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

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?

Andrew

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.

-Eli

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
>