[Bug sanitizer/55975] asan does not work with 46 bit address space on PowerPC64

["kcc at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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.