[SimplifyCFG] Support trunc nuw in chain of comparisons.

Author: andjo403

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -612,6 +612,15 @@ struct ConstantComparesGatherer {
   /// If CompValue is already set, the function is expected to fail if a match
   /// is found but the value compared to is different.
   bool matchInstruction(Instruction *I, bool isEQ) {
+    Value *Val;
+    if (match(I, m_NUWTrunc(m_Value(Val)))) {
+      // If we already have a value for the switch, it has to match!
+      if (!setValueOnce(Val))
+        return false;
+      UsedICmps++;
+      Vals.push_back(ConstantInt::get(cast<IntegerType>(Val->getType()), isEQ));
+      return true;
+    }
     // If this is an icmp against a constant, handle this as one of the cases.
     ICmpInst *ICI;
     ConstantInt *C;

llvm/test/Transforms/SimplifyCFG/switch_create.ll

@@ -1195,11 +1195,12 @@ if.end:
 
 define void @and_chain_trunc_nuw_i1_condition(i8 %x) {
 ; CHECK-LABEL: @and_chain_trunc_nuw_i1_condition(
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw i8 [[X:%.*]], -2
-; CHECK-NEXT:    [[ICMP:%.*]] = icmp ugt i8 [[ADD]], 2
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nuw i8 [[X]] to i1
-; CHECK-NEXT:    [[AND:%.*]] = select i1 [[ICMP]], i1 [[TRUNC]], i1 false
-; CHECK-NEXT:    br i1 [[AND]], label [[IF_THEN:%.*]], label [[COMMON_RET:%.*]]
+; CHECK-NEXT:    switch i8 [[X:%.*]], label [[IF_THEN:%.*]] [
+; CHECK-NEXT:      i8 4, label [[COMMON_RET:%.*]]
+; CHECK-NEXT:      i8 3, label [[COMMON_RET]]
+; CHECK-NEXT:      i8 2, label [[COMMON_RET]]
+; CHECK-NEXT:      i8 0, label [[COMMON_RET]]
+; CHECK-NEXT:    ]
 ; CHECK:       common.ret:
 ; CHECK-NEXT:    ret void
 ; CHECK:       if.then:
@@ -1220,9 +1221,8 @@ if.end:
 
 define void @or_chain_trunc_nuw_i1_condition(i8 %x) {
 ; CHECK-LABEL: @or_chain_trunc_nuw_i1_condition(
-; CHECK-NEXT:    [[ICMP:%.*]] = icmp eq i8 [[X:%.*]], 2
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nuw i8 [[X]] to i1
-; CHECK-NEXT:    [[SWITCH:%.*]] = select i1 [[ICMP]], i1 true, i1 [[TRUNC]]
+; CHECK-NEXT:    [[X_OFF:%.*]] = add i8 [[X:%.*]], -1
+; CHECK-NEXT:    [[SWITCH:%.*]] = icmp ult i8 [[X_OFF]], 2
 ; CHECK-NEXT:    br i1 [[SWITCH]], label [[IF_THEN:%.*]], label [[COMMON_RET:%.*]]
 ; CHECK:       common.ret:
 ; CHECK-NEXT:    ret void