[SimplifyCFG] Improve one case

Author: dianqk

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -5838,10 +5838,24 @@ bool SimplifyCFGOpt::turnSwitchRangeIntoICmp(SwitchInst *SI,
   bool IsWrapping = false;
   SmallVectorImpl<ConstantInt *> *ContiguousCases = &CasesA;
   SmallVectorImpl<ConstantInt *> *OtherCases = &CasesB;
+
+  // Only one icmp is needed when there is only one case.
+  if (!HasDefault && CasesA.size() == 1) {
+    ContiguousCasesMax = CasesA[0];
+    ContiguousCasesMin = CasesA[0];
+    ContiguousDest = DestA;
+    OtherDest = DestB;
+  } else if (CasesB.size() == 1) {
+    ContiguousCasesMax = CasesB[0];
+    ContiguousCasesMin = CasesB[0];
+    ContiguousDest = DestB;
+    OtherDest = DestA;
+    std::swap(ContiguousCases, OtherCases);
+  }
   // Correctness: Cases to the default destination cannot be contiguous cases.
-  if (!HasDefault && !CasesA.empty() &&
-      casesAreContiguous(SI->getCondition(), CasesA, ContiguousCasesMin,
-                         ContiguousCasesMax, IsWrapping)) {
+  else if (!HasDefault && !CasesA.empty() &&
+           casesAreContiguous(SI->getCondition(), CasesA, ContiguousCasesMin,
+                              ContiguousCasesMax, IsWrapping)) {
     ContiguousDest = DestA;
     OtherDest = DestB;
   } else if (casesAreContiguous(SI->getCondition(), CasesB, ContiguousCasesMin,

llvm/test/Transforms/SimplifyCFG/switch-range-to-icmp.ll

@@ -306,11 +306,11 @@ else:
 define i32 @one_case_1(i32 %x) {
 ; CHECK-LABEL: @one_case_1(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[X_OFF:%.*]] = add i32 [[X:%.*]], -5
-; CHECK-NEXT:    [[SWITCH:%.*]] = icmp ult i32 [[X_OFF]], 3
+; CHECK-NEXT:    [[X_OFF:%.*]] = add i32 [[X:%.*]], -10
+; CHECK-NEXT:    [[SWITCH:%.*]] = icmp ult i32 [[X_OFF]], 1
 ; CHECK-NEXT:    br i1 [[SWITCH]], label [[A:%.*]], label [[B:%.*]]
 ; CHECK:       common.ret:
-; CHECK-NEXT:    [[COMMON_RET_OP:%.*]] = phi i32 [ [[TMP0:%.*]], [[A]] ], [ [[TMP1:%.*]], [[B]] ]
+; CHECK-NEXT:    [[COMMON_RET_OP:%.*]] = phi i32 [ [[TMP0:%.*]], [[B]] ], [ [[TMP1:%.*]], [[A]] ]
 ; CHECK-NEXT:    ret i32 [[COMMON_RET_OP]]
 ; CHECK:       a:
 ; CHECK-NEXT:    [[TMP0]] = call i32 @f(i32 0)