Date: Sat, 19 Nov 2011 18:19:59 +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: <20111119161958.GA91681@reks> In-Reply-To: <CAMBSHm8TdZcEJQTVoier8LGA-5j7sjoPCQxKabWVsrdk0p3=ZA@mail.gmail.com> References: <20111119100150.GA1560@reks> <CAMBSHm8TdZcEJQTVoier8LGA-5j7sjoPCQxKabWVsrdk0p3=ZA@mail.gmail.com>
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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. Thanks, Gleb. > > Thanks, > matthew
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