* David Miller (davem@xxxxxxxxxxxxx) wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Date: Wed, 19 Jan 2011 10:33:26 -0500
>
> > I'm still unsure that __long_long_aligned is needed over __long_aligned though.
> > AFAIK, the only requirement we have for, e.g. tracepoints, is to align on the
> > pointer size (sizeof(long)), so RCU pointer updates are performed atomically.
> > Aligning on the pointer size also allows the architecture to efficiently read
> > the field content. What does aligning on sizeof(long long) bring to us ? Is it
> > that you are concerned about the fact that the "aligned" type attribute, when
> > applied to a structure, is only used as a lower-bound by the compiler ? In that
> > case, we might want to consider using "packed" too:
>
> My concern is that if there is ever a u64 or similarly "long long"
> typed member in these tracing structures, it will not be aligned
> sufficiently to avoid unaligned access traps on 32-bit systems.
Hrm, I'd like to see what kind of ill-conceived 32-bit architecture would
generate a unaligned access for a 32-bit aligned u64. Do you have examples in
mind ? By definition, the memory accesses should be at most 32-bit, no ? AFAIK,
gcc treats u64 as two distinct reads on all 32-bit architectures.
> If your suggestion defines the lowest possible alignment and GCC will
> do the right thing and "up-align" the structure if necessary, then
> fine.
Well, I must admit that my assumption is that aligning on the "long" size should
be the only alignment required, both on 32-bit and 64-bit. But I'm curious to
see if there are indeed architectures that break this assumption.
Ideally, I'd like to avoid letting gcc up-align a structure, because it is then
hard to know for sure what the alignment value of the section should be (in the
linker script, we can safely choose 32, but it's more a "safe choice" than
anything else). Moreover, I'm not convinced that gcc will choose to up-align the
structure with the exact same alignment values for both the type declaration and
the variable definition (I'm deeply distrusting gcc to do the right thing here).
> If you add "packed" it is going to screw everything up and we'll
> essentially be back to square one.
>
> On RISC like sparc64, "packed" causes even 16-bit words to be read and
> written a byte at a time.
>
> Never use "packed" under any circumstances unless absolutely
> unavoidable.
gcc on my sparc64 box (32-bit userland) disagrees with you here ;) Using
gcc (Debian 4.3.3-14) 4.3.3, here is a demonstration that, indeed, "packed"
generates aweful code, but that "packed, aligned(4 or 8)" generates pretty
decent code:
compiling for sparc32:
struct test {
unsigned long a;
unsigned long b;
};
Storing to test "a" field in a main() that returns 0, with -O0:
000104f0 <main>:
104f0: 9d e3 bf 90 save %sp, -112, %sp
104f4: 03 00 00 81 sethi %hi(0x20400), %g1
104f8: 84 10 63 9c or %g1, 0x39c, %g2 ! 2079c <blah>
104fc: 82 10 20 2a mov 0x2a, %g1
10500: c2 20 80 00 st %g1, [ %g2 ]
10504: 82 10 20 00 clr %g1
10508: b0 10 00 01 mov %g1, %i0
1050c: 81 e8 00 00 restore
10510: 81 c3 e0 08 retl
10514: 01 00 00 00 nop
__attribute__((packed))
000104f0 <main>:
104f0: 9d e3 bf 90 save %sp, -112, %sp
104f4: 03 00 00 81 sethi %hi(0x20400), %g1
104f8: 84 10 63 dc or %g1, 0x3dc, %g2 ! 207dc <blah>
104fc: c2 08 80 00 ldub [ %g2 ], %g1
10500: 82 08 60 00 and %g1, 0, %g1
10504: c2 28 80 00 stb %g1, [ %g2 ]
10508: c2 08 a0 01 ldub [ %g2 + 1 ], %g1
1050c: 82 08 60 00 and %g1, 0, %g1
10510: c2 28 a0 01 stb %g1, [ %g2 + 1 ]
10514: c2 08 a0 02 ldub [ %g2 + 2 ], %g1
10518: 82 08 60 00 and %g1, 0, %g1
1051c: c2 28 a0 02 stb %g1, [ %g2 + 2 ]
10520: c2 08 a0 03 ldub [ %g2 + 3 ], %g1
10524: 82 08 60 00 and %g1, 0, %g1
10528: 82 10 60 2a or %g1, 0x2a, %g1
1052c: c2 28 a0 03 stb %g1, [ %g2 + 3 ]
10530: 82 10 20 00 clr %g1
10534: b0 10 00 01 mov %g1, %i0
10538: 81 e8 00 00 restore
1053c: 81 c3 e0 08 retl
10540: 01 00 00 00 nop
__attribute__((packed, aligned(4)))
000104f0 <main>:
104f0: 9d e3 bf 90 save %sp, -112, %sp
104f4: 03 00 00 81 sethi %hi(0x20400), %g1
104f8: 84 10 63 9c or %g1, 0x39c, %g2 ! 2079c <blah>
104fc: 82 10 20 2a mov 0x2a, %g1
10500: c2 20 80 00 st %g1, [ %g2 ]
10504: 82 10 20 00 clr %g1
10508: b0 10 00 01 mov %g1, %i0
1050c: 81 e8 00 00 restore
10510: 81 c3 e0 08 retl
10514: 01 00 00 00 nop
__attribute__((packed, aligned(8)))
000104f0 <main>:
104f0: 9d e3 bf 90 save %sp, -112, %sp
104f4: 03 00 00 81 sethi %hi(0x20400), %g1
104f8: 84 10 63 a0 or %g1, 0x3a0, %g2 ! 207a0 <blah>
104fc: 82 10 20 2a mov 0x2a, %g1
10500: c2 20 80 00 st %g1, [ %g2 ]
10504: 82 10 20 00 clr %g1
10508: b0 10 00 01 mov %g1, %i0
1050c: 81 e8 00 00 restore
10510: 81 c3 e0 08 retl
10514: 01 00 00 00 nop
Now about :
struct test {
unsigned long long a;
unsigned long long b;
};
__attribute__((packed, aligned(8)))
(and without attribute)
000104f0 <main>:
104f0: 9d e3 bf 90 save %sp, -112, %sp
104f4: 03 00 00 81 sethi %hi(0x20400), %g1
104f8: 82 10 63 a0 or %g1, 0x3a0, %g1 ! 207a0 <blah>
104fc: 84 10 20 00 clr %g2
10500: 86 10 20 2a mov 0x2a, %g3
10504: c4 38 40 00 std %g2, [ %g1 ]
10508: 82 10 20 00 clr %g1
1050c: b0 10 00 01 mov %g1, %i0
10510: 81 e8 00 00 restore
10514: 81 c3 e0 08 retl
10518: 01 00 00 00 nop
1051c: 00 00 00 00 illtrap 0
__attribute__((packed, aligned(4)))
000104f0 <main>:
104f0: 9d e3 bf 90 save %sp, -112, %sp
104f4: 03 00 00 81 sethi %hi(0x20400), %g1
104f8: 84 10 63 9c or %g1, 0x39c, %g2 ! 2079c <blah>
104fc: 82 10 20 2a mov 0x2a, %g1
10500: c2 20 a0 04 st %g1, [ %g2 + 4 ]
10504: c0 20 80 00 clr [ %g2 ]
10508: 82 10 20 00 clr %g1
1050c: b0 10 00 01 mov %g1, %i0
10510: 81 e8 00 00 restore
10514: 81 c3 e0 08 retl
10518: 01 00 00 00 nop
1051c: 00 00 00 00 illtrap 0
So the packed, aligned(__alignof__(long)) options does not look that bad.
Mathieu
--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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