[LLVM-C][bindings/go] Get metadata from a value

[aykevl]

Summary

I added a new function that returns a metadata node directly instead of wrapping it in a value, which IIRC is now the recommended approach.

This is especially useful for getting debug info metadata from global variables.

Note that this is a breaking change for the Go bindings. I think this is fine because having a metadata-wrapped-as-value wasn't really usable anyway from the API and the old API was inconsistent with SetMetadata (which accepts a Metadata instead of a Value object). Also, I couldn't find any uses of the Metadata() call in llgo and I can tell you that it is not used in TinyGo.

[CodaFi]

Why not change the behavior of LLVMGetMetadata instead? It's an ABI-compatible change, and expanding the API to cover more cases instead of crashing is API-compatible.

[deadalnix]

llvm/include/llvm-c/Core.h

3071

Shouldn't there be a setter too?

Maybe add a release note saying that LLVMGetMetadata is deprecated?

If this is now the advised way to do this, then porting the echo test to use that API instead of the previous one would definitively help.

llvm/bindings/go/llvm/ir.go

 func (v Value) SetName(name string) {
   cname := C.CString(name)
   defer C.free(unsafe.Pointer(cname))
   C.LLVMSetValueName(v.C, cname)
 }
 func (v Value) Dump()                       { C.LLVMDumpValue(v.C) }
 func (v Value) ReplaceAllUsesWith(nv Value) { C.LLVMReplaceAllUsesWith(v.C, nv.C) }
 func (v Value) HasMetadata() bool           { return C.LLVMHasMetadata(v.C) != 0 }
-func (v Value) Metadata(kind int) (rv Value) {
-  rv.C = C.LLVMGetMetadata(v.C, C.unsigned(kind))
+func (v Value) Metadata(kind int) (md Metadata) {
+  md.C = C.LLVMGetMetadata2(v.C, C.unsigned(kind))
   return
 }
 func (v Value) SetMetadata(kind int, node Metadata) {
   C.LLVMSetMetadata2(v.C, C.unsigned(kind), node.C)
 }
 
 // Conversion functions.
 // Return the input value if it is an instance of the specified class, otherwise NULL.

llvm/include/llvm-c/Core.h

  */
 
 /**
  * Determine whether an instruction has any metadata attached.
  */
 int LLVMHasMetadata(LLVMValueRef Val);
 
 /**
- * Return metadata associated with an instruction value.
+ * Return metadata associated with an instruction value. It returns a metadata
+ * object wrapped as a value.
  */
 LLVMValueRef LLVMGetMetadata(LLVMValueRef Val, unsigned KindID);
 
+/**
+ * Return metadata associated with an instruction value or global object.
+ */
+LLVMMetadataRef LLVMGetMetadata2(LLVMValueRef Val, unsigned KindID);
+
 /**
  * Set metadata associated with an instruction value.
  */
 void LLVMSetMetadata(LLVMValueRef Val, unsigned KindID, LLVMValueRef Node);
 
 /**
  * Add metadata to a global object.
  */
 void LLVMGlobalObjectAddMetadata(LLVMValueRef Global, unsigned KindID, LLVMMetadataRef MD);
 
 /**
  * Returns the metadata associated with an instruction value, but filters out

llvm/lib/IR/Core.cpp

 LLVMValueRef LLVMGetMetadata(LLVMValueRef Inst, unsigned KindID) {
   auto *I = unwrap<Instruction>(Inst);
   assert(I && "Expected instruction");
   if (auto *MD = I->getMetadata(KindID))
     return wrap(MetadataAsValue::get(I->getContext(), MD));
   return nullptr;
 }
 
+LLVMMetadataRef LLVMGetMetadata2(LLVMValueRef Val, unsigned KindID) {
+  auto *V = unwrap<Value>(Val);
+  if (auto *I = dyn_cast<Instruction>(V)) {
+    if (auto *MD = I->getMetadata(KindID))
+      return wrap(MD);
+    return nullptr;
+  }
+  if (auto *G = dyn_cast<GlobalObject>(V)) {
+    if (auto *MD = G->getMetadata(KindID))
+      return wrap(MD);
+    return nullptr;
+  }
+  return nullptr;
+}
+
 // MetadataAsValue uses a canonical format which strips the actual MDNode for
 // MDNode with just a single constant value, storing just a ConstantAsMetadata
 // This undoes this canonicalization, reconstructing the MDNode.
 static MDNode *extractMDNode(MetadataAsValue *MAV) {
   Metadata *MD = MAV->getMetadata();
   assert((isa<MDNode>(MD) || isa<ConstantAsMetadata>(MD)) &&
       "Expected a metadata node or a canonicalized constant");