[IR] Fix Module::setModuleFlag for uniqued metadata (llvm#164580)

`Module::setModuleFlag` is supposed to change a single module. However,
when an `MDNode` has the same value in more than one module in the same
`LLVMContext`, such `MDNode` is shared (uniqued) across all of them.
Therefore `MDNode::replaceOperandWith` changes all modules that share
the same `MDNode`.

This used to cause problems for llvm#86212, where a module is marked as
"upgraded" via a module flag. When this flag is shared across multiple
modules, all of them are marked, yet some may not have been processed at
all.

After the patch we now construct a new `MDNode` and replace the old one.

Author: asavonic

llvm/lib/IR/Module.cpp

@@ -403,9 +403,14 @@ void Module::setModuleFlag(ModFlagBehavior Behavior, StringRef Key,
                            Metadata *Val) {
   NamedMDNode *ModFlags = getOrInsertModuleFlagsMetadata();
   // Replace the flag if it already exists.
-  for (MDNode *Flag : ModFlags->operands()) {
+  for (unsigned i = 0; i < ModFlags->getNumOperands(); ++i) {
+    MDNode *Flag = ModFlags->getOperand(i);
     if (cast<MDString>(Flag->getOperand(1))->getString() == Key) {
-      Flag->replaceOperandWith(2, Val);
+      Type *Int32Ty = Type::getInt32Ty(Context);
+      Metadata *Ops[3] = {
+          ConstantAsMetadata::get(ConstantInt::get(Int32Ty, Behavior)),
+          MDString::get(Context, Key), Val};
+      ModFlags->setOperand(i, MDNode::get(Context, Ops));
       return;
     }
   }

llvm/unittests/IR/ModuleTest.cpp

@@ -103,6 +103,36 @@ TEST(ModuleTest, setModuleFlagInt) {
   EXPECT_EQ(Val2, A2->getZExtValue());
 }
 
+TEST(ModuleTest, setModuleFlagTwoMod) {
+  LLVMContext Context;
+  Module MA("MA", Context);
+  Module MB("MB", Context);
+  StringRef Key = "Key";
+  uint32_t Val1 = 1;
+  uint32_t Val2 = 2;
+
+  // Set a flag to MA
+  EXPECT_EQ(nullptr, MA.getModuleFlag(Key));
+  MA.setModuleFlag(Module::ModFlagBehavior::Error, Key, Val1);
+  auto A1 = mdconst::extract_or_null<ConstantInt>(MA.getModuleFlag(Key));
+  EXPECT_EQ(Val1, A1->getZExtValue());
+
+  // Set a flag to MB
+  EXPECT_EQ(nullptr, MB.getModuleFlag(Key));
+  MB.setModuleFlag(Module::ModFlagBehavior::Error, Key, Val1);
+  auto B1 = mdconst::extract_or_null<ConstantInt>(MB.getModuleFlag(Key));
+  EXPECT_EQ(Val1, B1->getZExtValue());
+
+  // Change the flag of MA
+  MA.setModuleFlag(Module::ModFlagBehavior::Error, Key, Val2);
+  auto A2 = mdconst::extract_or_null<ConstantInt>(MA.getModuleFlag(Key));
+  EXPECT_EQ(Val2, A2->getZExtValue());
+
+  // MB should keep the original flag value
+  auto B2 = mdconst::extract_or_null<ConstantInt>(MB.getModuleFlag(Key));
+  EXPECT_EQ(Val1, B2->getZExtValue());
+}
+
 const char *IRString = R"IR(
   !llvm.module.flags = !{!0}