[vm/ffi] Rewrite Pointer.elementAt calls in CFE

This rewrites `elementAt` calls in the CFE to skip the runtime entry
for `sizeOf` when the type argument is constant.

The runtime entry is still used when the type argument is generic.
Forcing the type argument to be constant and removing the runtime entry
will be done in follow up CLs.

Bug: #44621
Bug: #38721

TEST=tests/ffi/data_test.dart

Change-Id: I480db43e7c115c24bd45f0ddab0cfea7eb8cfa58
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/182263
Reviewed-by: Clement Skau <[email protected]>

Author: dcharkes

pkg/vm/lib/transformations/ffi.dart

@@ -206,6 +206,7 @@ class FfiTransformer extends Transformer {
   final Field pragmaOptions;
   final Procedure listElementAt;
   final Procedure numAddition;
+  final Procedure numMultiplication;
 
   final Library ffiLibrary;
   final Class allocatorClass;
@@ -267,6 +268,7 @@ class FfiTransformer extends Transformer {
         pragmaOptions = coreTypes.pragmaOptions,
         listElementAt = coreTypes.index.getMember('dart:core', 'List', '[]'),
         numAddition = coreTypes.index.getMember('dart:core', 'num', '+'),
+        numMultiplication = coreTypes.index.getMember('dart:core', 'num', '*'),
         ffiLibrary = index.getLibrary('dart:ffi'),
         allocatorClass = index.getClass('dart:ffi', 'Allocator'),
         nativeFunctionClass = index.getClass('dart:ffi', 'NativeFunction'),

pkg/vm/lib/transformations/ffi_use_sites.dart

@@ -31,7 +31,6 @@ import 'ffi.dart'
         FfiTransformerData,
         NativeType,
         FfiTransformer,
-        optimizedTypes,
         nativeTypeSizes,
         WORD_SIZE,
         UNKNOWN,
@@ -432,20 +431,26 @@ class _FfiUseSiteTransformer extends FfiTransformer {
 
         _warningNativeTypeValid(nativeType, node);
 
+        // TODO(http://dartbug.com/38721): Change this to an error.
         if (nativeType is TypeParameterType) {
           // Do not rewire generic invocations.
           return node;
         }
-        final Class nativeClass = (nativeType as InterfaceType).classNode;
-        final NativeType nt = getType(nativeClass);
-        if (optimizedTypes.contains(nt)) {
-          final typeArguments = [
-            if (nt == NativeType.kPointer) _pointerTypeGetTypeArg(nativeType)
-          ];
-          return StaticInvocation(
-              elementAtMethods[nt],
-              Arguments([node.receiver, node.arguments.positional[0]],
-                  types: typeArguments));
+
+        Expression inlineSizeOf = _inlineSizeOf(nativeType);
+        if (inlineSizeOf != null) {
+          // Generates `receiver.offsetBy(inlineSizeOfExpression)`.
+          return MethodInvocation(
+              node.receiver,
+              offsetByMethod.name,
+              Arguments([
+                MethodInvocation(
+                    node.arguments.positional.single,
+                    numMultiplication.name,
+                    Arguments([inlineSizeOf]),
+                    numMultiplication)
+              ], types: node.arguments.types),
+              offsetByMethod);
         }
       }
     } on _FfiStaticTypeError {