A6: WIP

6 files changed, 602 insertions, 50 deletions

diff --git a/a6/Makefile b/a6/Makefile
new file mode 100644
index 0000000..61ea860
--- /dev/null
+++ b/a6/Makefile
@@ -0,0 +1,15 @@
+all: noalloc pdf
+
+noalloc: noalloc.cc Makefile
+	g++ -g -O0 -Wall -Werror -Wextra -Wconversion -std=c++20 $< -o $@
+
+pdf: au_BentBisballeNyeng_A6.pdf
+au_BentBisballeNyeng_A6.bbl: au_BentBisballeNyeng_A6.bcf
+	biber $<
+
+au_BentBisballeNyeng_A6.bcf: au_BentBisballeNyeng_A6.tex
+	xelatex -halt-on-error $<
+
+au_BentBisballeNyeng_A6.pdf: au_BentBisballeNyeng_A6.tex au_BentBisballeNyeng_A6.bbl
+	xelatex -halt-on-error $<
+	xelatex -halt-on-error $<
diff --git a/a6/au_BentBisballeNyeng_A6.tex b/a6/au_BentBisballeNyeng_A6.tex
new file mode 100644
index 0000000..ad4cc0d
--- /dev/null
+++ b/a6/au_BentBisballeNyeng_A6.tex
@@ -0,0 +1,154 @@
+\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}
diff --git a/a6/exercise.tex b/a6/exercise.tex
deleted file mode 100644
index 2cdace6..0000000
--- a/a6/exercise.tex
+++ /dev/null
@@ -1,50 +0,0 @@
-\title{Essay: Applying contemporary C++ in applications for embedded
-microcontrollers.}
-\documentclass[11pt]{article}
-\usepackage{graphicx}
-%\usepackage{xcolor}
-\usepackage{fancyhdr}
-\usepackage{listings}
-\usepackage{subfig}
-\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
-
-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.
-
-
-
-\end{document}
diff --git a/a6/fragmentation.svg b/a6/fragmentation.svg
new file mode 100644
index 0000000..4aa629d
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+       style="fill:#0000ff;stroke:#0000ff;stroke-width:0.260804;stroke-dashoffset:4.53543"
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+  </g>
+</svg>
diff --git a/a6/noalloc.cc b/a6/noalloc.cc
new file mode 100644
index 0000000..0da4025
--- /dev/null
+++ b/a6/noalloc.cc
@@ -0,0 +1,68 @@
+#include <iostream>
+#include <functional>
+
+// Universal allocator
+namespace memory
+{
+void* ptr{};
+}
+
+void* operator new([[maybe_unused]]std::size_t n) // throw(std::bad_alloc) - don't throw
+{
+	std::cout << "new\n";
+	// Just return the supplied stack pointer
+	return memory::ptr;
+}
+
+void operator delete(void*) throw()
+{
+	std::cout << "delete\n";
+	// Do nothing. actual memory is allocated on the stack
+}
+
+void operator delete(void*, std::size_t) throw()
+{
+	std::cout << "delete[]\n";
+	// Do nothing. actual memory is allocated on the stack
+}
+
+//void foo() __attribute__((noinline))
+int main()
+{
+	char buf[256];
+	memory::ptr = buf;
+
+	std::cout << " ** strings:\n";
+	{ // strings
+		// now this is ok:
+		std::string str(32, 'a');
+		std::cout << str << '\n';
+
+		std::string str2(24, 'b');
+		std::cout << str << '\n'; // the contents of str has been overwritten
+
+		// this will also allocate, but supply the same buffer - ok
+		str = "hello world hello world hello world";
+
+		// this is also ok, but due to SSO
+		std::string sso{"hello"};
+	}
+
+	std::cout << " ** lambdas:\n";
+	{ // lambdas
+		std::function<int()> f;
+		{
+			char foo[16]{};
+			f = [=]()__attribute__((noinline)) // capture up 16 bytes - ok
+				{
+					int i = 0;
+					for(auto v : foo)
+					{
+						i += v;
+					}
+					return i;
+				}; // capture foo by copy - inlined
+		}
+		[[maybe_unused]]auto x = f();
+	}
+}
diff --git a/a6/references.bib b/a6/references.bib
new file mode 100644
index 0000000..5e0e27a
--- /dev/null
+++ b/a6/references.bib
@@ -0,0 +1,44 @@
+@book{tannenbaum,
+  added-at = {2011-05-03T00:00:00.000+0200},
+  author = {Tanenbaum, Andrew S.},
+  biburl = {https://www.bibsonomy.org/bibtex/2e90b050159c1629022a54a2bf947f318/dblp},
+  interhash = {c97d1fa1663502cf42aaf310cc07056f},
+  intrahash = {e90b050159c1629022a54a2bf947f318},
+  isbn = {978-0-13-852872-0},
+  keywords = {dblp},
+  pages = {I-XVII, 1-587},
+  publisher = {Prentice Hall},
+  timestamp = {2011-05-04T11:32:41.000+0200},
+  title = {Structured computer organization, 3rd Edition.},
+  year = 1990
+}
+
+@article{belson,
+  author = {Belson, Bruce and Xiang, Wei and Holdsworth, Jason and Philippa, Bronson},
+  year = {2020},
+  month = {02},
+  pages = {1-1},
+  title = {C++20 Coroutines on Microcontrollers -What We Learned},
+  volume = {PP},
+  journal = {IEEE Embedded Systems Letters},
+  doi = {10.1109/LES.2020.2973397}
+}
+
+@website{elbeno,
+  author = {Ben Deane},
+	title = {Why is a raven like a writing desk? Another myth, about C++ lambdas},
+  url = {https://www.elbeno.com/blog/?p=1068}
+}
+
+@website{weis,
+  author = {Andreas Weis},
+	title = {Taming dynamic memory - Andreas Weis - Meeting C++ 2018},
+  url = {https://github.com/ComicSansMS/presentations/releases/download/meetingcpp2018/taming_dynamic_memory.pdf},
+%  url = {https://www.youtube.com/watch?v=R3cBbvIFqFk}
+}
+
+@website{craig,
+  author = {Ben Craig},
+	title = {P2268R0 - Freestanding Roadmap},
+	url = {https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2268r0.html}
+}
\ No newline at end of file