Store sub-class lists as a bit vector.

This uses less memory and it reduces the complexity of sub-class
operations:

- hasSubClassEq() and friends become O(1) instead of O(N).

- getCommonSubClass() becomes O(N) instead of O(N^2).

In the future, TableGen will infer register classes.  This makes it
cheap to add them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140898 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 13 insertions, 32 deletions

diff --git a/lib/Target/TargetRegisterInfo.cpp b/lib/Target/TargetRegisterInfo.cpp
index c4b084dfc8..67239b830e 100644
--- a/lib/Target/TargetRegisterInfo.cpp
+++ b/lib/Target/TargetRegisterInfo.cpp
@@ -100,42 +100,23 @@ BitVector TargetRegisterInfo::getAllocatableSet(const MachineFunction &MF,
 const TargetRegisterClass *
 TargetRegisterInfo::getCommonSubClass(const TargetRegisterClass *A,
                                       const TargetRegisterClass *B) const {
-  // First take care of the trivial cases
+  // First take care of the trivial cases.
   if (A == B)
     return A;
   if (!A || !B)
     return 0;
 
-  // If B is a subclass of A, it will be handled in the loop below
-  if (B->hasSubClass(A))
-    return A;
+  // Register classes are ordered topologically, so the largest common
+  // sub-class it the common sub-class with the smallest ID.
+  const unsigned *SubA = A->getSubClassMask();
+  const unsigned *SubB = B->getSubClassMask();
 
-  const TargetRegisterClass *Best = 0;
-  for (TargetRegisterClass::sc_iterator I = A->subclasses_begin();
-       const TargetRegisterClass *X = *I; ++I) {
-    if (X == B)
-      return B;                 // B is a subclass of A
-
-    // X must be a common subclass of A and B
-    if (!B->hasSubClass(X))
-      continue;
-
-    // A superclass is definitely better.
-    if (!Best || Best->hasSuperClass(X)) {
-      Best = X;
-      continue;
-    }
-
-    // A subclass is definitely worse
-    if (Best->hasSubClass(X))
-      continue;
-
-    // Best and *I have no super/sub class relation - pick the larger class, or
-    // the smaller spill size.
-    int nb = std::distance(Best->begin(), Best->end());
-    int ni = std::distance(X->begin(), X->end());
-    if (ni>nb || (ni==nb && X->getSize() < Best->getSize()))
-      Best = X;
-  }
-  return Best;
+  // We could start the search from max(A.ID, B.ID), but we are only going to
+  // execute 2-3 iterations anyway.
+  for (unsigned Base = 0, BaseE = getNumRegClasses(); Base < BaseE; Base += 32)
+    if (unsigned Common = *SubA++ & *SubB++)
+      return getRegClass(Base + CountTrailingZeros_32(Common));
+
+  // No common sub-class exists.
+  return NULL;
 }