[NFC][SimplifyCFG] Refactor passingValueIsAlwaysUndefined to work on Use (#125519)

Address comment
#125383 (comment)

Author: dtcxzyw

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -8175,8 +8175,8 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValu
   if (C->isNullValue() || isa<UndefValue>(C)) {
     // Only look at the first use we can handle, avoid hurting compile time with
     // long uselists
-    auto FindUse = llvm::find_if(I->users(), [](auto *U) {
-      auto *Use = cast<Instruction>(U);
+    auto FindUse = llvm::find_if(I->uses(), [](auto &U) {
+      auto *Use = cast<Instruction>(U.getUser());
       // Change this list when we want to add new instructions.
       switch (Use->getOpcode()) {
       default:
@@ -8199,26 +8199,28 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValu
         return true;
       }
     });
-    if (FindUse == I->user_end())
+    if (FindUse == I->use_end())
       return false;
-    auto *Use = cast<Instruction>(*FindUse);
-    // Bail out if Use is not in the same BB as I or Use == I or Use comes
-    // before I in the block. The latter two can be the case if Use is a
+    auto &Use = *FindUse;
+    auto *User = cast<Instruction>(Use.getUser());
+    // Bail out if User is not in the same BB as I or User == I or User comes
+    // before I in the block. The latter two can be the case if User is a
     // PHI node.
-    if (Use->getParent() != I->getParent() || Use == I || Use->comesBefore(I))
+    if (User->getParent() != I->getParent() || User == I ||
+        User->comesBefore(I))
       return false;
 
     // Now make sure that there are no instructions in between that can alter
     // control flow (eg. calls)
     auto InstrRange =
-        make_range(std::next(I->getIterator()), Use->getIterator());
+        make_range(std::next(I->getIterator()), User->getIterator());
     if (any_of(InstrRange, [](Instruction &I) {
           return !isGuaranteedToTransferExecutionToSuccessor(&I);
         }))
       return false;
 
     // Look through GEPs. A load from a GEP derived from NULL is still undefined
-    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
+    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(User))
       if (GEP->getPointerOperand() == I) {
         // The current base address is null, there are four cases to consider:
         // getelementptr (TY, null, 0)                 -> null
@@ -8235,7 +8237,7 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValu
       }
 
     // Look through return.
-    if (ReturnInst *Ret = dyn_cast<ReturnInst>(Use)) {
+    if (ReturnInst *Ret = dyn_cast<ReturnInst>(User)) {
       bool HasNoUndefAttr =
           Ret->getFunction()->hasRetAttribute(Attribute::NoUndef);
       // Return undefined to a noundef return value is undefined.
@@ -8249,56 +8251,45 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValu
     }
 
     // Load from null is undefined.
-    if (LoadInst *LI = dyn_cast<LoadInst>(Use))
+    if (LoadInst *LI = dyn_cast<LoadInst>(User))
       if (!LI->isVolatile())
         return !NullPointerIsDefined(LI->getFunction(),
                                      LI->getPointerAddressSpace());
 
     // Store to null is undefined.
-    if (StoreInst *SI = dyn_cast<StoreInst>(Use))
+    if (StoreInst *SI = dyn_cast<StoreInst>(User))
       if (!SI->isVolatile())
         return (!NullPointerIsDefined(SI->getFunction(),
                                       SI->getPointerAddressSpace())) &&
                SI->getPointerOperand() == I;
 
     // llvm.assume(false/undef) always triggers immediate UB.
-    if (auto *Assume = dyn_cast<AssumeInst>(Use)) {
+    if (auto *Assume = dyn_cast<AssumeInst>(User)) {
       // Ignore assume operand bundles.
       if (I == Assume->getArgOperand(0))
         return true;
     }
 
-    if (auto *CB = dyn_cast<CallBase>(Use)) {
+    if (auto *CB = dyn_cast<CallBase>(User)) {
       if (C->isNullValue() && NullPointerIsDefined(CB->getFunction()))
         return false;
       // A call to null is undefined.
       if (CB->getCalledOperand() == I)
         return true;
 
-      if (C->isNullValue()) {
-        for (const llvm::Use &Arg : CB->args())
-          if (Arg == I) {
-            unsigned ArgIdx = CB->getArgOperandNo(&Arg);
-            if (CB->isPassingUndefUB(ArgIdx) &&
-                CB->paramHasAttr(ArgIdx, Attribute::NonNull)) {
-              // Passing null to a nonnnull+noundef argument is undefined.
-              return !PtrValueMayBeModified;
-            }
-          }
-      } else if (isa<UndefValue>(C)) {
+      if (CB->isArgOperand(&Use)) {
+        unsigned ArgIdx = CB->getArgOperandNo(&Use);
+        // Passing null to a nonnnull+noundef argument is undefined.
+        if (C->isNullValue() && CB->isPassingUndefUB(ArgIdx) &&
+            CB->paramHasAttr(ArgIdx, Attribute::NonNull))
+          return !PtrValueMayBeModified;
         // Passing undef to a noundef argument is undefined.
-        for (const llvm::Use &Arg : CB->args())
-          if (Arg == I) {
-            unsigned ArgIdx = CB->getArgOperandNo(&Arg);
-            if (CB->isPassingUndefUB(ArgIdx)) {
-              // Passing undef to a noundef argument is undefined.
-              return true;
-            }
-          }
+        if (isa<UndefValue>(C) && CB->isPassingUndefUB(ArgIdx))
+          return true;
       }
     }
     // Div/Rem by zero is immediate UB
-    if (match(Use, m_BinOp(m_Value(), m_Specific(I))) && Use->isIntDivRem())
+    if (match(User, m_BinOp(m_Value(), m_Specific(I))) && User->isIntDivRem())
       return true;
   }
   return false;