[MLIR] Enable converting dialect attributes on external functions

[skatrak]

Summary

This patch modifies the MLIR-to-LLVMIR translation pass to enable dialect attributes attached to external functions being processed by the corresponding dialect's translation interface via amendOperation().

[ftynse]

Please add a test.

[skatrak]

Add unit test.

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

LogicalResult ModuleTranslation::convertFunctionSignatures() {
   }
 
   return success();
 }
 
 LogicalResult ModuleTranslation::convertFunctions() {
   // Convert functions.
   for (auto function : getModuleBody(mlirModule).getOps<LLVMFuncOp>()) {
-    // Ignore external functions.
-    if (function.isExternal())
+    // Do not convert external functions, but do process dialect attributes
+    // attached to them.
+    if (function.isExternal()) {
+      if (failed(convertDialectAttributes(function)))
+        return failure();
       continue;
+    }
 
     if (failed(convertOneFunction(function)))
       return failure();
   }
 
   return success();
 }
 

mlir/test/Target/LLVMIR/external-func-dialect-attr.mlir

+// RUN: mlir-translate -mlir-to-llvmir %s | FileCheck %s
+
+// Check that dialect attributes are processed for external functions.
+// This might not be an intended use case for `nvvm.minctasm`, but it enables
+// testing this feature easily.
+
+module {
+  llvm.func external @f() attributes { nvvm.minctasm = 10 : i32 }
+  // CHECK: !nvvm.annotations = !{![[NVVM:[0-9]+]]}
+  // CHECK: ![[NVVM]] = !{ptr @f, !"minctasm", i32 10}
+}