Date: Fri, 2 Dec 2011 01:56:52 -0800 From: Stanislav Sedov <stas@FreeBSD.org> 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: <20111202015652.475ee54e.stas@FreeBSD.org> In-Reply-To: <20111119100150.GA1560@reks> References: <20111119100150.GA1560@reks>
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--Signature=_Fri__2_Dec_2011_01_56_52_-0800_VCl/=6NAmU9YiIfo Content-Type: text/plain; charset=US-ASCII Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sat, 19 Nov 2011 12:01:50 +0200 Gleb Kurtsou <gleb.kurtsou@gmail.com> mentioned: > Hi, >=20 > 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) >=20 > -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. >=20 > 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 do= wn > to a simple test I could share but without much success. >=20 > 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. >=20 > -O2 -fno-omit-frame-pointer -fno-inline is buggy > -O2 -fno-omit-frame-pointer -frename-registers is buggy >=20 > 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=3D679924 > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D47893 >=20 > I'll be glad to help debugging it and will be hanging on #bsddev during > weekend as glk. >=20 Hi! I'm not sure this is relevant to your case, but our base gcc used to have a bug with strict aliasing, which was fixed only in a GPLv3 version of it. That's why we have -fno-strict-aliasing in default CFALGS. So you might try to build using -fno-strict-aliasing. --=20 Stanislav Sedov ST4096-RIPE () ascii ribbon campaign - against html e-mail=20 /\ www.asciiribbon.org - against proprietary attachments --Signature=_Fri__2_Dec_2011_01_56_52_-0800_VCl/=6NAmU9YiIfo Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- iQIcBAEBAgAGBQJO2KDpAAoJEL8lojEJL9nw0HIP/38ws0TRzJQEXXiY2oLz/uKd r7nJ+UZ87kDI6EIMs2zYcb19lZw3vhRDLfmHxX3XTqhhQvfDhTu6fcAuDNl1VVOi qoYv+aowD7Hl7aLBUi4L28NOc8mz1hZosCfoJGzxetewYm9r1NRPXPPVQFVkR3rw yJuPF10NFTjnZJS7K/t/NQ6ibcl8Pr54VoaynfNxx6KO725wYoCBKAItpL/Ymrbl fthkbYOW2ZnFxDrfUdJiVh9ACRUyeGaZMlL3tYDvo/QpsiWnTqsjZUfD9Wa2L+vu 8Tq7oo5wWY/hM76oe+abn0lnM58Jc8iWg0GZpvNBu00LsI0yNMiCc7mbRBY6e691 dhJ3vKdGIJiHGws7mO0RnTEx3/F22rWBLqbta61a5lW5J4Znwipfhxic2B/sXXbR r3ztDCw5CS1+VJyYvdLLTkHSqgpBnr4pETGP/oil4uY5a3Gq3MsPlYaOJMBRcHFN 0nyLFGzqoMsOvOBn3tbkQZoJGqZx/qgQwbwsEl5smkL+7gsbfCKX6lktCkoY4LrY +wptXTS1vR5sqqPQuMWuJiOmje0qiFOvXQ7WyfGF6ikdMYmtZIw4945iA3+LtITZ AknNdgxWM/sjEOyacdNHdIRzPoHy8fg1Nrc69W36G1pQkIHV4q2lhmXv2ZWJKDj4 Rvcvw3KDgDBG4ExEpbfS =wi30 -----END PGP SIGNATURE----- --Signature=_Fri__2_Dec_2011_01_56_52_-0800_VCl/=6NAmU9YiIfo--
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