[IR] Allow alignstack attribute on return values

Author: AlexMaclean

llvm/docs/LangRef.rst

@@ -1616,12 +1616,12 @@ Currently, only the following parameter attributes are defined:
 
 ``alignstack(<n>)``
     This indicates the alignment that should be considered by the backend when
-    assigning this parameter to a stack slot during calling convention
-    lowering. The enforcement of the specified alignment is target-dependent,
-    as target-specific calling convention rules may override this value. This
-    attribute serves the purpose of carrying language specific alignment
-    information that is not mapped to base types in the backend (for example,
-    over-alignment specification through language attributes).
+    assigning this parameter or return value to a stack slot during calling
+    convention lowering. The enforcement of the specified alignment is
+    target-dependent, as target-specific calling convention rules may override
+    this value. This attribute serves the purpose of carrying language specific
+    alignment information that is not mapped to base types in the backend (for
+    example, over-alignment specification through language attributes).
 
 ``allocalign``
     The function parameter marked with this attribute is the alignment in bytes of the

llvm/include/llvm/IR/Attributes.td

@@ -291,7 +291,7 @@ def SExt : EnumAttr<"signext", IntersectPreserve, [ParamAttr, RetAttr]>;
 
 /// Alignment of stack for function (3 bits)  stored as log2 of alignment with
 /// +1 bias 0 means unaligned (different from alignstack=(1)).
-def StackAlignment : IntAttr<"alignstack", IntersectPreserve, [FnAttr, ParamAttr]>;
+def StackAlignment : IntAttr<"alignstack", IntersectPreserve, [FnAttr, ParamAttr, RetAttr]>;
 
 /// Function can be speculated.
 def Speculatable : EnumAttr<"speculatable", IntersectAnd, [FnAttr]>;

llvm/test/CodeGen/NVPTX/param-overalign.ll

@@ -45,7 +45,7 @@ define float @caller_md(float %a, float %b) {
   ret float %r
 }
 
-define float @callee_md(%struct.float2 %a) {
+define float @callee_md(%struct.float2 alignstack(8) %a) {
 ; CHECK-LABEL: .visible .func  (.param .b32 func_retval0) callee_md(
 ; CHECK-NEXT:         .param .align 8 .b8 callee_md_param_0[8]
 ; CHECK-NEXT: )
@@ -105,5 +105,10 @@ define float @callee(%struct.float2 alignstack(8) %a ) {
   ret float %2
 }
 
-!nvvm.annotations = !{!0}
-!0 = !{ptr @callee_md, !"align", i32 u0x00010008}
+define alignstack(8) %struct.float2 @aligned_return(%struct.float2 %a ) {
+; CHECK-LABEL: .visible .func  (.param .align 8 .b8 func_retval0[8]) aligned_return(
+; CHECK-NEXT:         .param .align 4 .b8 aligned_return_param_0[8]
+; CHECK-NEXT: )
+; CHECK-NEXT: {
+  ret %struct.float2 %a
+}