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

[Gleb Kurtsou]

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.

-O2 -fno-omit-frame-pointer -fno-inline is buggy
-O2 -fno-omit-frame-pointer -frename-registers is buggy

I found similar issue with gcc 4.6, but I'm not able to reproduce it
with gcc test case:
https://bugzilla.redhat.com/show_bug.cgi?id=679924
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47893

I'll be glad to help debugging it and will be hanging on #bsddev during
weekend as glk.


Thanks,
Gleb.