From 645f8bbc6f3f97699b2ff8e849ead34519d11251 Mon Sep 17 00:00:00 2001
From: David Srbecky <dsrbecky@google.com>
Date: Thu, 25 Jan 2018 12:21:59 +0000
Subject: [PATCH] Emit the frame for DEX PC before the frame for native PC.

This results in more natural backtrace.

Test: ART's 137-cfi test

Change-Id: Ic472db2733a330f8dbc722887809273b77d9dfb5
---
 libbacktrace/UnwindStack.cpp | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/libbacktrace/UnwindStack.cpp b/libbacktrace/UnwindStack.cpp
index 91bcc6688..c5dd45b86 100644
--- a/libbacktrace/UnwindStack.cpp
+++ b/libbacktrace/UnwindStack.cpp
@@ -128,9 +128,28 @@ bool Backtrace::Unwind(unwindstack::Regs* regs, BacktraceMap* back_map,
   }
   frames->resize(total_frames);
   size_t cur_frame = 0;
-  for (size_t i = num_ignore_frames; i < unwinder.NumFrames(); i++, cur_frame++) {
+  for (size_t i = num_ignore_frames; i < unwinder.NumFrames(); i++) {
     auto frame = &unwinder_frames[i];
-    backtrace_frame_data_t* back_frame = &frames->at(cur_frame);
+
+    // Inject extra 'virtual' frame that represents the dex pc data.
+    // The dex pc is magic register defined in the Mterp interpreter,
+    // and thus it will be restored/observed in the frame after it.
+    // Adding the dex frame first here will create something like:
+    //   #7 pc 006b1ba1 libartd.so  ExecuteMterpImpl+14625
+    //   #8 pc 0015fa20 core.vdex   java.util.Arrays.binarySearch+8
+    //   #9 pc 0039a1ef libartd.so  art::interpreter::Execute+719
+    if (frame->dex_pc != 0) {
+      backtrace_frame_data_t* dex_frame = &frames->at(cur_frame++);
+      dex_frame->num = cur_frame;
+      dex_frame->pc = frame->dex_pc;
+      dex_frame->rel_pc = frame->dex_pc;
+      dex_frame->sp = frame->sp;
+      dex_frame->stack_size = 0;
+      dex_frame->func_offset = 0;
+      FillInDexFrame(stack_map, frame->dex_pc, dex_frame);
+    }
+
+    backtrace_frame_data_t* back_frame = &frames->at(cur_frame++);
 
     back_frame->num = cur_frame;
 
@@ -147,19 +166,6 @@ bool Backtrace::Unwind(unwindstack::Regs* regs, BacktraceMap* back_map,
     back_frame->map.offset = frame->map_offset;
     back_frame->map.load_bias = frame->map_load_bias;
     back_frame->map.flags = frame->map_flags;
-
-    // Inject a frame that represents the dex pc data.
-    if (frame->dex_pc != 0) {
-      cur_frame++;
-      backtrace_frame_data_t* dex_frame = &frames->at(cur_frame);
-      dex_frame->num = cur_frame;
-      dex_frame->pc = frame->dex_pc;
-      dex_frame->rel_pc = frame->dex_pc;
-      dex_frame->sp = back_frame->sp;
-      dex_frame->stack_size = 0;
-      dex_frame->func_offset = 0;
-      FillInDexFrame(stack_map, frame->dex_pc, dex_frame);
-    }
   }
 
   return true;