[compiler] Improve contract for Compiler::CompileDebugCode.

src/compiler.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler.h"
 
 #include <algorithm>
 
 #include "src/ast/ast-numbering.h"
 #include "src/ast/prettyprinter.h"
@@ skipping 415 matching lines @@
     info->shared_info()->set_feedback_metadata(*feedback_metadata);
   }
 
   // It's very important that recompiles do not alter the structure of the type
   // feedback vector. Verify that the structure fits the function literal.
   CHECK(!info->shared_info()->feedback_metadata()->SpecDiffersFrom(
       info->literal()->feedback_vector_spec()));
 }
 
 bool UseIgnition(CompilationInfo* info) {
-  if (info->is_debug()) return false;
+  DCHECK(info->has_shared_info());
+
+  // When requesting debug code as a replacement for existing code, we provide
+  // the same kind as the existing code (to prevent implicit tier-change).
+  if (info->is_debug() && info->shared_info()->is_compiled()) {
+    return info->shared_info()->HasBytecodeArray();
+  }
+
+  // For generator or async functions we might avoid Ignition wholesale.
   if (info->shared_info()->is_resumable() && !FLAG_ignition_generators) {
     return false;
   }
 
   // Since we can't OSR from Ignition, skip Ignition for asm.js functions.
   if (info->shared_info()->asm_function()) {
     return false;
   }
 
   // Checks whether top level functions should be passed by the filter.
@@ skipping 55 matching lines @@
 
 void InstallSharedScopeInfo(CompilationInfo* info,
                             Handle<SharedFunctionInfo> shared) {
   Handle<ScopeInfo> scope_info =
       ScopeInfo::Create(info->isolate(), info->zone(), info->scope());
   shared->set_scope_info(*scope_info);
 }
 
 void InstallSharedCompilationResult(CompilationInfo* info,
                                     Handle<SharedFunctionInfo> shared) {
+  // TODO(mstarzinger): Compiling for debug code might be used to reveal inner
+  // functions via {FindSharedFunctionInfoInScript}, in which case we end up
+  // regenerating existing bytecode. Fix this!
+  if (info->is_debug() && info->has_bytecode_array()) {
+    shared->ClearBytecodeArray();
+  }
   // Assert that we are not overwriting (possibly patched) debug code.
   DCHECK(!shared->HasDebugInfo());
   DCHECK(!info->code().is_null());
   shared->ReplaceCode(*info->code());
   if (info->has_bytecode_array()) {
     DCHECK(!shared->HasBytecodeArray());  // Only compiled once.
     shared->set_bytecode_array(*info->bytecode_array());
   }
 }
 
@@ skipping 1269 matching lines @@
     DCHECK(shared->is_compiled());
     function->set_literals(cached.literals);
   } else if (shared->is_compiled()) {
     // TODO(mvstanton): pass pretenure flag to EnsureLiterals.
     JSFunction::EnsureLiterals(function);
   }
 }
 
 }  // namespace internal
 }  // namespace v8