Add a CoveringSubRegIndices field to SubRegIndex records.

This can be used to tell TableGen to use a specific SubRegIndex instead
of synthesizing one when discovering all sub-registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161982 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 18 insertions, 0 deletions

diff --git a/include/llvm/Target/Target.td b/include/llvm/Target/Target.td
index 24be2b1..cc76714 100644
--- a/include/llvm/Target/Target.td
+++ b/include/llvm/Target/Target.td
@@ -28,6 +28,24 @@ class SubRegIndex<list<SubRegIndex> comps = []> {
   // ComposedOf - A list of two SubRegIndex instances, [A, B].
   // This indicates that this SubRegIndex is the result of composing A and B.
   list<SubRegIndex> ComposedOf = comps;
+
+  // CoveringSubRegIndices - A list of two or more sub-register indexes that
+  // cover this sub-register.
+  //
+  // This field should normally be left blank as TableGen can infer it.
+  //
+  // TableGen automatically detects sub-registers that straddle the registers
+  // in the SubRegs field of a Register definition. For example:
+  //
+  //   Q0    = dsub_0 -> D0, dsub_1 -> D1
+  //   Q1    = dsub_0 -> D2, dsub_1 -> D3
+  //   D1_D2 = dsub_0 -> D1, dsub_1 -> D2
+  //   QQ0   = qsub_0 -> Q0, qsub_1 -> Q1
+  //
+  // TableGen will infer that D1_D2 is a sub-register of QQ0. It will be given
+  // the synthetic index dsub_1_dsub_2 unless some SubRegIndex is defined with
+  // CoveringSubRegIndices = [dsub_1, dsub_2].
+  list<SubRegIndex> CoveringSubRegIndices = [];
 }
 
 // RegAltNameIndex - The alternate name set to use for register operands of