524 lines
16 KiB
TeX
524 lines
16 KiB
TeX
% vim: spell spelllang=en
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\documentclass[11pt,xcolor={usenames,dvipsnames}]{beamer}
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\usetheme{Warsaw}
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\lstset{basicstyle=\footnotesize\ttfamily}
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\newcommand{\thenalert}[1]{\only<1>{#1}\only<2>{\alert{#1}}}
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\newcommand{\slidecountline}{
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{}
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{\insertframenumber/\inserttotalframenumber}}
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\newcommand{\sectionline}{
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{}
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{\Roman{section}~-- \insertsection}}
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\AtBeginSection[]{
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\begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
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\lstdefinelanguage{gdb}{
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morekeywords={gdb},
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sensitive=false,
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}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\author[\slidecountline]{Théophile \textsc{Bastian} \\
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\footnotesize{under the supervision of Francesco Zappa Nardelli}}
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\title[\sectionline]
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{Speeding up stack unwinding by compiling DWARF debug data}
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\date{March\ --\ August 2018}
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%\subject{}
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%\logo{}
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\institute{Team PARKAS, INRIA, Paris}
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\begin{document}
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\begin{frame}
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\addtocounter{framenumber}{-1}
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\titlepage{}
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\vspace{-2em}
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\begin{center}
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\begin{align*}
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\text{Slides: } &\text{\url{https://tobast.fr/m2/slides.pdf}} \\
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\text{Report: } &\text{\url{https://tobast.fr/m2/report.pdf}}
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\end{align*}
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\end{center}
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\end{frame}
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\begin{frame}{~}
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\addtocounter{framenumber}{-1}
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\tableofcontents[hideallsubsections]
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Stack unwinding data}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Introduction}
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\begin{frame}[fragile]{We often use stack unwinding!}
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\begin{columns}[c]
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\begin{column}{0.70\textwidth}
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\begin{lstlisting}[language=gdb, numbers=none, escapechar=|]
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Program received signal SIGSEGV.
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0x54625 in fct_b at segfault.c:5
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5 printf("%l\n", *b);
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|\pause| (gdb) backtrace
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#0 0x54625 in fct_b at segfault.c:5
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#1 0x54663 in fct_a at segfault.c:10
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#2 0x54674 in main at segfault.c:14
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|\pause| (gdb) frame 1
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#1 0x54663 in fct_a at segfault.c:10
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10 fct_b((int*) a);
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|\pause| (gdb) print a
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$1 = 84
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\end{lstlisting}
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\vspace{-1em}
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\pause{}
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\begin{center}
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\textbf{\Large How does it work?!}
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\end{center}
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\end{column}
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\begin{column}{0.35\textwidth}
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\pause{}
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\includegraphics[width=0.95\linewidth]{img/call_stack}
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\end{column}
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\end{columns}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Stack frames and unwinding}
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\begin{frame}{Call stack and registers}
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\begin{columns}[c]
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\begin{column}{0.55\textwidth}
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\begin{center}
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\large\bf
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How do we get the grandparent RA\@?
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\medskip
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Isn't it as trivial as \texttt{pop()}?
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\vspace{2em}
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\only<2>{We only have \reg{rsp} and \reg{rip}.}
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\end{center}
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\end{column}
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\begin{column}{0.45\textwidth}
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\includegraphics[width=0.95\linewidth]{img/call_stack}
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\end{column}
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\end{columns}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{DWARF tables}
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\newcolumntype{a}{>{\columncolor{RedOrange}}l}
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\begin{frame}{DWARF unwinding data}
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\vspace{2em}
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\tt \footnotesize
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\begin{tabular}{
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>{\columncolor{YellowGreen}}l
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>{\columncolor{Thistle}}l
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l l l l l l
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>{\columncolor{Apricot}}l}
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~LOC & CFA & rbx & rbp & r12 & r13 & r14 & r15 & ra \\
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0084950 & rsp+8 & u & u & u & u & u & u & c-8 \\
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0084952 & rsp+16 & u & u & u & u & u & c-16 & c-8 \\
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0084954 & rsp+24 & u & u & u & u & c-24 & c-16 & c-8 \\
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0084956 & rsp+32 & u & u & u & c-32 & c-24 & c-16 & c-8 \\
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0084958 & rsp+40 & u & u & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084959 & rsp+48 & u & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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\rowcolor{Aquamarine} 008495a & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084962 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a19 & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a1d & rsp+48 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a1e & rsp+40 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a20 & rsp+32 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a22 & rsp+24 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a24 & rsp+16 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a26 & rsp+8 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a30 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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\end{tabular}
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\pause{}
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\vspace{-3cm}
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\hfill\includegraphics[height=3cm, angle=45, origin=c]{img/dwarf_logo}
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\hspace{-1cm}
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\end{frame}
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\begin{frame}[t, fragile]{The real DWARF}
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\begin{lstlisting}[numbers=none, language=]
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00009b30 48 009b34 FDE cie=0000 pc=0084950..0084b37
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DW_CFA_advance_loc: 2 to 0000000000084952
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DW_CFA_def_cfa_offset: 16
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DW_CFA_offset: r15 (r15) at cfa-16
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DW_CFA_advance_loc: 2 to 0000000000084954
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DW_CFA_def_cfa_offset: 24
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DW_CFA_offset: r14 (r14) at cfa-24
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DW_CFA_advance_loc: 2 to 0000000000084956
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DW_CFA_def_cfa_offset: 32
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DW_CFA_offset: r13 (r13) at cfa-32
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DW_CFA_advance_loc: 2 to 0000000000084958
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DW_CFA_def_cfa_offset: 40
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DW_CFA_offset: r12 (r12) at cfa-40
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DW_CFA_advance_loc: 1 to 0000000000084959
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[...]
