Clarify intent.

[oota-llvm.git] / utils / TableGen / CodeGenRegisters.cpp

diff --git a/utils/TableGen/CodeGenRegisters.cpp b/utils/TableGen/CodeGenRegisters.cpp
index e902ce031fd8b65308e7b0c6fca9d7fb44bd2357..4238340ff068f80ba23ddfa419a08db5ac4eaba4 100644 (file)
--- a/utils/TableGen/CodeGenRegisters.cpp
+++ b/utils/TableGen/CodeGenRegisters.cpp
@@ -1198,6 +1198,12 @@ void CodeGenRegBank::computeSubRegIndexLaneMasks() {
     if (Idx->getComposites().empty()) {
       Idx->LaneMask = 1u << Bit;
       // Share bit 31 in the unlikely case there are more than 32 leafs.
+      //
+      // Sharing bits is harmless; it allows graceful degradation in targets
+      // with more than 32 vector lanes. They simply get a limited resolution
+      // view of lanes beyond the 32nd.
+      //
+      // See also the comment for getSubRegIndexLaneMask().
       if (Bit < 31) ++Bit;
     } else {
       Idx->LaneMask = 0;