Sean> Does anyone know if using packed when it's not needed
Sean> results in less efficient code?
Yes, it definitely does on some (non-mainstream) architectures. We
talked about this before I think...
...ah yes: http://article.gmane.org/gmane.linux.drivers.openib/8396
The assembly there came from compiling with no optimization, but if
anything the packed version in that code:
struct foo { int a; };
struct bar { int b; } __attribute__((packed));
int c(struct foo *x) { return x->a; }
int d(struct bar *x) { return x->b; }
looks worse with -O2. ia64 compiled with -O2 goes from one bundle to six:
0000000000000000 <c>:
0: 13 40 00 40 10 10 [MBB] ld4 r8=[r32]
6: 00 00 00 00 10 80 nop.b 0x0
c: 08 00 84 00 br.ret.sptk.many b0;;
0000000000000010 <d>:
10: 09 70 00 40 00 21 [MMI] mov r14=r32
16: f0 10 80 00 42 00 adds r15=2,r32
1c: 34 00 01 84 adds r32=3,r32;;
20: 19 80 04 1c 00 14 [MMB] ld1 r16=[r14],1
26: f0 00 3c 00 20 00 ld1 r15=[r15]
2c: 00 00 00 20 nop.b 0x0;;
30: 09 70 00 1c 00 10 [MMI] ld1 r14=[r14]
36: 80 00 80 00 20 e0 ld1 r8=[r32]
3c: f1 78 bd 53 shl r15=r15,16;;
40: 01 00 00 00 01 00 [MII] nop.m 0x0
46: e0 70 dc ee 29 00 shl r14=r14,8
4c: 81 38 9d 53 shl r8=r8,24;;
50: 0b 70 40 1c 0e 20 [MMI] or r14=r16,r14;;
56: f0 70 3c 1c 40 00 or r15=r14,r15
5c: 00 00 04 00 nop.i 0x0;;
60: 11 00 00 00 01 00 [MIB] nop.m 0x0
66: 80 78 20 1c 40 80 or r8=r15,r8
6c: 08 00 84 00 br.ret.sptk.many b0;;
and sparc64 goes similarly crazy:
0000000000000000 <c>:
0: 81 c3 e0 08 retl
4: d0 42 00 00 ldsw [ %o0 ], %o0
8: 30 68 00 06 b,a %xcc, 20 <d>
0000000000000020 <d>:
20: c6 0a 00 00 ldub [ %o0 ], %g3
24: c2 0a 20 01 ldub [ %o0 + 1 ], %g1
28: c4 0a 20 02 ldub [ %o0 + 2 ], %g2
2c: 87 28 f0 18 sllx %g3, 0x18, %g3
30: d0 0a 20 03 ldub [ %o0 + 3 ], %o0
34: 83 28 70 10 sllx %g1, 0x10, %g1
38: 82 10 40 03 or %g1, %g3, %g1
3c: 85 28 b0 08 sllx %g2, 8, %g2
40: 84 10 80 01 or %g2, %g1, %g2
44: 90 12 00 02 or %o0, %g2, %o0
48: 81 c3 e0 08 retl
4c: 91 3a 20 00 sra %o0, 0, %o0
50: 30 68 00 04 b,a %xcc, 60 <d+0x40>
Note that mainstream architectures that handle unaligned accesses
sanely do fine with packed. eg ppc64:
0000000000000000 <.c>:
0: e8 63 00 02 lwa r3,0(r3)
4: 4e 80 00 20 blr
0000000000000014 <.d>:
14: e8 63 00 02 lwa r3,0(r3)
18: 4e 80 00 20 blr
x86_64:
0000000000000000 <c>:
0: 8b 07 mov (%rdi),%eax
2: c3 retq
0000000000000010 <d>:
10: 8b 07 mov (%rdi),%eax
12: c3 retq
- R.
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