llvm dev - Jul 2010 - [LLVMdev] different layout of structs for llc vs. llvm-gcc

On Wed, Jul 14, 2010 at 10:54 AM, Eli Friedman <eli.friedman at gmail.com>
wrote:
> On Wed, Jul 14, 2010 at 8:33 AM, Andrew Lenharth <andrewl at
lenharth.org> wrote:
>> On Wed, Jul 14, 2010 at 10:26 AM, Eli Friedman <eli.friedman at
gmail.com> wrote:
>>> On Wed, Jul 14, 2010 at 3:20 AM, Torvald Riegel
>>> <torvald at se.inf.tu-dresden.de> wrote:
>>>> On Tuesday 13 July 2010 19:48:25 you wrote:
>>>>> On Tue, Jul 13, 2010 at 10:09 AM, Torvald Riegel
>>>>> > I thought that the layout of structs was supposed to
be preserved (wrong
>>>>> > assumption?). Otherwise, any ideas why this happens?
>>>>>
>>>>> It should be preserved in general;
>>>>
>>>> Is this a "should" or a "must"? Are there
any cases in which structure layout
>>>> must be preserved besides for volatile accesses and if the data
goes out to
>>>> external code?
>>>
>>> LLVM generally doesn't attempt to preserve the layout of
structs which
>>> aren't externally visible or used by a volatile load/store.
>>>
>>>> I've seen code like the one above quite often to put data
on different
>>>> cachelines, so even if it's a "should" and not a
"must" it might be good to
>>>> preserve the padding. Otherwise, is there a portable way to
ensure that
>>>> globals end up on a separate cacheline (without making all
accesses to them
>>>> volatile)?
>>>
>>> I can't think of any way of doing it without essentially trying
to
>>> trick the compiler... although there are many ways to trick LLVM.
>>>
>>> Anyone else have ideas?
>>
>> Is alignment on a field propagated when the struct is split up?
>
> Irrelevant here; the struct in question only has 4 byte alignment.
Not meant to be irrelevant because the though not explained.  What I
meant to say was if the padding was accomplished with an align
directive on the field, would the alignment be propagated to the
global after being split up?

Andrew
>
> -Eli
>

On Wednesday 14 July 2010 17:57:26 Andrew Lenharth
wrote:
> On Wed, Jul 14, 2010 at 10:54 AM, Eli Friedman <eli.friedman at
gmail.com>
wrote:
> > On Wed, Jul 14, 2010 at 8:33 AM, Andrew Lenharth <andrewl at
lenharth.org>
wrote:
> >> On Wed, Jul 14, 2010 at 10:26 AM, Eli Friedman <eli.friedman at
gmail.com>
wrote:
> >> Is alignment on a field propagated when the struct is split up?
> > 
> > Irrelevant here; the struct in question only has 4 byte alignment.
> 
> Not meant to be irrelevant because the though not explained.  What I
> meant to say was if the padding was accomplished with an align
> directive on the field, would the alignment be propagated to the
> global after being split up?
I tried this already (in the form of { uint32_t field; uint8_t pad[60]; 
uint32_field2;}). Alignment seems to be properly propagated, but pad[] is still 
removed, so I believe that other globals without custom alignment constraints 
could still be located in the same cachelines as field or field2.

Torvald

I believe he was asking if adding __attribute__((aligned(NN))) to the
fields worked.  It's probably a cleaner away of accomplishing
alignment than padding.

Reid

On Wed, Jul 14, 2010 at 2:03 PM, Torvald Riegel
<torvald at se.inf.tu-dresden.de> wrote:
> On Wednesday 14 July 2010 17:57:26 Andrew Lenharth wrote:
>> On Wed, Jul 14, 2010 at 10:54 AM, Eli Friedman <eli.friedman at
gmail.com>
> wrote:
>> > On Wed, Jul 14, 2010 at 8:33 AM, Andrew Lenharth <andrewl at
lenharth.org>
> wrote:
>> >> On Wed, Jul 14, 2010 at 10:26 AM, Eli Friedman
<eli.friedman at gmail.com>
> wrote:
>> >> Is alignment on a field propagated when the struct is split
up?
>> >
>> > Irrelevant here; the struct in question only has 4 byte alignment.
>>
>> Not meant to be irrelevant because the though not explained.  What I
>> meant to say was if the padding was accomplished with an align
>> directive on the field, would the alignment be propagated to the
>> global after being split up?
>
> I tried this already (in the form of { uint32_t field; uint8_t pad[60];
> uint32_field2;}). Alignment seems to be properly propagated, but pad[] is
still
> removed, so I believe that other globals without custom alignment
constraints
> could still be located in the same cachelines as field or field2.
>
> Torvald
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>