Print the correct return address, even on architectures where StackFrame::instruction is offset.

a=bruce.dawson, r=jimblandy


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1105 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/google_breakpad/processor/stack_frame.h

@@ -83,11 +83,31 @@ struct StackFrame {
     }
   };
 
-  // The program counter location as an absolute virtual address.  For the
-  // innermost called frame in a stack, this will be an exact program counter
-  // or instruction pointer value.  For all other frames, this will be within
-  // the instruction that caused execution to branch to a called function,
-  // but may not necessarily point to the exact beginning of that instruction.
+  // Return the actual return address, as saved on the stack or in a
+  // register. See the comments for 'instruction', below, for details.
+  virtual u_int64_t ReturnAddress() const { return instruction; }
+
+  // The program counter location as an absolute virtual address.
+  //
+  // - For the innermost called frame in a stack, this will be an exact
+  //   program counter or instruction pointer value.
+  //
+  // - For all other frames, this address is within the instruction that
+  //   caused execution to branch to this frame's callee (although it may
+  //   not point to the exact beginning of that instruction). This ensures
+  //   that, when we look up the source code location for this frame, we
+  //   get the source location of the call, not of the point at which
+  //   control will resume when the call returns, which may be on the next
+  //   line. (If the compiler knows the callee never returns, it may even
+  //   place the call instruction at the very end of the caller's machine
+  //   code, such that the "return address" (which will never be used)
+  //   immediately after the call instruction is in an entirely different
+  //   function, perhaps even from a different source file.)
+  //
+  // On some architectures, the return address as saved on the stack or in
+  // a register is fine for looking up the point of the call. On others, it
+  // requires adjustment. ReturnAddress returns the address as saved by the
+  // machine.
   u_int64_t instruction;
 
   // The module in which the instruction resides.