Reapply "IRGen: Fix select_enum for single-payload, multi-empty-case enums."

This reapplies 8643d14 with a test update to change the data type to
Int32 so that it will pass on 32- and 64-bit targets with the same IR
generated.

Author: rudkx

lib/IRGen/GenEnum.cpp

@@ -1539,13 +1539,20 @@ namespace {
       std::tie(payloadTag, extraTag) = getNoPayloadCaseValue(Case);
 
       auto &ti = getFixedPayloadTypeInfo();
-      bool hasExtraInhabitants = ti.getFixedExtraInhabitantCount(IGF.IGM) > 0;
-
+      
       llvm::Value *payloadResult = nullptr;
-      if (hasExtraInhabitants)
-        payloadResult = payload.emitCompare(IGF,
+      // We can omit the payload check if this is the only case represented with
+      // the particular extra tag bit pattern set.
+      //
+      // TODO: This logic covers the most common case, when there's exactly one
+      // more no-payload case than extra inhabitants in the payload. This could
+      // be slightly generalized to cases where there's multiple tag bits and
+      // exactly one no-payload case in the highest used tag value.
+      if (!tagBits ||
+        ElementsWithNoPayload.size() != getFixedExtraInhabitantCount(IGF.IGM)+1)
+          payloadResult = payload.emitCompare(IGF,
                                         ti.getFixedExtraInhabitantMask(IGF.IGM),
-                                            payloadTag);
+                                        payloadTag);
 
       // If any tag bits are present, they must match.
       llvm::Value *tagResult = nullptr;

test/IRGen/select_enum_single_payload.sil

@@ -0,0 +1,34 @@
+// RUN: %target-swift-frontend %s -emit-ir | FileCheck %s
+sil_stage canonical
+
+import Builtin
+
+enum ManyEmptyCases {
+    case A
+    case B
+    case C(Builtin.Int32)
+}
+
+// CHECK-LABEL: define i1 @select_enum_A(i32, i1)
+// CHECK:         [[PAYLOAD:%.*]] = icmp eq i32 %0, 0
+// CHECK:         [[EXTRA:%.*]] = and i1 %1, [[PAYLOAD]]
+// CHECK:         ret i1 [[EXTRA]]
+sil @select_enum_A : $@convention(thin) (ManyEmptyCases) -> Builtin.Int1 {
+entry(%0 : $ManyEmptyCases):
+  %4 = integer_literal $Builtin.Int1, -1          // user: %6
+  %5 = integer_literal $Builtin.Int1, 0           // user: %6
+  %6 = select_enum %0 : $ManyEmptyCases, case #ManyEmptyCases.A!enumelt: %4, default %5 : $Builtin.Int1
+  return %6 : $Builtin.Int1
+}
+
+// CHECK-LABEL: define i1 @select_enum_B(i32, i1)
+// CHECK:         [[PAYLOAD:%.*]] = icmp eq i32 %0, 1
+// CHECK:         [[EXTRA:%.*]] = and i1 %1, [[PAYLOAD]]
+// CHECK:         ret i1 [[EXTRA]]
+sil @select_enum_B : $@convention(thin) (ManyEmptyCases) -> Builtin.Int1 {
+entry(%0 : $ManyEmptyCases):
+  %4 = integer_literal $Builtin.Int1, -1          // user: %6
+  %5 = integer_literal $Builtin.Int1, 0           // user: %6
+  %6 = select_enum %0 : $ManyEmptyCases, case #ManyEmptyCases.B!enumelt: %4, default %5 : $Builtin.Int1
+  return %6 : $Builtin.Int1
+}