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

[Gleb Kurtsou]

On (19/11/2011 09:11), mdf at FreeBSD.org wrote:
> 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:

It could be related but doesn't fix bug I observe. I've installed fresh
9.0-RC2 virtual machine and reran tests in clean environment.

Do you plan committing it?

> 
> 
> 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