From b4e8a41aca5a22fc02da3b1a29ca10e4d472b6c7 Mon Sep 17 00:00:00 2001
From: Bent Bisballe Nyeng <deva@aasimon.org>
Date: Fri, 4 Aug 2023 21:30:35 +0200
Subject: A6: WIP

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+\title{Essay: Applying Contemporary C++ in Enviroments Without
+Free-Store}
+\documentclass[11pt]{article}
+\usepackage{graphicx}
+%\usepackage{xcolor}
+\usepackage{fancyhdr}
+\usepackage{listings}
+\usepackage{subfig}
+\usepackage{biblatex}
+\addbibresource{references.bib}
+\renewcommand{\floatpagefraction}{.8}%
+%\renewcommand{\thesubfigure}{Figure \arabic{subfigure}}
+\captionsetup[subfigure]{labelformat=simple, labelsep=colon}
+\pagestyle{fancy}
+\author{Bent Bisballe Nyeng <deva@aasimon.org> University of Aarhus}
+\begin{document}
+\maketitle
+
+\begin{abstract}
+%\section*{Abstract}
+
+In this essay I want to examine to which extend it is possible to use
+free-store allocating constructs from the standard template library
+(STL) and C++ core-language in enviroments without access to a
+free-store.
+\end{abstract}
+
+\section{Introduction}
+C++ contains a lot of helpful constructs that can be widely used,
+including in environments without a free-store, such
+as \texttt{concepts}, \texttt{module}s, \texttt{template}s in general,
+and functions from \texttt{algorithm} in particular but some parts of
+the language and the STL is off-limits when building applications in
+environments without free-store such as the perhaps obvious, but
+useful \texttt{std::vector} or \texttt{std::string}, but also the less
+obvious co-routines\cite{belson} or storing lambdas
+in \texttt{std::function}s\cite{elbeno}.
+This also inherently means that RAII cannot be used for managing memory
+allocations (such as smart-pointers), but can still be used for
+managing other types of resources, such as locks or hardware
+peripheral access.
+
+\subsection{Dynamic Memory Allocation}
+
+There can be many reasons for not allocating on the free-store, either
+by convention; ``no allocations allowed after the engines has
+started'', or because the hardware or operating system doesn't
+have a virtual memory abstraction, ie. doesn't have a memory
+management unit (MMU)\cite{tannenbaum}, and therefore, over time, is
+at risk at fragmenting the memory available ultimately leading to
+memory depletion\cite{weis}.
+
+In the case of memory fragmentation one might argue that it is not the
+allocation that is the problem but rather the free'ing since this is
+when the fragmentation happens. This problem is shown in
+figure \ref{frag}, which might be possible to circumvent in singular
+concrete cases, but cannot be solved in general without the page
+indirections of the virtual memory\cite{weis}.
+
+\begin{figure}
+\makebox[\textwidth][c]{%
+\includegraphics[scale=0.8]{fragmentation.pdf}}
+\caption{\textit{(a) visualizes the full, free, memory of a system.
+Then, in (b), 4 equal-sized chunks of memory has been allocated
+filling up the whole memory. In (c) chunk 2 and 4 has been free'd and
+finally, in (d), a chunk which can fit in the total amount of
+free memory, is being allocated but fails because of memory
+fragmentation.}}
+\label{frag}
+\end{figure}
+
+This can to some degree be prevented by monotonic allocations which
+might work for  not very practical in real-world
+software and certainly not for the dynamic allocations in the STL or
+the core-language.
+
+In particular, a lambda stored in a \texttt{std::function} might
+allocate memory on the free-store if the lambda exceeds the size of
+the (compiler dependent) small-buffer optimization (SBO) buffer inside
+the \texttt{std::functions}\cite{elbeno}. In much the same way as
+the \texttt{std::string} has its small string optimization (also
+compiler dependent).
+
+\subsection{Free-Standing}
+
+Work is being done to modify the ``free-standing C++'' towards, among
+other things, making it run on systems without free-store by isolating
+the parts of the STL that can be used entirely without allocating
+along with not supporting exceptions and run-time type
+information\cite{craig}.
+
+Working with the resulting small sub-set of the available components,
+however, is not well suited for making contemporary C++ applications.
+The ideal solution would be to find ways to be able to use all (or at
+least most) features, but with a potential known set of restictions or
+limitations.
+
+\section{Method}
+In the following, 3 methods for managing memory allocations will be
+investigated, and their suitability for real-life applications be
+evaluated:
+\begin{itemize}
+\item Using Custom Allocators for the STL components that supports it.
+\item Overloading \texttt{new}/\texttt{delete} to use stack allocated
+      memory instead of the free-store for all allocation.
+\item Support from the compiler to fail compilation if an
+      unintentional \texttt{new} or \texttt{delete} is being called,
+      at least preventing accidental allocations.
+\end{itemize}
+
+Each of the three will be evaluated with large lambda
+captures, \texttt{std::vector} allocation and some simple co-routines.
+The epxeriments are done on a linux PC using the gcc-11.2 compiler.
+
+\section{Experiments}
+
+
+----------------------------------------
+
+
+No access to MMU, implicitly, prohibits calls to delete after
+initialization phase. Otherwise this will lead to memory fragmentation
+which again might lead to free-store depletion and ultimately
+application failure.
+
+Writing a custom allocator is only a solution to a sub-set of the
+allocations in an application, for example if all allocations are
+guaranteed to always be of the same size, in which can no
+fragmentation will occur.
+
+But for most applications (or at least most parts on an application)
+this is not the case, and therefore others means need to be taken into
+use.
+
+The most common way of addressing this, is simply to only use stack
+allocation, or store all objects in as static globals.
+But in certain areas of the C++ language dynamic allocation might
+occur without the developer knowing about it.
+\texttt{std::string}s of sizes that doesn't fit in the SSO buffer is
+one example, but even more devious is the capture clause of a
+lambda, which might allocate extra memory, if more than $N$ members
+are captured, where $N$ is compiler dependent.
+
+No way of telling the compiler that ``no allocations allowed, fail if
+one is made'' exists, but one could wish for such a mechanism in the
+wake of the ``free-standing C++'' subset work.
+One thing is to prohibit use of language constructs that are
+guaranteed to allocate, but quite another is to allow using constructs
+in ways that doesn't make them allocate.
+This, I think, is not part of the ``free-standing C++'' work.
+
+\printbibliography
+
+\end{document}
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