[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;
+
----------------
majnemer wrote:
> chandlerc wrote:
> > 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.
> I'm still not sure why we would want to use TypeSizeInBits instead of the store size...
> What if the type is an i63? We would actually store 8 bytes...
I think it is at least reasonable to start with type size == store size. Among other things, when that isn't true, the shifting and extension logic will be at least a little more complicated.

Still, Wei, this might make sense as a follow-up patch to work on to extend this to handle more complicated merged stores. As a potentially useful set of examples you could look at the generated code for:

  struct S {
    unsigned x : 27;
  };
  
  std::pair<S, float> g1();
  std::pair<float, S> g2();

  void f(std::pair<float, unsigned> &result) {
    auto p1 = g1();
    auto p2 = g2();
    result.first = p1.second + p2.first;
    result.second = p1.first + p2.second;
  }

Or something similar... But I'm happy to do this as a follow-up generalization now that the correctness issues are addressed.

That seem reasonable David?


================
Comment at: test/CodeGen/X86/split-store.ll:100-119
+; CHECK-LABEL: int31_int31_pair
+; CHECK: andl $2147483647, %edi
+; CHECK: movl %edi, (%rdx)
+; CHECK: andl $2147483647, %esi
+; CHECK: movl %esi, 4(%rdx)
+define void @int31_int31_pair(i31 %tmp1, i31 %tmp2, i64* %ref.tmp) {
+entry:
----------------
I would also include more negative tests:

1) non-symmetric merges
2) non-power-of-two merged store sizes where type size == store size (i24, i48, etc)
3) non-power-of-two merged store sizes where type size != store size (i14)


Repository:
  rL LLVM

https://reviews.llvm.org/D25914