http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55975
--- Comment #21 from Kostya Serebryany <kcc at gcc dot gnu.org> 2013-01-18 15:33:33 UTC --- While we are at it... It is possible to use zero shadow offset of powerpc somehow? on x86_64 linux, when I build the code with -fPIC -pie the mapping looks like this: 7fb6dda49000-7fb6ddbfe000 r-xp 00000000 fc:00 1208459 /lib/x86_64-linux-gnu/libc-2.15.so ... 7fff01eba000-7fff01edb000 rw-p 00000000 00:00 0 [stack] 7fff01f15000-7fff01f16000 r-xp 00000000 00:00 0 [vdso] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] which allows us to occupy 16T of memory starting from zero for the shadow. On my power pc box, however, I get this mapping: 36ec0000-36ed0000 r-xp 00000000 08:02 3213429 /tmp/a.out 36ed0000-36ee0000 rw-p 00000000 08:02 3213429 /tmp/a.out fff7f360000-fff7f370000 rw-p 00000000 00:00 0 ... fffe70d0000-fffe7100000 rw-p 00000000 00:00 0 [stack] So, I can't use zero-based shadow. On x86_64 using zero-based shadow gives 5% speedup. It's worth trying on ppc as well, I think.
