phd-defense/slides/20_foundations/main.tex

\section{Foundations}

\begin{frame}{Bird's eye view of a CPU}
    \centering
    \includegraphics[height=0.94\textheight]{cpu_big_picture.svg}
\end{frame}

\begin{frame}{Possible bottlenecks}
    \begin{columns}
        \begin{column}{0.4\textwidth}
            \begin{center}
                \includegraphics[width=\textwidth]{cpu_big_picture_truncate.svg}
            \end{center}
        \end{column}
        \hfill
        \begin{column}{0.58\textwidth}
            \begin{tightitemize}{0pt}
                \begin{itemize}
                    \item \alert{Frontend:} \uops{} issued not fast enough;
                        issuing faster would speed up computation;
                        \bigskip

                    \item \alert{Backend:} saturated execution units; adding
                        more units would speed up computation;
                        \bigskip

                    \item \alert{Dependencies:} computation is stalled waiting
                        for previous results; removing data dependencies would
                        speed up computation.
                \end{itemize}
            \end{tightitemize}
        \end{column}
    \end{columns}
\end{frame}

\begin{frame}{Dependencies and the ROB}
    \begin{itemize}
        \item Dependencies can stall execution
        \item Maybe instructions further down can be executed right now?
        \item ROB: circular buffer of \uops{}
        \item First possible instruction is issued
    \end{itemize}
\end{frame}

\begin{frame}{How do we get insights from this complex system?}
    \textbf{Hardware counters}
    \begin{itemize}
        \item Built-in hardware, counters gathered at runtime
        \item Very accurate
        \item Available data varies from model to model
        \item May not even be available at all
    \end{itemize}

    \textbf{Simulation?}
    \begin{itemize}
        \item A modern CPU is \alert{$\sim$\,100e9 transistors}: very complex
            models!
        \item Very expensive, even for manufacturers for design validation
        \item CPU design is industrial secret $\leadsto$ not available anyway
        \item \ldots{}\ie{} not feasible.
    \end{itemize}
\end{frame}

\begin{frame}{Enter code analyzers}
    \begin{itemize}
        \item Tools that predict performance of a piece of assembly code on a
            given CPU
        \item Features microarchitectural models
        \item Most often static analyzers
        \item May derive further useful metrics, \eg{} bottlenecks, by
            inspecting their model at will
    \end{itemize}
\end{frame}

\begin{frame}{What can be analyzed?}
    Pieces of code referred as \alert{``microkernels''}:

    \begin{itemize}
        \item body of an (assumed) infinite loop;
        \item in steady-state;
        \item straight-line code (branches assumed not taken);
        \item L1-resident (memory model is out of scope).
    \end{itemize}

    Reasonable hypotheses for the category of codes worth optimizing this way!
\end{frame}

\begin{frame}{Existing code analyzers}
    \begin{itemize}
        \item Intel \alert{\iaca{}}: proprietary and only compatible with
            Intel. First ``good'' code analyzer, now deprecated. Was (is?)
            widely used.
        \item \alert{\llvmmca{}}: FOSS, production-grade, many
            microarchitectures. Based on data from the \texttt{llvm} compiler.
        \item \alert{\uica{}} and \alert{\uopsinfo{}}: research, good accuracy.
            Intel.
        \item \alert{\ithemal{}}: machine-learning based. Not so accurate.
        \item \alert{\gus{}}: instrumentation-based code analyzer (not
            static) $\leadsto$ slow. Access to mode information. Made in the
            CORSE team.
    \end{itemize}

    Except Ithemal, \alert{all} are (to some extent) based on manually-made
    models!
\end{frame}