On Wed, Oct 15, 2008 at 23:39:52 +0300, Niko Tyni wrote: > This is reproducible on sperger.d.o. > > It's an alignment problem that only shows up with an optimization level > of at least -O2. > > Program received signal SIGBUS, Bus error. > 0xf7ec1134 in decode_json (string=0x192280, json=0x39f18, > offset_return=0xff85b96c) at XS.xs:1443 > 1443 SvGROW (string, SvCUR (string) + 1); // should basically be a NOP > (gdb) bt > #0 0xf7ec1134 in decode_json (string=0x192280, json=0x39f18, > offset_return=0xff85b96c) at XS.xs:1443 > #1 0xf7ec2444 in XS_JSON__XS_incr_parse (my_perl=0x22008, cv=0x126d58) at > XS.xs:1828 > #2 0xf7dfab48 in Perl_pp_entersub () from /usr/lib/libperl.so.5.10 > #3 0xf7df8f9c in Perl_runops_standard () from /usr/lib/libperl.so.5.10 > #4 0xf7df4298 in perl_run () from /usr/lib/libperl.so.5.10 > #5 0x00010b88 in main () > > The instruction that causes the bus error is a double-word load (ldd): > > 0xf7ec1130 <decode_json+132>: ld [ %l1 ], %g1 > 0xf7ec1134 <decode_json+136>: ldd [ %g1 + 8 ], %g2 > 0xf7ec1138 <decode_json+140>: inc %g2 > 0xf7ec113c <decode_json+144>: cmp %g3, %g2 > > The preprocessor output for the above SvGROW() macro invocation is > > (((XPV*) (string)->sv_any)->xpv_len < (((XPV*) (string)->sv_any)->xpv_cur + > 1) ? Perl_sv_grow(((PerlInterpreter > *)pthread_getspecific((*Perl_Gthr_key_ptr(((void *)0))))), string,((XPV*) > (string)->sv_any)->xpv_cur + 1) : ((string)->sv_u.svu_pv)); > > and the above assembly is the compiled form of the first comparison > at -O2. The compiler is using a double-word instruction to load both > xpv_cur and xpv_len in one go. > > Now, this apparently requires that ((XPV*) (string)->sv_any)->xpv_cur > is aligned on a double-word (64-bit) boundary, which fails here: > [...] > > This suggests to me that gcc is being too aggressive on optimizing here. > I don't see how it can consider the double word instruction safe. > __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.
Cheers, Julien -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
