[llvm] [Reassociate] Distribute multiply over add to enable factorization (PR #178201)

[Yingwei Zheng via llvm-commits]

================
@@ -963,6 +963,69 @@ static BinaryOperator *convertOrWithNoCommonBitsToAdd(Instruction *Or) {
   return New;
 }
 
+/// Return true if Mul is of the form (X+Y)*C or (X-Y)*C where C is a
+/// constant, and there exists a sibling instruction of the form X*C' or Y*C'
+/// in the same expression — indicating that distribution followed by
+/// factoring will reduce the instruction count.
+static bool ShouldBreakUpDistribution(Instruction *Mul) {
+  Value *A, *B;
+  if (!match(Mul, m_c_Mul(m_OneUse(m_CombineOr(m_Add(m_Value(A), m_Value(B)),
+                                               m_Sub(m_Value(A), m_Value(B)))),
+                          m_ImmConstant())))
+    return false;
+
+  if (!Mul->hasOneUse())
+    return false;
+
+  auto *MulUser = dyn_cast<Instruction>(Mul->user_back());
+  // The parent MUST be an Add or Sub to ensure the tree is flattened
+  if (!MulUser || (MulUser->getOpcode() != Instruction::Add &&
+                   MulUser->getOpcode() != Instruction::Sub))
+    return false;
+
+  for (Value *Sibling : MulUser->operands()) {
+    if (Sibling == Mul || !Sibling->hasOneUse())
+      continue;
+
+    // Sibling must be NonConst * C'.
+    Value *SibNC;
+    if (match(Sibling, m_Mul(m_Value(SibNC), m_Constant()))) {
+      if ((SibNC == A || SibNC == B) && !isa<Constant>(SibNC))
+        return true;
+    }
----------------
dtcxzyw wrote:

```suggestion
    if (match(Sibling, m_Mul(m_Value(SibNC), m_Constant())) && (SibNC == A || SibNC == B) && !isa<Constant>(SibNC))
        return true;
    
```

https://github.com/llvm/llvm-project/pull/178201