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\end{lstlisting}
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\begin{itemize}
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\item[\textbf{$\longrightarrow$}] \textbf{\alert{constructed} on-demand
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by a \alert{Turing-complete bytecode}!}
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\end{itemize}
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\pause{}
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\vspace{-5.5cm}
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\begin{center}
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\bf \fontsize{8cm}{1cm}\colorbox{white}{\alert{Slow!}}
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\end{center}
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\end{frame}
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\begin{frame}{Why does slow matter?}
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\begin{itemize}
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\item{} After all, we're talking about \alert{debugging procedures} ran
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by a \alert{human being} (slower than the machine).
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\ldots{}or are we?
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\end{itemize}
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\pause{}
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\begin{center}
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\textbf{\Large{}No!}
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\end{center}
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\begin{itemize}
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\pause{}\item{} Pretty much any \alert{program analysis tool}
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\pause{}\item{} \alert{Profiling} with polling profilers
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\pause{}\item{} \alert{Exception handling} in C++
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\end{itemize}
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\vspace{2em}
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\begin{center}
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\textbf{\Large{}Debug data is not only for debugging}
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\end{center}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Compiling stack unwinding data ahead-of-time}
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\subsection*{}
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\begin{frame}{Compilation overview}
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\begin{itemize}
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\item Compiled to \alert{C code}
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\item C code then \alert{compiled to native binary} (gcc)
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\begin{itemize}
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\item[$\leadsto$] gcc optimisations for free
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\end{itemize}
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\item Compiled as \alert{separate \texttt{.so} files}, called \ehelfs{}
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\bigskip{}
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\item Morally a \alert{monolithic switch} on IPs
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\item Each case contains assembly that computes a \alert{row of the
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table}
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\end{itemize}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Example}
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\begin{frame}{Compilation example: original C, DWARF}
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\lstinputlisting[language=C]{src/fib7/fib7.cfde}
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\end{frame}
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\begin{frame}[shrink]{Compilation example: generated C}
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\lstinputlisting[language=C]{src/fib7/fib7.eh_elf_basic.c}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Compilation Strategy}
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\begin{frame}{Compilation choices}
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\textbf{In order to keep the compiler \alert{simple} and \alert{easily
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testable}, the whole DWARF5 instruction set is not supported.}
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\begin{itemize}
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\item Focus on \alert{x86\_64}
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\item Focus on unwinding return address \\
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\vspace{0.3ex}
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$\leadsto$ \textit{Allows building a backtrace}
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\begin{itemize}
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\item \alert{suitable for perf, not for gdb}
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\item Only supports \alert{unwinding registers}: \reg{rip}, \reg{rsp},
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\reg{rbp}, \reg{rbx}
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\item Supports the \alert{wide majority} ($> 99.9\%$) of instructions
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used
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\item Among \alert{4000} randomly sampled filed, only \alert{24}
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containing unsupported instructions
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\end{itemize}
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\end{itemize}
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\end{frame}
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\begin{frame}{Interface: libunwind}
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\begin{itemize}
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\item \alert{libunwind}: \textit{de facto} standard library for
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unwinding
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\item Relies on DWARF
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\bigskip{}
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\item \texttt{libunwind-eh\_elf}: alternative implementation using
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\ehelfs{}
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\item[$\leadsto$] \alert{alternative implementation} of libunwind,
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almost plug-and-play for existing projects!
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\begin{itemize}
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\item[$\leadsto$] It is \alert{easy} to use \ehelfs{}: just
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link against the right library!
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\end{itemize}
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\end{itemize}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Outlining}
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\begin{frame}{Size optimisation: outlining}
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\begin{itemize}
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\item This \alert{works}, but \alert{takes space}: about \alert{7 times
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larger in size} than regular DWARF\@.
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\item DWARF optimisation strategy: \alert{alter previous row}. \\
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Causes slowness: we cannot do that.
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\item Remark: a lot of lines appear often.
