[llvm] [llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq 0, (and X, V - 1)" (PR #152851)

[Yingwei Zheng via llvm-commits]

================
@@ -1320,6 +1320,67 @@ Instruction *InstCombinerImpl::foldICmpWithZero(ICmpInst &Cmp) {
   return nullptr;
 }
 
+// Fold icmp eq (num + (val - 1)) & -val, num
+//      to
+//      icmp eq 0, (and num, val - 1)
+// For value being power of two
+Instruction *InstCombinerImpl::foldNextMultiply(ICmpInst &Cmp) {
+  Value *Op0 = Cmp.getOperand(0);
+  Value *Neg, *Add, *Num, *Mask, *Value;
+  CmpPredicate Pred;
+  const APInt *NegConst, *MaskConst;
+
+  // Match num + neg
+  if (!match(Op0, m_OneUse(m_c_And(m_Value(Add), m_Value(Neg)))))
+    return nullptr;
+
+  // Match num & mask and handle commutative care
+  if (!match(Op0, m_c_And(m_c_Add(m_Value(Num), m_Value(Mask)), m_Value(Neg))))
+    return nullptr;
+
+  // Check the constant case
+  if (match(Neg, m_APInt(NegConst)) && match(Mask, m_APInt(MaskConst))) {
+    // Mask + 1 should be a power-of-two
+    if (!(*MaskConst + 1).isPowerOf2())
+      return nullptr;
+
+    // Neg = -(Mask + 1)
+    if (*NegConst != -(*MaskConst + 1))
+      return nullptr;
+  } else {
+    // Match neg = sub 0, val
+    if (!match(Neg, m_Sub(m_Zero(), m_Value(Value))))
+      return nullptr;
+
+    // mask = add %val, -1. No commutative here, since it's canonical
+    // representation for sub %val, -1
+    if (!match(Mask, m_Add(m_Value(Value), m_AllOnes()))) {
+      if (match(Num, m_Add(m_Value(Value), m_AllOnes()))) {
+        std::swap(Mask, Num);
+      } else {
+        return nullptr;
+      }
+    }
+
+    // Value should be a known power-of-two.
+    if (!isKnownToBeAPowerOfTwo(Value, false, &Cmp))
+      return nullptr;
+  }
+
+  // Verify that Add and Num are connected by ICmp.
+  if (!match(&Cmp, m_c_ICmp(Pred, m_Value(Add), m_Specific(Num))))
+    return nullptr;
----------------
dtcxzyw wrote:

```suggestion
```
Remember not to overwrite an assigned value with `m_Value`.


https://github.com/llvm/llvm-project/pull/152851