gcc 4.2 miscompilation with -O2 -fno-omit-frame-pointer on amd64

mdf at FreeBSD.org mdf at FreeBSD.org
Sat Nov 19 15:26:45 UTC 2011


On Sat, Nov 19, 2011 at 2:01 AM, Gleb Kurtsou <gleb.kurtsou at gmail.com> wrote:
> Hi,
>
> I was lucky to write a bit of code which gcc 4.2 fails to compile
> correctly with -O2. Too keep long story short the code fails for gcc
> from base system and last gcc 4.2 snapshot from ports. It works with gcc
> 4.3, gcc 4.4 on FreeBSD and Linux. Clang from base is also good. -O and
> -Os optimization levels are fine (I've tried with all -f* flags
> mentioned in documentation)
>
> -O2 -fno-omit-frame-pointer combination is troublesome on amd64. I
> presume i386 should be fine. These options are also used for
> compilation of kernel (with debugging enabled) and modules.
>
> I'm not able to share the code, but have a test case reproducing the
> bug. I've encountered the issue over a week ago and tried narrowing it down
> to a simple test I could share but without much success.
>
> The code itself is very common: initialize two structs on stack, call a
> function with pointers to those stucts as arguments. A number of inlined
> assertion functions. gcc fails to correctly optimize struct assignments
> with -fno-omit-frame-pointer, I have a number of small structs assigned,
> gcc decides not to use data coping but to assign fields directly. I've
> tried disabling sra, tweaking sra parameters -- no luck in forcing it
> to copy data. Replacing one particular assignment with memcpy produces
> correct code, but that's not a solution.

How small are the structs?  gcc has an optimization for structs that
are no larger than a register, but it's buggy in 4.2 and we disabled
it at $WORK.  I can dig up the patch if this is the problem.

Thanks,
matthew


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