Date: Sun, 20 Nov 2011 20:24:30 +0200 From: Gleb Kurtsou <gleb.kurtsou@gmail.com> To: mdf@FreeBSD.org Cc: freebsd-hackers@freebsd.org Subject: Re: gcc 4.2 miscompilation with -O2 -fno-omit-frame-pointer on amd64 Message-ID: <20111120182430.GA1672@reks> In-Reply-To: <CAMBSHm-WbYxPvc5p43%2B78kY4mehRCY7r5OwjsCVh_sYaUdbv=A@mail.gmail.com> References: <20111119100150.GA1560@reks> <CAMBSHm8TdZcEJQTVoier8LGA-5j7sjoPCQxKabWVsrdk0p3=ZA@mail.gmail.com> <20111119161958.GA91681@reks> <CAMBSHm-WbYxPvc5p43%2B78kY4mehRCY7r5OwjsCVh_sYaUdbv=A@mail.gmail.com>
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On (19/11/2011 09:11), mdf@FreeBSD.org wrote: > On Sat, Nov 19, 2011 at 8:19 AM, Gleb Kurtsou <gleb.kurtsou@gmail.com> wrote: > > On (19/11/2011 07:26), mdf@FreeBSD.org wrote: > >> On Sat, Nov 19, 2011 at 2:01 AM, Gleb Kurtsou <gleb.kurtsou@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
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