Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6552,6 +6552,20 @@
     // Note: We call removeProdecessor later since we need to be able to get the
     // PHI value for the default case in case we're using a bit mask.
   } else {
+    const APInt *MaskC;
+    if (UseSwitchConditionAsTableIndex &&
+        match(TableIndex, m_And(m_Value(), m_APInt(MaskC)))) {
+      APInt SCVPlus1 = *MaskC + 1;
+      const uint64_t *RawData = MaskC->getRawData();
+      // Expand the size of the table when the MaskC < 64.
+      if (SCVPlus1.isPowerOf2() && SCVPlus1.logBase2() <= 6 && RawData) {
+        assert(TableSize <= RawData[0] && "unexpect mask value");
+        // The range check will be deleted later when we enlarge the lookup
+        // table because the check always return true.
+        TableSize = RawData[0] + 1;
+      }
+    }
+
     Value *Cmp = Builder.CreateICmpULT(
         TableIndex, ConstantInt::get(MinCaseVal->getType(), TableSize));
     RangeCheckBranch =
Index: llvm/test/Transforms/SimplifyCFG/switch_mask.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SimplifyCFG/switch_mask.ll
@@ -0,0 +1,33 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --scrub-attributes
+; RUN: opt -passes=simplifycfg --switch-to-lookup -S < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; https://alive2.llvm.org/ce/z/2bNvhg
+define noundef i1 @switch_lookup_with_small_mask(i64 noundef %x) {
+; CHECK-LABEL: @switch_lookup_with_small_mask(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i64 [[X:%.*]], 15
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i64 [[AND]], 16
+; CHECK-NEXT:    [[SWITCH_CAST:%.*]] = trunc i64 [[AND]] to i16
+; CHECK-NEXT:    [[SWITCH_SHIFTAMT:%.*]] = mul nuw nsw i16 [[SWITCH_CAST]], 1
+; CHECK-NEXT:    [[SWITCH_DOWNSHIFT:%.*]] = lshr i16 1030, [[SWITCH_SHIFTAMT]]
+; CHECK-NEXT:    [[SWITCH_MASKED:%.*]] = trunc i16 [[SWITCH_DOWNSHIFT]] to i1
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[TMP0]], i1 [[SWITCH_MASKED]], i1 false
+; CHECK-NEXT:    ret i1 [[TMP1]]
+;
+entry:
+  %and = and i64 %x, 15
+  switch i64 %and, label %default [
+  i64 10, label %lor.end
+  i64 1, label %lor.end
+  i64 2, label %lor.end
+  ]
+
+default:                                          ; preds = %entry
+  br label %lor.end
+
+lor.end:                                          ; preds = %entry, %entry, %entry, %default
+  %0 = phi i1 [ true, %entry ], [ false, %default ], [ true, %entry ], [ true, %entry ]
+  ret i1 %0
+}