[mlir][llvmir] Fixed MDNode uniquing during TBAA translation.

[vzakhari]

Summary

In the process of creating the MDNodes for the TBAA tag operations
we used to produce incomplete MDNodes like:

@__tbaa::@tbaa_tag_4 => !{!null, !null, i64 0}
@__tbaa::@tbaa_tag_7 => !{!null, !null, i64 0}

This caused the two tags to map to the same incomplete MDNode due to uniquing.
To prevent this, we have to use temporary MDNodes instead of !null's.

[vitalybuka]

Looks broken by this patch https://lab.llvm.org/buildbot/#/builders/5/builds/30788

[vzakhari]

In D141726#4060230, @vitalybuka wrote:

Looks broken by this patch https://lab.llvm.org/buildbot/#/builders/5/builds/30788

Thank you, Vitaly! I will take a look.

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

for (auto metadata : getModuleBody(mlirModule).getOps<LLVM::MetadataOp>()) {
     for (auto &op : metadata.getBody().getOps()) {
       SmallVector<llvm::Metadata *> operands;
       if (auto rootOp = dyn_cast<LLVM::TBAARootMetadataOp>(op)) {
         operands.push_back(llvm::MDString::get(ctx, rootOp.getIdentity()));
       } else if (auto tdOp = dyn_cast<LLVM::TBAATypeDescriptorOp>(op)) {
         operands.push_back(llvm::MDString::get(
             ctx, tdOp.getIdentity().value_or(llvm::StringRef{})));
         for (int64_t offset : tdOp.getOffsets()) {
-          operands.push_back(nullptr); // Placeholder for the member type.
+          // Use temporary MDNode as the placeholder for the member type
+          // to prevent uniquing the type descriptor nodes until they are
+          // finalized.
+          operands.push_back(
+              llvm::MDNode::getTemporary(ctx, std::nullopt).release());
           operands.push_back(llvm::ConstantAsMetadata::get(
               llvm::ConstantInt::get(offsetTy, offset)));
         }
       } else if (auto tagOp = dyn_cast<LLVM::TBAATagOp>(op)) {
-        operands.push_back(nullptr); // Placeholder for the base type.
-        operands.push_back(nullptr); // Placeholder for the access type.
+        // Use temporary MDNode's as the placeholders for the base and access
+        // types to prevent uniquing the tag nodes until they are finalized.
+        operands.push_back(
+            llvm::MDNode::getTemporary(ctx, std::nullopt).release());
+        operands.push_back(
+            llvm::MDNode::getTemporary(ctx, std::nullopt).release());
         operands.push_back(llvm::ConstantAsMetadata::get(
             llvm::ConstantInt::get(offsetTy, tagOp.getOffset())));
         if (tagOp.getConstant())
           operands.push_back(llvm::ConstantAsMetadata::get(
               llvm::ConstantInt::get(offsetTy, 1)));
       }
 
       if (operands.empty())

