[InstCombine] Canonicalize (a + 1 == 0) ? -1 : a + 1 -> uadd.sat(a, 1)

This is because this actually is just:

// ((X + Y) u< X) ? -1 : (X + Y) --> uadd.sat(X, Y)
// ((X + Y) u< Y) ? -1 : (X + Y) --> uadd.sat(X, Y)

However, ult 1 is canonicalized to eq 0.

Alive2: https://alive2.llvm.org/ce/z/Y4eHav

Author: AZero13

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1013,12 +1013,19 @@ static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
 
   // uge -1 is canonicalized to eq -1 and requires special handling
   // (a == -1) ? -1 : a + 1 -> uadd.sat(a, 1)
+  // ult 1 is canonicalized to eq 0
+  // (a + 1 == 0) ? -1 : a + 1 -> uadd.sat(a, 1)
   if (Pred == ICmpInst::ICMP_EQ) {
     if (match(FVal, m_Add(m_Specific(Cmp0), m_One())) &&
-        match(Cmp1, m_AllOnes())) {
+        match(Cmp1, m_AllOnes()))
       return Builder.CreateBinaryIntrinsic(
           Intrinsic::uadd_sat, Cmp0, ConstantInt::get(Cmp0->getType(), 1));
-    }
+
+    if (match(Cmp1, m_Zero()) && match(Cmp0, m_Add(m_Value(X), m_One())) &&
+        match(FVal, m_Add(m_Specific(X), m_One())))
+      return Builder.CreateBinaryIntrinsic(Intrinsic::uadd_sat, X,
+                                           ConstantInt::get(X->getType(), 1));
+
     return nullptr;
   }
 

llvm/test/Transforms/InstCombine/saturating-add-sub.ll

@@ -2352,9 +2352,7 @@ define i8 @fold_add_umax_to_usub_multiuse(i8 %a) {
 
 define i32 @add_check_zero(i32 %num) {
 ; CHECK-LABEL: @add_check_zero(
-; CHECK-NEXT:    [[ADD:%.*]] = add i32 [[NUM:%.*]], 1
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[ADD]], 0
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[CMP]], i32 -1, i32 [[ADD]]
+; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[NUM:%.*]], i32 1)
 ; CHECK-NEXT:    ret i32 [[COND]]
 ;
   %add = add i32 %num, 1