[PATCH] D25914: Redo store splitting in CodeGenPrepare

[Wei Mi via llvm-commits]

On Mon, Nov 28, 2016 at 2:13 PM, Chandler Carruth via Phabricator
<reviews at reviews.llvm.org> wrote:
> chandlerc added inline comments.
>
>
> ================
> Comment at: lib/CodeGen/CodeGenPrepare.cpp:5300-5301
> +                                const TargetLowering &TLI) {
> +  unsigned HalfValBitSize =
> +      DL.getTypeSizeInBits(SI.getValueOperand()->getType()) / 2;
> +
> ----------------
> wmi wrote:
>> majnemer wrote:
>> > wmi wrote:
>> > > majnemer wrote:
>> > > > This looks wrong. Shouldn't it be `getTypeStoreSizeInBits` instead of `getTypeSizeInBits`?
>> > > They are the same here because the type of the store value must have power of 2 size if it is a merged store. But you remind me to add some testcases: @int31_float_pair, @int15_float_pair, @int7_float_pair added in the testcase.
>> > But aren't i1, i2 and i4 powers of two?
>> >
>> > If we stored an i1, `HalfValBitSize` would be zero which sounds problematic.
>> The store val will at least be i2 because it is a merged val from two smaller vals.
>>
>> If the store val is an i2 which are combined from an {i1, i1} pair, and we use getTypeStoreSizeInBits to compute the HalfValBitSize, the HalfValBitSize will be 4. It means the split store size will be 4 bits. It is not what we expect. We expect the type of split store is i1.
>>
>> I cannot add add an i1_i1_pair test because now the target query will return false for int pair. But I have verified that the i1_i1_pair test worked correctly to use getTypeSizeInBits by disabling the target query temporarily.
>>
>>
> I don't think storing i1s makes any sense here.
>
> I think you should add a check that the type store size == the type size both before and after splitting and not split unless that is satisfied, regardless of what the target says.
>
> And to make this easier to test, I suggest adding a flag that forces us to split everything we can split. Otherwise covering interesting inputs is too target dependent.
>

It is a nice suggestion and makes things easier. Will do that.

Thanks,
Wei.