[mlir] Add sret and byval argument attribute support to LLVM-IR dialect

[schweitz]

Summary

Adds argument attributes for using LLVM's sret and byval attributes to
the conversion of LLVM IR dialect. These attributes are used in FIR to
support the lowering of Fortran using target-specific calling
conventions.

Add roundtrip tests.

[ftynse]

I have two requests:

1. use UnitAttr instead of BoolAttr
2. add tests for any user-visible error messages

(Potentially for a follow-up, we should check attributes in the op verifier, not in the translation, but I see there are other attributes checked in the translation).

[schweitz]

In D94052#2478970, @ftynse wrote:

I have two requests:

1. use UnitAttr instead of BoolAttr
2. add tests for any user-visible error messages

(Potentially for a follow-up, we should check attributes in the op verifier, not in the translation, but I see there are other attributes checked in the translation).

Sure. The use of BoolAttr was done to be consistent with the "noalias" attribute, fwiw.

[schweitz]

Changes requested in review.

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

if (auto attr = func.getArgAttrOfType<IntegerAttr>(
       auto argTy = mlirArg.getType();
       if (!argTy.isa<LLVM::LLVMPointerType>())
         return func.emitError(
             "llvm.align attribute attached to LLVM non-pointer argument");
       llvmArg.addAttrs(
           llvm::AttrBuilder().addAlignmentAttr(llvm::Align(attr.getInt())));
     }
 
+    if (auto attr = func.getArgAttrOfType<UnitAttr>(argIdx, "llvm.sret")) {
+      auto argTy = mlirArg.getType().dyn_cast<LLVM::LLVMType>();
+      if (!argTy.isa<LLVM::LLVMPointerType>())
+        return func.emitError(
+            "llvm.sret attribute attached to LLVM non-pointer argument");
+      llvmArg.addAttr(llvm::Attribute::AttrKind::StructRet);
+    }
+
+    if (auto attr = func.getArgAttrOfType<UnitAttr>(argIdx, "llvm.byval")) {
+      auto argTy = mlirArg.getType().dyn_cast<LLVM::LLVMType>();
+      if (!argTy.isa<LLVM::LLVMPointerType>())
+        return func.emitError(
+            "llvm.byval attribute attached to LLVM non-pointer argument");
+      llvmArg.addAttr(llvm::Attribute::AttrKind::ByVal);
+    }
+
     valueMapping[mlirArg] = &llvmArg;
     argIdx++;
   }
 
   // Check the personality and set it.
   if (func.personality().hasValue()) {
     llvm::Type *ty = llvm::Type::getInt8PtrTy(llvmFunc->getContext());
     if (llvm::Constant *pfunc =

mlir/test/Dialect/LLVMIR/func.mlir

module {
 
   // CHECK: llvm.func @roundtrip12(%{{.*}}: !llvm.ptr<i32> {llvm.noalias = true})
   // CHECK: attributes {foo = 42 : i32}
   llvm.func @roundtrip12(%arg0: !llvm.ptr<i32> {llvm.noalias = true})
   attributes {foo = 42 : i32} {
     llvm.return
   }
 
+  // CHECK: llvm.func @byvalattr(%{{.*}}: !llvm.ptr<i32> {llvm.byval})
+  llvm.func @byvalattr(%arg0: !llvm.ptr<i32> {llvm.byval}) {
+    llvm.return
+  }
+
+  // CHECK: llvm.func @sretattr(%{{.*}}: !llvm.ptr<i32> {llvm.sret})
+  llvm.func @sretattr(%arg0: !llvm.ptr<i32> {llvm.sret}) {
+    llvm.return
+  }
+
   // CHECK: llvm.func @variadic(...)
   llvm.func @variadic(...)
 
   // CHECK: llvm.func @variadic_args(!llvm.i32, !llvm.i32, ...)
   llvm.func @variadic_args(!llvm.i32, !llvm.i32, ...)
 
   //
   // Check that functions can have linkage attributes.

mlir/test/Target/llvmir-invalid.mlir

 // RUN: mlir-translate -verify-diagnostics -split-input-file -mlir-to-llvmir %s
 
 // expected-error @+1 {{unsupported module-level operation}}
 func @foo() {
   llvm.return
 }
 
 // -----
 
 // expected-error @+1 {{llvm.noalias attribute attached to LLVM non-pointer argument}}
 llvm.func @invalid_noalias(%arg0 : !llvm.float {llvm.noalias = true}) -> !llvm.float {
   llvm.return %arg0 : !llvm.float
 }
 
 // -----
 
+// expected-error @+1 {{llvm.sret attribute attached to LLVM non-pointer argument}}
+llvm.func @invalid_noalias(%arg0 : !llvm.float {llvm.sret}) -> !llvm.float {
+  llvm.return %arg0 : !llvm.float
+}
+// -----
+
+// expected-error @+1 {{llvm.byval attribute attached to LLVM non-pointer argument}}
+llvm.func @invalid_noalias(%arg0 : !llvm.float {llvm.byval}) -> !llvm.float {
+  llvm.return %arg0 : !llvm.float
+}
+
+// -----
+
 // expected-error @+1 {{llvm.align attribute attached to LLVM non-pointer argument}}
 llvm.func @invalid_align(%arg0 : !llvm.float {llvm.align = 4}) -> !llvm.float {
   llvm.return %arg0 : !llvm.float
 }
 
 // -----
 
 llvm.func @no_nested_struct() -> !llvm.array<2 x array<2 x array<2 x struct<(i32)>>>> {