avx512 mask register spilling

[Richard Henderson <rth at redhat dot com>]

On 08/06/2013 03:57 AM, Kirill Yukhin wrote:
> On 05 Aug 09:55, Richard Henderson wrote:
>> On 08/05/2013 08:07 AM, Kirill Yukhin wrote:
>>> Hello Richard, Vlad,
>>>
>>> On 31 Jul 06:26, Richard Henderson wrote:
>>>> On 07/31/2013 05:02 AM, Kirill Yukhin wrote:
>>>>>
>>>>> There's ICE (max. number of generated reload insns per insn is achieved (90)),
>>>>> when LRA tried to save mask register before call. This was caused by fact that  split_reg function
>>>>> in lra-constraints.c allocates memory for saved reg in SECONDARY_MEMORY_NEEDED_MODE which 
>>>>
>>>> I've told you before that it's not SECONDARY_MEMORY that you want, but
>>>> a secondary reload.  You should be patching ix86_secondary_reload, right
>>>> below where we handle QImode spills from non-Q registers for x86-32.
>>>
>>> Trying to do that with no success so far.
>>> Could you pls correct me if I am wrong.
>>> What I am trying to do is to introduce 2 new `define_expand' for load and store.
>>
>> Huh?  You shouldn't need this.
>>
>> Give me a test case and I can have a look at it.
> 
> Hello,
> 
> I've squashed part 1 and 2 + rebased on recent trunk.
> Testcase is attached.
> To reproduce: build-x86_64-linux/gcc/xgcc -Bbuild-x86_64-linux/gcc -Ofast -mavx512f -march=core-avx2 repro.c  -S -o-  -ffixed-rsi  -ffixed-rdi  -ffixed-rbx -ffixed-rbp -m32
> 
> Thanks a lot for help!

You've found what I believe to be a bug in LRA.

Specifically, lra-constraints.c split_reg uses SECONDARY_MEMORY_NEEDED_MODE to
choose what mode to spill a caller-save register.  Given the existing
definition in i386.h, this tries to spill a MASK_CLASS register in SImode.  But
MASK_CLASS does not support SImode, only QI/HImode.  Which leads to substantial
confusion in the allocator trying to satisfy the move.

I believe the use of SECONDARY_MEMORY_NEEDED_MODE in split_reg is wrong.
What's the history behind that, Vlad?  Surely we can spill the value in
its current mode?

Certainly this patch fixes the crash from Kirill's reproducer...


r~

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