Date: Sat, 19 Nov 2011 07:26:44 -0800 From: mdf@FreeBSD.org To: Gleb Kurtsou <gleb.kurtsou@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: gcc 4.2 miscompilation with -O2 -fno-omit-frame-pointer on amd64 Message-ID: <CAMBSHm8TdZcEJQTVoier8LGA-5j7sjoPCQxKabWVsrdk0p3=ZA@mail.gmail.com> In-Reply-To: <20111119100150.GA1560@reks> References: <20111119100150.GA1560@reks>
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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. Thanks, matthew
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