Initial support for emitting unwinding information in perf jitdump.

src/unwinding-info.cc

+// Copyright 2016 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/unwinding-info.h"
+#include "src/objects-inl.h"
+#include "src/objects.h"
+
+namespace v8 {
+namespace internal {
+
+static const int DW_EH_PE_pcrel = 0x10;
+static const int DW_EH_PE_datarel = 0x30;
+static const int DW_EH_PE_udata4 = 0x03;
+static const int DW_EH_PE_sdata4 = 0x0b;
+
+static const int kSizeOfCIE = 0;
+
+EhFrameHdr::EhFrameHdr(Code* code) {

[rmcilroy, 2016/06/21 13:47:44]

Can we have a test (cctest or unittest) for this class.

[Stefano Sanfilippo, 2016/06/23 15:23:44]

Done. I wrote a cctest because we need to pass through the factory to
instantiate a Code object.

+  int code_size = code->is_crankshafted() ? code->safepoint_table_offset()
+                                          : code->instruction_size();
+  version_ = 1;
+  eh_frame_ptr_encoding_ = DW_EH_PE_sdata4 | DW_EH_PE_pcrel;
+  lut_size_encoding_ = DW_EH_PE_udata4;
+  lut_entries_encoding_ = DW_EH_PE_sdata4 | DW_EH_PE_datarel;
+
+  // .eh_frame pointer and LUT
+  if (code->has_unwinding_info()) {
+    DCHECK_GE(code->unwinding_info_size(), EhFrameHdr::kRecordSize);
+    int eh_frame_size = code->unwinding_info_size() - EhFrameHdr::kRecordSize;
+
+    offset_to_eh_frame_ = -(eh_frame_size + 4);
+    lut_entries_number_ = 1;
+    offset_to_procedure_ = -(RoundUp(code_size, 8) + eh_frame_size);
+    offset_to_fde_ = -(eh_frame_size - static_cast<int>(kSizeOfCIE));
+  } else {
+    // Create a dummy table
+    offset_to_eh_frame_ = 0;
+    lut_entries_number_ = 0;
+    offset_to_procedure_ = 0;
+    offset_to_fde_ = 0;
+  }
+}
+
+void PatchProcedureBoundariesInEhFrame(Code* code) {

[rmcilroy, 2016/06/21 13:47:44]

As mentioned above, please remove this from this CL for now, however when adding
it back, I'm not convinced it should live here. It should be part of a class
(maybe the Code class if it is messing with the code object? ) What is this
actually doing, and why can't it be done as part of generating the unwinding
info?

[Stefano Sanfilippo, 2016/06/23 15:23:44]

Removed. The issue was that in some circumstances the reported size of code 
after compilation would not match the one seen at the moment of emitting the
jitdump. This function calculates the code size and writes it to the placeholder
in the CIE just before dumping it, at the latest moment possible.

I am not sure where else this bit of code could go of if it should stay at all,
let me first investigate the root cause of the issue.

+  int boundaries_base_offset = kSizeOfCIE + 8;
+  int32_t code_size = code->is_crankshafted() ? code->safepoint_table_offset()
+                                              : code->instruction_size();
+  int32_t offset_to_text = -(RoundUp(code_size, 8) + boundaries_base_offset);
+
+  MemMove(code->unwinding_info_start() + boundaries_base_offset,
+          &offset_to_text, sizeof(offset_to_text));
+  MemMove(code->unwinding_info_start() + boundaries_base_offset +
+              sizeof(offset_to_text),
+          &code_size, sizeof(code_size));
+}
+
+}  // namespace internal
+}  // namespace v8