Optimize function at its entry instead of at exit.

runtime/vm/code_patcher.cc

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "vm/code_patcher.h"
+#include "vm/instructions.h"
+#include "vm/object.h"
+
+namespace dart {
+
+static void SwapCode(intptr_t num_bytes, char* a, char* b) {
+  for (intptr_t i = 0; i < num_bytes; i++) {
+    char tmp = *a;
+    *a = *b;
+    *b = tmp;
+    a++;
+    b++;
+  }
+}
+
+
+// The patch code buffer contains the jmp code which will be inserted at
+// entry point.
+void CodePatcher::PatchEntry(const Code& code) {
+  const uword patch_addr = code.GetPcForDeoptId(Isolate::kNoDeoptId,
+                                                PcDescriptors::kEntryPatch);
+  JumpPattern jmp_entry(patch_addr);
+  ASSERT(!jmp_entry.IsValid());
+  const uword patch_buffer = code.GetPatchCodePc();
+  ASSERT(patch_buffer != 0);
+  JumpPattern jmp_patch(patch_buffer);
+  ASSERT(jmp_patch.IsValid());
+  const uword jump_target = jmp_patch.TargetAddress();
+  SwapCode(jmp_patch.pattern_length_in_bytes(),
+           reinterpret_cast<char*>(patch_addr),
+           reinterpret_cast<char*>(patch_buffer));
+  jmp_entry.SetTargetAddress(jump_target);
+}
+
+
+// The entry point is a jmp instruction, the patch code buffer contains
+// original code, the entry point contains the jump instruction.
+void CodePatcher::RestoreEntry(const Code& code) {
+  const uword patch_addr = code.GetPcForDeoptId(Isolate::kNoDeoptId,
+                                                PcDescriptors::kEntryPatch);
+  JumpPattern jmp_entry(patch_addr);
+  ASSERT(jmp_entry.IsValid());
+  const uword jump_target = jmp_entry.TargetAddress();
+  const uword patch_buffer = code.GetPatchCodePc();
+  ASSERT(patch_buffer != 0);
+  // 'patch_buffer' contains original entry code.
+  JumpPattern jmp_patch(patch_buffer);
+  ASSERT(!jmp_patch.IsValid());
+  SwapCode(jmp_patch.pattern_length_in_bytes(),
+           reinterpret_cast<char*>(patch_addr),
+           reinterpret_cast<char*>(patch_buffer));
+  ASSERT(jmp_patch.IsValid());
+  jmp_patch.SetTargetAddress(jump_target);
+}
+
+
+bool CodePatcher::CodeIsPatchable(const Code& code) {
+  const uword patch_addr = code.GetPcForDeoptId(Isolate::kNoDeoptId,
+                                                PcDescriptors::kEntryPatch);
+  // kEntryPatch may not exist which means the function is not patchable.
+  if (patch_addr == 0) {
+    return true;
+  }
+  JumpPattern jmp_entry(patch_addr);
+  if (code.Size() < (jmp_entry.pattern_length_in_bytes() * 2)) {
+    return false;
+  }
+  const uword limit = patch_addr + jmp_entry.pattern_length_in_bytes();
+  // Check no object stored between patch_addr .. limit.
+  for (intptr_t i = 0; i < code.pointer_offsets_length(); i++) {
+    const uword obj_start = code.GetPointerOffsetAt(i) + code.EntryPoint();
+    const uword obj_end  = obj_start + kWordSize;
+    if ((obj_start < limit) && (obj_end > patch_addr)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+}  // namespace dart