[TargetRegisterInfo] Re-apply r265734.

Original commit message:
[TargetRegisterInfo] Refactor the code to use BitMaskClassIterator.

llvm-svn: 265764

1 files changed, 5 insertions, 12 deletions

diff --git a/llvm/lib/CodeGen/TargetRegisterInfo.cpp b/llvm/lib/CodeGen/TargetRegisterInfo.cpp
index fc88629b5c8..e1d90cb913e 100644
--- a/llvm/lib/CodeGen/TargetRegisterInfo.cpp
+++ b/llvm/lib/CodeGen/TargetRegisterInfo.cpp
@@ -112,18 +112,11 @@ TargetRegisterInfo::getAllocatableClass(const TargetRegisterClass *RC) const {
   if (!RC || RC->isAllocatable())
     return RC;
 
-  const unsigned *SubClass = RC->getSubClassMask();
-  for (unsigned Base = 0, BaseE = getNumRegClasses();
-       Base < BaseE; Base += 32) {
-    unsigned Idx = Base;
-    for (unsigned Mask = *SubClass++; Mask; Mask >>= 1) {
-      unsigned Offset = countTrailingZeros(Mask);
-      const TargetRegisterClass *SubRC = getRegClass(Idx + Offset);
-      if (SubRC->isAllocatable())
-        return SubRC;
-      Mask >>= Offset;
-      Idx += Offset + 1;
-    }
+  for (BitMaskClassIterator It(RC->getSubClassMask(), *this); It.isValid();
+       ++It) {
+    const TargetRegisterClass *SubRC = getRegClass(It.getID());
+    if (SubRC->isAllocatable())
+      return SubRC;
   }
   return nullptr;
 }