[LLVMdev] Poor register allocation (constants causing spilling)

[Robert Lougher]

Hi Quentin,

Sorry for the delay, I've been bogged down with other things today.

On 14 July 2015 at 18:48, Quentin Colombet <qcolombet at apple.com> wrote:
>>
>> * A rematerializable interval once split is no longer rematerializable *
>>
>> The isRematerializable check in CalcSpillWeights.cpp uses the target
>> instruction info to check that the machine instruction for the live
>> interval definition is trivially rematerializable.  In the case of
>> interval A above, the definition is a load from the constant-pool.
>> This is trivially rematerializable and the spill weight is halved.
>> However, in the remainder intervals B, C and D the definition is a
>> copy.  This is not trivially rematerializable, and the spill weight is
>> unadjusted.  This means these ranges are considered as expensive as
>> non-rematerializable ranges.
>
> This is the first think to fix. As long as a live-range is rematerializable, we should reflect that properly in the spill weight.
> Could you clean up your patch to fix that?

Unfortunately my change was an ugly hack that was good enough for my
experiment but not the right approach for a proper fix (it added an
is_remat bool to the interval class, and adjusted the weights and
propagated the flag after local splitting - as they're in the same
basic block they should remain rematerializable).  I'm now looking at
a proper fix, although it may take some time...

Thanks,
Rob.