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(-)
