Detect proper register sub-classes.

Some instructions require restricted register classes, but most of the
time that doesn't affect register allocation. For example, some
instructions don't work with the stack pointer, but that is a reserved
register anyway.

Sometimes it matters, GR32_ABCD only has 4 allocatable registers. For
such a proper sub-class, the register allocator should try to enable
register class inflation since that makes more registers available for
allocation.

Make sure only legal super-classes are considered. For example, tGPR is
not a proper sub-class in Thumb mode, but in ARM mode it is.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136981 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 18 insertions, 2 deletions

diff --git a/lib/CodeGen/RegisterClassInfo.cpp b/lib/CodeGen/RegisterClassInfo.cpp
index 5a77e47bc5..786d279c2b 100644
--- a/lib/CodeGen/RegisterClassInfo.cpp
+++ b/lib/CodeGen/RegisterClassInfo.cpp
@@ -99,11 +99,16 @@ void RegisterClassInfo::compute(const TargetRegisterClass *RC) const {
   // CSR aliases go after the volatile registers, preserve the target's order.
   std::copy(CSRAlias.begin(), CSRAlias.end(), &RCI.Order[N]);
 
+  // Check if RC is a proper sub-class.
+  if (const TargetRegisterClass *Super = TRI->getLargestLegalSuperClass(RC))
+    if (Super != RC && getNumAllocatableRegs(Super) > RCI.NumRegs)
+      RCI.ProperSubClass = true;
+
   DEBUG({
     dbgs() << "AllocationOrder(" << RC->getName() << ") = [";
     for (unsigned I = 0; I != RCI.NumRegs; ++I)
       dbgs() << ' ' << PrintReg(RCI.Order[I], TRI);
-    dbgs() << " ]\n";
+    dbgs() << (RCI.ProperSubClass ? " ] (sub-class)\n" : " ]\n");
   });
 
   // RCI is now up-to-date.
diff --git a/lib/CodeGen/RegisterClassInfo.h b/lib/CodeGen/RegisterClassInfo.h
index 90c0a7ee4a..2c1407096c 100644
--- a/lib/CodeGen/RegisterClassInfo.h
+++ b/lib/CodeGen/RegisterClassInfo.h
@@ -28,9 +28,10 @@ class RegisterClassInfo {
   struct RCInfo {
     unsigned Tag;
     unsigned NumRegs;
+    bool ProperSubClass;
     OwningArrayPtr<unsigned> Order;
 
-    RCInfo() : Tag(0), NumRegs(0) {}
+    RCInfo() : Tag(0), NumRegs(0), ProperSubClass(false) {}
     operator ArrayRef<unsigned>() const {
       return makeArrayRef(Order.get(), NumRegs);
     }
@@ -87,6 +88,16 @@ public:
     return get(RC);
   }
 
+  /// isProperSubClass - Returns true if RC has a legal super-class with more
+  /// allocatable registers.
+  ///
+  /// Register classes like GR32_NOSP are not proper sub-classes because %esp
+  /// is not allocatable.  Similarly, tGPR is not a proper sub-class in Thumb
+  /// mode because the GPR super-class is not legal.
+  bool isProperSubClass(const TargetRegisterClass *RC) const {
+    return get(RC).ProperSubClass;
+  }
+
   /// getLastCalleeSavedAlias - Returns the last callee saved register that
   /// overlaps PhysReg, or 0 if Reg doesn't overlap a CSR.
   unsigned getLastCalleeSavedAlias(unsigned PhysReg) const {