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

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[].

Differential Revision: https://reviews.llvm.org/D138341

Author: bjope

llvm/test/TableGen/ConcatenatedSubregs.td

@@ -96,12 +96,10 @@ def TestTarget : Target;
 // 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

llvm/utils/TableGen/CodeGenRegisters.cpp

@@ -1369,11 +1369,15 @@ getConcatSubRegIndex(const SmallVector<CodeGenSubRegIndex *, 8> &Parts) {
   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();
-    Size += Parts[i]->Size;
-    if (Parts[i]->Offset != (LastOffset + LastSize))
+    if (Size == UnknownSize || Parts[i]->Size == UnknownSize)
+      Size = UnknownSize;
+    else
+      Size += Parts[i]->Size;
+    if (LastSize == UnknownSize || Parts[i]->Offset != (LastOffset + LastSize))
       isContinuous = false;
     LastOffset = Parts[i]->Offset;
     LastSize = Parts[i]->Size;