mlir/test/Target/LLVMIR/tbaa.mlir

 // CHECK-DAG: ![[LTAG]] = !{![[AGG2T:[0-9]*]], ![[I64T:[0-9]*]], i64 8}
 // CHECK-DAG: ![[AGG2T]] = !{!"agg2_t", ![[I64T]], i64 0, ![[I64T]], i64 8}
 // CHECK-DAG: ![[I64T]] = !{!"long long", ![[CHART:[0-9]*]], i64 0}
 // CHECK-DAG: ![[CHART]] = !{!"omnipotent char", ![[ROOT:[0-9]*]], i64 0}
 // CHECK-DAG: ![[ROOT]] = !{!"Simple C/C++ TBAA"}
 // CHECK-DAG: ![[STAG]] = !{![[AGG1T:[0-9]*]], ![[I32T:[0-9]*]], i64 0, i64 1}
 // CHECK-DAG: ![[AGG1T]] = !{!"agg1_t", ![[I32T]], i64 0, ![[I32T]], i64 4}
 // CHECK-DAG: ![[I32T]] = !{!"int", ![[CHART]], i64 0}
+
+// -----
+
+// Verify that the MDNode's created for the access tags are not uniqued
+// before they are finalized. In the process of creating the MDNodes for
+// the tag operations we used to produce incomplete MDNodes like:
+//   @__tbaa::@tbaa_tag_4 => !{!null, !null, i64 0}
+//   @__tbaa::@tbaa_tag_7 => !{!null, !null, i64 0}
+// This caused the two tags to map to the same incomplete MDNode due to
+// uniquing. To prevent this, we have to use temporary MDNodes
+// instead of !null's.
+module {
+  llvm.metadata @__tbaa {
+    llvm.tbaa_root @tbaa_root_0 {id = "Simple C/C++ TBAA"}
+    llvm.tbaa_type_desc @tbaa_type_desc_1 {id = "omnipotent char", members = {<@tbaa_root_0, 0>}}
+    llvm.tbaa_type_desc @tbaa_type_desc_2 {id = "float", members = {<@tbaa_type_desc_1, 0>}}
+    llvm.tbaa_type_desc @tbaa_type_desc_3 {id = "agg2_t", members = {<@tbaa_type_desc_2, 0>, <@tbaa_type_desc_2, 4>}}
+    llvm.tbaa_tag @tbaa_tag_4 {access_type = @tbaa_type_desc_2, base_type = @tbaa_type_desc_3, offset = 0 : i64}
+    llvm.tbaa_type_desc @tbaa_type_desc_5 {id = "int", members = {<@tbaa_type_desc_1, 0>}}
+    llvm.tbaa_type_desc @tbaa_type_desc_6 {id = "agg1_t", members = {<@tbaa_type_desc_5, 0>, <@tbaa_type_desc_5, 4>}}
+    llvm.tbaa_tag @tbaa_tag_7 {access_type = @tbaa_type_desc_5, base_type = @tbaa_type_desc_6, offset = 0 : i64}
+  }
+  llvm.func @tbaa2(%arg0: !llvm.ptr, %arg1: !llvm.ptr) {
+    %0 = llvm.mlir.constant(0 : i32) : i32
+    %1 = llvm.mlir.constant(1 : i32) : i32
+    %2 = llvm.getelementptr inbounds %arg1[%0, 0] : (!llvm.ptr, i32) -> !llvm.ptr, !llvm.struct<"struct.agg2_t", (f32, f32)>
+    // CHECK: load float, ptr %{{.*}},{{.*}}!tbaa ![[LTAG:[0-9]*]]
+    %3 = llvm.load %2 {llvm.tbaa = [@__tbaa::@tbaa_tag_4]} : !llvm.ptr -> f32
+    %4 = llvm.fptosi %3 : f32 to i32
+    %5 = llvm.getelementptr inbounds %arg0[%0, 0] : (!llvm.ptr, i32) -> !llvm.ptr, !llvm.struct<"struct.agg1_t", (i32, i32)>
+    // CHECK: store i32 %{{.*}}, ptr %{{.*}},{{.*}}!tbaa ![[STAG:[0-9]*]]
+    llvm.store %4, %5 {llvm.tbaa = [@__tbaa::@tbaa_tag_7]} : i32, !llvm.ptr
+    llvm.return
+  }
+}
+
+// CHECK-DAG: ![[LTAG]] = !{![[AGG2T:[0-9]*]], ![[F32T:[0-9]*]], i64 0}
+// CHECK-DAG: ![[AGG2T]] = !{!"agg2_t", ![[F32T]], i64 0, ![[F32T]], i64 4}
+// CHECK-DAG: ![[I64T]] = !{!"float", ![[CHART:[0-9]*]], i64 0}
+// CHECK-DAG: ![[CHART]] = !{!"omnipotent char", ![[ROOT:[0-9]*]], i64 0}
+// CHECK-DAG: ![[ROOT]] = !{!"Simple C/C++ TBAA"}
+// CHECK-DAG: ![[STAG]] = !{![[AGG1T:[0-9]*]], ![[I32T:[0-9]*]], i64 0}
+// CHECK-DAG: ![[AGG1T]] = !{!"agg1_t", ![[I32T]], i64 0, ![[I32T]], i64 4}
+// CHECK-DAG: ![[I32T]] = !{!"int", ![[CHART]], i64 0}