[TableGen] Inherit properties from the nearest allocatable superclass. (#127018)

Previously isAlocatable was updated to allow inheritance from any
superclass for a generated register class, but other properties are
still inherited from its nearest superclass. This could cause
a generated regclass inherit undesired properties, e.g., tsflags, from
an unallocatable superclass due to the topological inheritance order.

This change updates to inherit properties from the nearest allocatable
superclass if possible and includes a test to demonstrate a potential
incorrect inheritance of tsflags.

llvm/test/TableGen/RegisterInfoEmitter-inherit-properties.td

@@ -0,0 +1,87 @@
+// RUN: llvm-tblgen -gen-register-info -I %p/../../include -I %p/Common %s | FileCheck %s
+
+// This file tests that a synthesized sub-regclass can inherit properties, e.g.,
+// tsflags in this case, from the correct super-regclass.
+
+include "llvm/Target/Target.td"
+
+class MyReg<string n, list<Register> subregs = []>
+  : Register<n> {
+  let Namespace = "Test";
+  let SubRegs = subregs;
+  let CoveredBySubRegs = 1;
+}
+
+class MyClass<int align, list<ValueType> types, dag registers>
+  : RegisterClass<"Test", types, align, registers> {
+  field bit isA = 0;
+  field bit isB = 0;
+  let TSFlags{0} = isA;
+  let TSFlags{1} = isB;
+}
+
+def sub0 : SubRegIndex<32, 0>;
+def sub1 : SubRegIndex<32, 32>;
+def sub2 : SubRegIndex<32, 64>;
+
+foreach Num=0-7 in {
+  def A#Num : MyReg<"a"#Num>;
+}
+
+foreach Num=0-3 in {
+  def B#Num : MyReg<"b"#Num>;
+}
+
+class AClass<int align, list<ValueType> types, dag registers>
+  : MyClass<align, types, registers> {
+  let isA = 1;
+}
+
+class BClass<int align, list<ValueType> types, dag registers>
+  : MyClass<align, types, registers> {
+  let isB = 1;
+}
+
+def APair : RegisterTuples<[sub0, sub1],
+                           [(add A0, A2, A4, A6), (add A1, A3, A5, A7)]>;
+def BPair : RegisterTuples<[sub0, sub1],
+                           [(add B0, B2), (add B1, B3)]>;
+def ARC2 : AClass<32, [untyped], (add APair)>;
+def BRC2 : BClass<32, [untyped], (add BPair)>;
+def ABRC2 : MyClass<32, [untyped], (add ARC2, BRC2)> {
+  let isAllocatable = 0;
+}
+
+def ATuple : RegisterTuples<[sub0, sub1, sub2],
+                             [(add A0, A1, A2, A3, A4, A5),
+                              (add A1, A2, A3, A4, A5, A6),
+                              (add A2, A3, A4, A5, A6, A7)]>;
+
+def BTuple : RegisterTuples<[sub0, sub1, sub2],
+                             [(add B0, B1), (add B1, B2), (add B2, B3)]>;
+
+def ARC3 : AClass<32, [untyped], (add ATuple)>;
+def BRC3 : BClass<32, [untyped], (add BTuple)>;
+def ABRC3 : MyClass<32, [untyped], (add ARC3, BRC3)> {
+  let isAllocatable = 0;
+}
+
+def TestTarget : Target;
+
+// CHECK:      static unsigned const ARC3_with_sub0_sub1Superclasses[] = {
+// CHECK-NEXT:   Test::ABRC3RegClassID,
+// CHECK-NEXT:   Test::ARC3RegClassID,
+// CHECK-NEXT:   Test::ABRC3_with_sub0_sub1RegClassID,
+// CHECK-NEXT: };
+
+// CHECK:      static unsigned const ARC3_with_sub1_sub2Superclasses[] = {
+// CHECK-NEXT:   Test::ABRC3RegClassID,
+// CHECK-NEXT:   Test::ARC3RegClassID,
+// CHECK-NEXT:   Test::ABRC3_with_sub1_sub2RegClassID,
+// CHECK-NEXT: };
+
+// CHECK: extern const TargetRegisterClass ARC3_with_sub0_sub1RegClass = {
+// CHECK:   0x01, /* TSFlags */
+
+// CHECK: extern const TargetRegisterClass ARC3_with_sub1_sub2RegClass = {
+// CHECK:   0x01, /* TSFlags */

llvm/utils/TableGen/Common/CodeGenRegisters.cpp

@@ -861,23 +861,27 @@ CodeGenRegisterClass::CodeGenRegisterClass(CodeGenRegBank &RegBank,
   }
 }
 
-// Compute inherited propertied for a synthesized register class.
+// Compute inherited properties for a synthesized register class.
 void CodeGenRegisterClass::inheritProperties(CodeGenRegBank &RegBank) {
   assert(!getDef() && "Only synthesized classes can inherit properties");
   assert(!SuperClasses.empty() && "Synthesized class without super class");
 
-  // The last super-class is the smallest one.
-  CodeGenRegisterClass &Super = *SuperClasses.back();
+  // The last super-class is the smallest one in topological order. Check for
+  // allocatable super-classes and inherit from the nearest allocatable one if
+  // any.
+  auto NearestAllocSCRIt =
+      find_if(reverse(SuperClasses),
+              [&](const CodeGenRegisterClass *S) { return S->Allocatable; });
+  CodeGenRegisterClass &Super = NearestAllocSCRIt == SuperClasses.rend()
+                                    ? *SuperClasses.back()
+                                    : **NearestAllocSCRIt;
 
   // Most properties are copied directly.
   // Exceptions are members, size, and alignment
   Namespace = Super.Namespace;
   VTs = Super.VTs;
   CopyCost = Super.CopyCost;
-  // Check for allocatable superclasses.
-  Allocatable = any_of(SuperClasses, [&](const CodeGenRegisterClass *S) {
-    return S->Allocatable;
-  });
+  Allocatable = Super.Allocatable;
   AltOrderSelect = Super.AltOrderSelect;
   AllocationPriority = Super.AllocationPriority;
   GlobalPriority = Super.GlobalPriority;