Arne> Sure - I wasn't aware that gcc will produce different code
Arne> even if it doesn't have to insert padding bytes.
Yes, it gets paranoid about alignment. For example, if you build the
following on ia64:
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; }
Then c() compiles to:
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;;
and d() compiles to:
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;;
We should use __attribute__((packed)) only when it's really needed.
- R.
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