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\begin{itemize}
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\item[$\leadsto$] \textbf{\emph{outline} them!}
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\end{itemize}
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\pause{}
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\item On libc, $20\,827$ rows $\rightarrow$ $302$ outlined ($1.5\,\%$)
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\item Turn the big switch into a binary search \alert{if/else tree}
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\end{itemize}
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\pause{}
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\bigskip{}
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\begin{center}
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$\leadsto$ only \textbf{2.5 times bigger than DWARF}
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\end{center}
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\end{frame}
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\begin{frame}{Example with outlining}
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\lstinputlisting[language=C]{src/fib7/fib7.eh_elf_outline.c}
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\end{frame}
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\subsection{A word on formalization}
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\begin{frame}[t]{A word on formalization}
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\begin{itemize}
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\item First task: \alert{writing semantics} for DWARF, written as
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mapping to C code.
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\item DWARF5 specification: \alert{plain English}, no proper semantics
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\item Compiled code is in substance equivalent to semantics
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\item What remains to prove is mostly \alert{simple or classic
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optimisations}
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\end{itemize}
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\pause{}
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\vspace{-3cm}
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\begin{center}
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\includegraphics[width=0.8\linewidth, angle=10]{img/dw_spec.png}
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\end{center}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Benchmarking}
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\begin{frame}{Benchmarking requirements}
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\begin{enumerate}
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\item Thousands of samples (single unwind: $10\,\mu{}s$)
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\item Interesting enough program to unwind: nested functions, complex
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FDEs
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\item Mitigate caching: don't always unwind from the \emph{same} point
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\item Yet be fair: don't always unwind from totally different places
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\item Distribute evenly: if possible, also from within libraries
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\end{enumerate}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\begin{frame}{perf instrumentation}
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\textbf{\alert{perf} is the state-of-the-art polling profiler for Linux.}
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\begin{itemize}
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\item{} used to get readings of the time spent in each function
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\item{} works by regularly stopping the program, unwinding its stack,
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then aggregating the gathered data
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\end{itemize}
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\pause{}\bigskip{}
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\textbf{Instrumenting perf matches all the requirements!}
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\begin{itemize}
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\item{} \alert{Plug \ehelfs{} into perf}: use \ehelfs{} instead of
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DWARF to unwind the stack
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\item{} Implement \alert{unwinding performance counters} inside perf
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\bigskip{}
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\item{} Use perf on \alert{hackbench}, a kernel stress-test program
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\begin{itemize}
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\item Small program
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\item Lots of calls
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\item Relies on libc, libpthread
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\end{itemize}
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\end{itemize}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Results}
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\begin{frame}{Time performance}
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\small
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\centering
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\begin{tabular}{l r r r r r}
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\toprule
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\thead{Unwinding method} & \thead{Frames \\ unwound}
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& \thead{Tot.\ time \\ ($\mu s$)}
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& \thead{Avg. \\ time / frame \\ ($ns$)}
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& \thead{Time \\ ratio} \\
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\midrule
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\alert{\ehelfs{}}
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& 23506 % Frames unwound
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& 14837 % Total time
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& 631 % Avg time
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& 1
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\\
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\prog{libunwind}, \alert{cached}
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& 27058 % Frames unwound
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& 441601 % Total time
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& 16320 % Avg time
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& \alert{25.9}
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\\
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\prog{libunwind}, \alert{uncached}
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& 27058 % Frames unwound
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& 671292 % Total time
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& 24809 % Avg time
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& \alert{39.3}
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\\
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\bottomrule
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\end{tabular}
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\end{frame}
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\begin{frame}{Space performance}
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\begin{center}
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\begin{tabular}{r r r r r r}
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\toprule
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\thead{Object}
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& \thead{\% of binary size}
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& \thead{Growth factor} \\
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\midrule
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libc
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& 21.88 & 2.41 \\
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libpthread
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& 43.71 & 2.19 \\
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ld
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& 22.09 & 2.97 \\
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hackbench
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& 93.87 & 4.99 \\
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\midrule
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Total
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& 22.81 & \alert{2.44} \\
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\bottomrule
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\end{tabular}
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\end{center}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section*{}
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\setcounter{section}{0}
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\begin{frame}{What next?}
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\begin{itemize}
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\item Implement a release-ready, packageable, easy to use version of
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perf with \ehelfs{} and submit it for inclusion
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\item{} Measure \alert{C++ exceptions overhead} precisely in common
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software
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\item{} Implement \alert{\ehelfs{}} support for \alert{C++ runtime}
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exception handling, and other systems where unwinding is a
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performance bottleneck
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\medskip
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\item \alert{Outlining} was effective for
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compactness\ldots{} Try outlining DWARF bytecode\@?
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\end{itemize}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\begin{frame}
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\vspace{5mm}
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\includegraphics[width=\linewidth]{img/keep_breathing}
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\vspace{-1cm}
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\begin{center}
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\large
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\begin{align*}
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\textbf{Slides: } &\text{\url{https://tobast.fr/m2/slides.pdf}} \\
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\textbf{Report: } &\text{\url{https://tobast.fr/m2/report.pdf}}
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\end{align*}
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\end{center}
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\end{frame}
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\end{document}
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