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Postings from November 2008
On Sun, Nov 16, 2008 at 04:16:14AM -0800, Niko Tyni wrote: > As seen in <http://bugs.debian.org/505415>, calling sv_chop() or otherwise > upgrading an SV can result in an unaligned 64-bit access on the sparc > architecture. > Calling the SvGROW() macro afterwards will cast sv->sv_any to an XPV > pointer, which should be 64-bit aligned according to __alignof__(). > With -O2, gcc will compile the code using a doubleword load instruction > (ldd) that fails with a SIGBUS if the 64-bit alignment has been lost. > However, it seems to me that this is a more general problem that should > be fixed by making sv_upgrade() somehow retain the alignment of the > existing SV. Yes, it's a more general problem. > The instruction that causes the bus error is a double-word load (ldd) > during the first comparison. The compiler (gcc -O2) is using the > double-word instruction to load both xpv_cur and xpv_len in one go. > Quoting Julien Cristau in the Debian bug: > > > __alignof__(XPV) is 8, so gcc is allowed to assume that any XPV is > > 64-bit aligned, as far as I can tell. xpv_cur's offset is 8, so it > > should also be 64-bit aligned. What is __alignof__(XPVIV) and sizeof(XPVIV) on the same system? > cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include' I see that -fno-strict-aliasing is on. That might prove relevant in my reply to your reply. Nicholas ClarkThread Previous | Thread Next