[TableGen] Fix SubRegIndex size computation for concatenated subregs of unknown size

[bjope]

Summary

When calculating the size of concatenated subregisters, and at least
one of the subregisters involved has an unknown size (-1), then the
concatenated size should be set to -1 as well.

This bug was found for an out-of-tree target.

Looking at lib/Target the only in-tree target that has a subregister
with unknown size is X86:

X86RegisterInfo.td:  def sub_mask_0   : SubRegIndex<-1>;

But it looks like sub_mask_0 don't result in any concatenated subreg
index with faulty size if looking at X86SubRegIdxRanges[].

[arsenm]

LGTM

llvm/test/TableGen/ConcatenatedSubregs.td

 
 // CHECK-LABEL: SubRegIndex ssub1:
 // CHECK:        Offset, Size: 16, 16
 // CHECK-LABEL: SubRegIndex sub0:
 // CHECK-LABEL: SubRegIndex sub1:
 // CHECK-LABEL: SubRegIndex sub2:
 // Check inferred indexes:
 // CHECK-LABEL: SubRegIndex ssub1_ssub2:
-// FIXME: Size should be unknown (65535).
-// CHECK:         Offset, Size: 16, 15
+// CHECK:         Offset, Size: 16, 65535
 // CHECK-LABEL: SubRegIndex ssub3_ssub4:
 // CHECK-LABEL: SubRegIndex ssub0_ssub1_ssub2_ssub3:
-// FIXME: Size should be unknown (65535).
-// CHECK:         Offset, Size: 65535, 30
+// CHECK:         Offset, Size: 65535, 65535
 // CHECK-LABEL: SubRegIndex ssub1_ssub2_ssub3_ssub4:
 
 // Check that all subregs are generated on some examples
 // CHECK-LABEL: Register D0:
 // CHECK: HasDisjunctSubRegs: 1
 // CHECK-NEXT: SubReg ssub0 = S0
 // CHECK-NEXT: SubReg ssub1 = S1
 

llvm/utils/TableGen/CodeGenRegisters.cpp

#endif
 
   // None exists, synthesize one.
   std::string Name = Parts.front()->getName();
   // Determine whether all parts are contiguous.
   bool isContinuous = true;
   unsigned Size = Parts.front()->Size;
   unsigned LastOffset = Parts.front()->Offset;
   unsigned LastSize = Parts.front()->Size;
+  unsigned UnknownSize = (uint16_t)-1;
   for (unsigned i = 1, e = Parts.size(); i != e; ++i) {
     Name += '_';
     Name += Parts[i]->getName();
+    if (Size == UnknownSize || Parts[i]->Size == UnknownSize)
+      Size = UnknownSize;
+    else
       Size += Parts[i]->Size;
-    if (Parts[i]->Offset != (LastOffset + LastSize))
+    if (LastSize == UnknownSize || Parts[i]->Offset != (LastOffset + LastSize))
       isContinuous = false;
     LastOffset = Parts[i]->Offset;
     LastSize = Parts[i]->Size;
   }
   Idx = createSubRegIndex(Name, Parts.front()->getNamespace());
   Idx->Size = Size;
   Idx->Offset = isContinuous ? Parts.front()->Offset : -1;
   Idx->ConcatenationOf.assign(Parts.begin(), Parts.end());