[llvm] [InstCombine] Canonicalize signed saturated additions (PR #153053)

[Yingwei Zheng via llvm-commits]

================
@@ -1130,6 +1128,105 @@ static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
   return nullptr;
 }
 
+static Value *canonicalizeSaturatedAddSigned(ICmpInst *Cmp, Value *TVal,
+                                             Value *FVal,
+                                             InstCombiner::BuilderTy &Builder) {
+  // Match saturated add with constant.
+  Value *Cmp0 = Cmp->getOperand(0);
+  Value *Cmp1 = Cmp->getOperand(1);
+  ICmpInst::Predicate Pred = Cmp->getPredicate();
+  Value *X;
+  const APInt *C;
+
+  // Canonicalize INT_MAX to true value of the select.
+  if (match(FVal, m_MaxSignedValue())) {
+    std::swap(TVal, FVal);
+    Pred = CmpInst::getInversePredicate(Pred);
+  }
+
+  if (!match(TVal, m_MaxSignedValue()))
+    return nullptr;
+
+  // sge maximum signed value is canonicalized to eq maximum signed value and
+  // requires special handling (a == INT_MAX) ? INT_MAX : a + 1 -> sadd.sat(a,
+  // 1)
+  if (Pred == ICmpInst::ICMP_EQ) {
+    if (match(FVal, m_Add(m_Specific(Cmp0), m_One())) && Cmp1 == TVal) {
+      return Builder.CreateBinaryIntrinsic(
+          Intrinsic::sadd_sat, Cmp0, ConstantInt::get(Cmp0->getType(), 1));
+    }
+    return nullptr;
+  }
+
+  // (X > Y) ? INT_MAX : (X + C) --> sadd.sat(X, C)
+  // (X >= Y) ? INT_MAX : (X + C) --> sadd.sat(X, C)
+  // where Y is INT_MAX - C or INT_MAX - C - 1, and C > 0
+  if ((Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SGE) &&
+      match(FVal, m_Add(m_Specific(Cmp0), m_APIntAllowPoison(C))) &&
+      C->isStrictlyPositive()) {
----------------
dtcxzyw wrote:

```suggestion
      match(FVal, m_Add(m_Specific(Cmp0), m_StrictlyPositive(C)))) {
```

https://github.com/llvm/llvm-project/pull/153053