David> For the cases where it is no actually necessary, the code
David> generation cost on RISC cpus is very high. Byte loads and
David> stores will be used to access _every_ member of such
David> structures on RISC machines because the compiler cannot
David> guarentee the alignment of any data object when packed is
David> specified.
Agreed (although not really RISC -- sparc64 and ia64 have this
problem, while ppc is fine with unaligned access). However
__attribute__((packed,aligned)) has just been brought to my attention.
For example, on sparc64,
struct foo { char x; int a; } __attribute__((packed,aligned));
struct bar { char x; int b; } __attribute__((packed));
int c(struct foo *x) { return x->a; }
int d(struct bar *x) { return x->b; }
compiles to:
0000000000000000 <c>:
0: d0 5a 00 00 ldx [ %o0 ], %o0
4: 91 2a 30 08 sllx %o0, 8, %o0
8: 81 c3 e0 08 retl
c: 91 3a 30 20 srax %o0, 0x20, %o0
10: 30 68 00 04 b,a %xcc, 20 <d>
14: 01 00 00 00 nop
18: 01 00 00 00 nop
1c: 01 00 00 00 nop
0000000000000020 <d>:
20: c6 0a 20 01 ldub [ %o0 + 1 ], %g3
24: c2 0a 20 02 ldub [ %o0 + 2 ], %g1
28: c4 0a 20 03 ldub [ %o0 + 3 ], %g2
2c: 87 28 f0 18 sllx %g3, 0x18, %g3
30: d0 0a 20 04 ldub [ %o0 + 4 ], %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>
54: 01 00 00 00 nop
58: 01 00 00 00 nop
5c: 01 00 00 00 nop
which suggests that adding "aligned" is a good idea for many of the
uses of "packed". However I don't know how all gcc version do with
this. Anyone have any comments on "__attribute__((packed,aligned))"?
Thanks,
Roland
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