[PATCH] D25914: Redo store splitting in CodeGenPrepare

[Chandler Carruth via Phabricator via llvm-commits]

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D25914