Pass rep directly to borrow when component implements the resource type

Author: lukewagner

design/mvp/CanonicalABI.md

@@ -968,10 +968,18 @@ def lower_own(cx, src, rt):
 
 def lower_borrow(cx, src, rt):
   assert(isinstance(src, Handle))
+  if cx.inst is rt.impl:
+    return src.rep
   cx.borrow_scope.add(src)
   h = BorrowHandle(src.rep, rt, 0, cx.borrow_scope)
   return cx.inst.handles.insert(h)
 ```
+The special case in `lower_borrow` is an optimization, recognizing that, when
+a borrowed handle is passed to the component that implemented the resource
+type, the only thing the borrowed handle is good for is calling
+`resource.rep`, so lowering might as well avoid the overhead of creating an
+intermediate borrow handle.
+
 Note that the `rt` value that is stored in the runtime `Handle` captures what
 is statically known about the handle right before losing this information in
 the homogeneous `HandleTable`. Moreoever, as described above, distinct type

design/mvp/canonical-abi/definitions.py

@@ -844,6 +844,8 @@ def lower_own(cx, src, rt):
 
 def lower_borrow(cx, src, rt):
   assert(isinstance(src, Handle))
+  if cx.inst is rt.impl:
+    return src.rep
   cx.borrow_scope.add(src)
   h = BorrowHandle(src.rep, rt, 0, cx.borrow_scope)
   return cx.inst.handles.insert(h)

design/mvp/canonical-abi/run_tests.py

@@ -384,9 +384,10 @@ def dtor(x):
     nonlocal dtor_value
     dtor_value = x
 
-  rt = ResourceType(ComponentInstance(), dtor)
+  rt = ResourceType(ComponentInstance(), dtor) # usable in imports and exports
 
   inst = ComponentInstance()
+  rt2 = ResourceType(inst, dtor) # only usable in exports
   opts = mk_opts()
 
   def host_import(args):
@@ -400,10 +401,11 @@ def host_import(args):
   def core_wasm(args):
     nonlocal dtor_value
 
-    assert(len(args) == 3)
+    assert(len(args) == 4)
     assert(args[0].t == 'i32' and args[0].v == 0)
     assert(args[1].t == 'i32' and args[1].v == 1)
     assert(args[2].t == 'i32' and args[2].v == 2)
+    assert(args[3].t == 'i32' and args[3].v == 13)
     assert(canon_resource_rep(inst, rt, 0) == 42)
     assert(canon_resource_rep(inst, rt, 1) == 43)
     assert(canon_resource_rep(inst, rt, 2) == 44)
@@ -436,8 +438,8 @@ def core_wasm(args):
 
     return [Value('i32', 0), Value('i32', 1), Value('i32', 3)]
 
-  ft = FuncType([Own(rt),Own(rt),Borrow(rt)],[Own(rt),Own(rt),Own(rt)])
-  args = [OwnHandle(42, rt, 0), OwnHandle(43, rt, 0), OwnHandle(44, rt, 0)]
+  ft = FuncType([Own(rt),Own(rt),Borrow(rt),Borrow(rt2)],[Own(rt),Own(rt),Own(rt)])
+  args = [OwnHandle(42, rt, 0), OwnHandle(43, rt, 0), OwnHandle(44, rt, 0), OwnHandle(13, rt2, 0)]
   got,post_return = canon_lift(opts, inst, core_wasm, ft, args)
 
   assert(len(got) == 3)