From 36de838a892c8c5e7924c0991cab268f7a85ab21 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophile=20Bastian?= <contact@tobast.fr>
Date: Thu, 2 Aug 2018 13:28:36 +0200
Subject: [PATCH] Add unwinding usage

---
 report/report.tex               | 47 ++++++++++++++++++++++++++++++---
 report/src/segfault/gdb_session | 10 +++++++
 report/src/segfault/segfault.c  | 17 ++++++++++++
 3 files changed, 70 insertions(+), 4 deletions(-)
 create mode 100644 report/src/segfault/gdb_session
 create mode 100644 report/src/segfault/segfault.c

diff --git a/report/report.tex b/report/report.tex
index 11c73d7..d1dcbdb 100644
--- a/report/report.tex
+++ b/report/report.tex
@@ -87,6 +87,49 @@ necessary to have additional data to perform stack unwinding. This data is
 often stored among the debugging informations of a program, and one common
 format of debugging data is DWARF\@.
 
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\subsection{Unwinding usage and frequency}
+
+Stack unwinding is a more common operation that one might think at first. The
+most commonly thought use-case is simply to get a stack trace of a program, and
+provide a debugger with the information it needs: for instance, when inspecting
+a stack trace in \prog{gdb}, it is quite common to jump to a previous frame:
+
+\lstinputlisting{src/segfault/gdb_session}
+
+To be able to do this, \texttt{gdb} must be able to restore \lstc{fct_a}'s
+context, by unwinding \lstc{fct_b}'s frame.
+
+\medskip
+
+Yet, stack unwinding (and thus debugging data) \emph{is not limited to
+debugging}.
+
+Another common usage is profiling. A profiling tool, such as \prog{perf} under
+Linux, is used to measure and analyze in which functions a program spends its
+time, identify bottlenecks and find out which parts are critical to optimize.
+To do so, modern profilers pause the traced program at regular, short
+intervals, inspect their stack, and determine which function is currently being
+run. They also often perform a stack unwinding to determine the call path to
+this function, to determine which function indirectly takes time: \eg, a
+function \lstc{fct_a} can call both \lstc{fct_b} and \lstc{fct_c}, which are
+quite heavy; spend practically no time directly in \lstc{fct_a}, but spend a
+lot of time in calls to the other two functions that were made by \lstc{fct_a}.
+
+Exception handling also requires a stack unwinding mechanism in most languages.
+Indeed, an exception is completely different from a \lstc{return}: while the
+latter returns to the previous function, the former can be caught by virtually
+any function in the call path, at any point of the function. It is thus
+necessary to be able to unwind frames, one by one, until a suitable
+\lstc{catch} block is found. The C++ language, for one, includes a
+stack-unwinding library similar to \prog{libunwind} in its runtime.
+
+In both of these two previous cases, performance \emph{can} be a problem. In
+the latter, a slow unwinding directly impacts the overall program performance,
+particularly if a lot of exceptions are thrown and caught far away in their
+call path. In the former, profiling \emph{is} performance-heavy and often quite
+slow when analyzing large programs anyway.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{DWARF format}
 
@@ -168,10 +211,6 @@ drastically the unwinding process.
 \subsection{How big are FDEs?}
 \todo{}
 
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsection{Unwinding usage and frequency}
-\todo{}
-
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{Unwinding state-of-the-art}
 \todo{}
diff --git a/report/src/segfault/gdb_session b/report/src/segfault/gdb_session
new file mode 100644
index 0000000..3a3187b
--- /dev/null
+++ b/report/src/segfault/gdb_session
@@ -0,0 +1,10 @@
+(gdb) backtrace
+#0  0x0000555555554625 in fct_b (m=0x5c) at segfault.c:5
+#1  0x0000555555554663 in fct_a (n=42) at segfault.c:10
+#2  0x0000555555554674 in main () at segfault.c:14
+(gdb) frame 1
+#1  0x0000555555554663 in fct_a (n=42) at segfault.c:10
+10          fct_b((int*)(some_fct_a_var + 8));
+(gdb) print some_fct_a_var
+$1 = 84
+
diff --git a/report/src/segfault/segfault.c b/report/src/segfault/segfault.c
new file mode 100644
index 0000000..681e45a
--- /dev/null
+++ b/report/src/segfault/segfault.c
@@ -0,0 +1,17 @@
+#include <stdio.h>
+#include <stdint.h>
+
+void fct_b(int* m) {
+    printf("%l\n", *m);
+}
+
+void fct_a(int n) {
+    uintptr_t some_fct_a_var = n * 2;
+    fct_b((int*)(some_fct_a_var + 8));
+}
+
+int main(void) {
+    fct_a(42);
+    return 0;
+}
+