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

[mdf at FreeBSD.org]

On Sat, Nov 19, 2011 at 8:19 AM, Gleb Kurtsou <gleb.kurtsou at gmail.com> wrote:
> On (19/11/2011 07:26), mdf at FreeBSD.org wrote:
>> 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.
> struct sockaddr_in in this particular test. 16 bytes.
>
> Register size structs are rather common, e.g. struct in_addr.
>
> I could test the patch. Adding -finline-functions seems to fix the issue
> for me.

I can't find the thing I'm thinking of.  The only potentially relevant
patch I see in our gcc sources is this:


Index: opts.c
===================================================================
--- opts.c	(.../vendor.branches/freebsd/stable/7/src/contrib/gcc/opts.c)	(revision
211574)
+++ opts.c	(.../head/src/contrib/gcc/opts.c)	(revision 211574)
@@ -457,11 +457,7 @@
       flag_tree_dse = 1;
       flag_tree_ter = 1;
       flag_tree_live_range_split = 1;
+      /**
+       * 7dot1MERGE: tree-sra in gcc 4.2.x is buggy and
+       * breaks bitfield structs.
+       */
+      flag_tree_sra = 0;
-      flag_tree_sra = 1;
       flag_tree_copyrename = 1;
       flag_tree_fre = 1;
       flag_tree_copy_prop = 1;

Thanks,
matthew