2 +-
  arch/s390/include/asm/bug.h     |    2 +-
  arch/x86/include/asm/bug.h      |    4 ++--
  include/asm-generic/bug.h       |    4 ++--
  include/linux/compiler-gcc4.h   |   14 ++++++++++++++
  include/linux/compiler.h        |    5 +++++
  12 files changed, 34 insertions(+), 15 deletions(-)