Date: Wed, 7 Dec 2011 15:28:33 -0500 From: Arnaud Lacombe <lacombar@gmail.com> To: Gleb Kurtsou <gleb.kurtsou@gmail.com> Cc: freebsd-hackers@freebsd.org, mdf@freebsd.org Subject: Re: gcc 4.2 miscompilation with -O2 -fno-omit-frame-pointer on amd64 Message-ID: <CACqU3MXf%2BsbTpZMbqugmMKKb1BEbp6sNzeTkXfvnQtZ1E4ukEA@mail.gmail.com> In-Reply-To: <20111119100150.GA1560@reks> References: <20111119100150.GA1560@reks>
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Hi, On Sat, Nov 19, 2011 at 5:01 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. > > -O2 -fno-omit-frame-pointer -fno-inline is buggy > -O2 -fno-omit-frame-pointer -frename-registers is buggy > > I found similar issue with gcc 4.6, but I'm not able to reproduce it > with gcc test case: > https://bugzilla.redhat.com/show_bug.cgi?id=679924 > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47893 > this PR seems highly irrelevant, the cause has been identified to a commit made in mid-2010, that's 3 years older than gcc in base. > I'll be glad to help debugging it and will be hanging on #bsddev during > weekend as glk. > at least, can you share the testcase and miscompilation details ? Thanks, - Arnaud
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