http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55454
--- Comment #3 from Siarhei Siamashka <siarhei.siamashka at gmail dot com>
2012-11-25 19:32:02 UTC ---
Also fails with GCC trunk (gcc version 4.8.0 20120518 (experimental))
The disassembly listing for "init_buffer" function:
00000000 <init_buffer>:
0: 7d 80 42 a6 mfvrsave r12
4: 94 21 ff e0 stwu r1,-32(r1)
8: 91 81 00 1c stw r12,28(r1)
c: 65 8c 80 00 oris r12,r12,32768
10: 7d 80 43 a6 mtvrsave r12
14: 3d 40 00 00 lis r10,0
18: 7c 00 18 ce lvx v0,r0,r3
1c: 39 20 00 0a li r9,10
20: 39 4a 00 00 addi r10,r10,0
24: 7c 0a 49 ce stvx v0,r10,r9
^^^^
Here it happily tries to use STVX instruction. And using this instruction just
silently aligns the address down to 16 byte boundary, effectively doing the
write at &buffer[0] instead of &buffer[10].
28: 81 81 00 1c lwz r12,28(r1)
2c: 7d 80 43 a6 mtvrsave r12
30: 38 21 00 20 addi r1,r1,32
34: 4e 80 00 20 blr
And by the way, the memcpy workaround mentioned above is also broken in GCC
4.8, because it tries to be clever and generates exactly the same code relying
on STVX :)
With GCC 4.7.2, at least memcpy variant used to work correctly:
00000000 <init_buffer>:
0: 3d 40 00 00 lis r10,0
4: 80 a3 00 00 lwz r5,0(r3)
8: 80 c3 00 04 lwz r6,4(r3)
c: 80 e3 00 08 lwz r7,8(r3)
10: 39 2a 00 0a addi r9,r10,10
14: 81 03 00 0c lwz r8,12(r3)
18: 90 aa 00 0a stw r5,10(r10)
1c: 90 c9 00 04 stw r6,4(r9)
20: 90 e9 00 08 stw r7,8(r9)
24: 91 09 00 0c stw r8,12(r9)
28: 4e 80 00 20 blr
