[LLVMdev] x86 Frame Pointer with AVX

Cameron McInally cameron.mcinally at nyu.edu
Tue Oct 23 09:55:05 PDT 2012

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On Tue, Oct 23, 2012 at 11:43 AM, Cameron McInally <cameron.mcinally at nyu.edu
> wrote:

> On Mon, Oct 22, 2012 at 5:49 PM, Eric Christopher <echristo at gmail.com>wrote:
>> > In trunk, the frame pointer is always set up when an AVX register is
>> used in
>> > a function. This is done in case 32-byte spill code is later introduced
>> into
>> > the function and hence dynamic stack realignment is needed. Needless to
>> say,
>> > it's a big hammer. This regression seems particularly painful in
>> > small-to-medium sized routines that are called frequently in some codes.
>> >
>> >
>> > Is this issue already known? Is there a plan to fix this regression? If
>> not,
>> > does anyone have a suggestion on the best way to remedy this issue?
>> >
>> You'd need to change the default stack alignment of the platform to
>> deal with it effectively.
> Hey Eric,
> Thanks for replying so quickly. Would you elaborate on this further?
> It seems costly to change the default stack alignment on the platform,
> since that would require recompiling all of the system and user libraries
> to also adhere to 32-byte stack alignment. Depending on an alignment not
> specified by the ABI would also limit our compiler's interoperability with
> other compilers installed on the system.
> I suppose that the stack could be aligned dynamically at main(...) and
> other visible entry points, but that too seems costly compared to the
> current M.O..
> Maybe I do not fully understand all the issues involved, but I suppose I
> should be able to dynamically align the stack only when AVX registers are
> spilled in a function, right? Seems reasonable with my limited knowledge.
> Do you have any intuition built? It could be possible that the
> prologue/epilogue emitters run prior to the spilling decisions. I am not so
> sure of the ordering here.
> Also, and this might be asking a lot, but do you have any insight into why
> this behaviour changed sometime around the LLVM 3.0 release? I have not
> been able to find much history.
> Thanks again,
> Cameron
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