Add spacial bench and FDE size distribution

report/data/fde_line_count.csv

@@ -0,0 +1,51 @@
+lines,proportion
+1,0.23499301373163092
+2,0.017110093953264273
+3,0.07736352686099734
+4,0.056643700313177545
+5,0.03385015658877379
+6,0.03386942905324018
+7,0.04987713803902674
+8,0.0671876656227415
+9,0.008098289568778607
+10,0.033187183811129846
+11,0.04311443025776921
+12,0.07292700554083353
+13,0.005184292941459889
+14,0.0267906528547338
+15,0.017582269332690918
+16,0.11287304264032763
+17,0.01497277764394122
+18,0.0167882437966755
+19,0.00454444712117562
+20,0.013413635268609973
+21,0.00499927728258251
+22,0.001603469043603951
+23,0.007791857383762949
+24,0.01650879306191279
+25,0.002752107925801012
+26,0.00184822934232715
+27,0.001376053962900506
+28,0.0025921464707299446
+29,0.001821247892074199
+30,0.0021218983377499397
+31,0.0018925560105998555
+32,0.003746567092266924
+33,0.0007882437966754999
+34,0.0008653336545410745
+35,0.0006649000240905806
+36,0.00109082148879788
+37,0.0005839556733317273
+38,0.0005068658154661527
+39,0.0006109371235846784
+40,0.0015591423753312456
+41,0.000439412189833775
+42,0.0004914478438930378
+43,0.0003257046494820525
+44,0.0005916646591182848
+45,0.00028330522765598654
+46,0.0004586846543001686
+47,0.00034112262105516744
+48,0.00045290291496025056
+49,0.00024861479161647796
+50,0.00027174174897615035

report/fiche_synthese.tex

@@ -135,10 +135,9 @@ In most cases of everyday's life, a slow stack unwinding is not a problem, or
 even an annoyance. Yet, having a 25 times speed-up on stack unwinding-heavy
 tasks, such as profiling, can be really useful to profile heavy programs,
 particularly if one wants to profile many times in order to analyze the impact
-of multiple changes. It can also be useful for exception-heavy
-programs~\qtodo{cite Stephen's software?}. Thus, it might be interesting to
-implement a more stable version, and try to interface it cleanly with
-mainstream tools, such as \prog{perf}.
+of multiple changes. It can also be useful for exception-heavy programs. Thus,
+it might be interesting to implement a more stable version, and try to
+interface it cleanly with mainstream tools, such as \prog{perf}.
 
 Another question worth exploring might be whether it is possible to shrink even
 more the original DWARF unwinding data, which would be stored in a format not

report/report.tex

@@ -18,6 +18,7 @@ Under supervision of Francesco Zappa-Nardelli\\
 \usepackage{makecell}
 \usepackage{booktabs}
 \usepackage{wrapfig}
+\usepackage{pgfplots}
 %\usepackage[backend=biber,style=alphabetic]{biblatex}
 \usepackage[backend=biber]{biblatex}
 
@@ -194,6 +195,12 @@ 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.
 
+One of the causes that inspired this internship were also Stephen Kell's
+\prog{libcrunch}~\cite{kell2016libcrunch}, which makes a heavy use of stack
+unwinding through \prog{libunwind} and was forced to force \prog{gcc} to use a
+frame pointer (\reg{rbp}) everywhere through \lstbash{-fno-omit-frame-pointer}
+in order to mitigate the slowness.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{DWARF format}
 
@@ -291,7 +298,40 @@ unwinding process.
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{How big are FDEs?}
-\todo{}
+
+\begin{figure}[h]
+    \centering
+    \begin{tikzpicture}
+        \begin{axis}[
+                width=0.9\linewidth, height=4cm,
+                grid=major,
+                grid style={dashed,gray!30},
+                xlabel=FDE row count,
+                ylabel=Proportion,
+                %legend style={at={(0.5,-0.2)},anchor=north},
+                xtick distance=5,
+                ybar, %added here
+            ]
+            \addplot[blue,fill] table[x=lines,y=proportion, col sep=comma]
+            {data/fde_line_count.csv};
+
+        \end{axis}
+    \end{tikzpicture}
+    \caption{FDE line count density}\label{fig:fde_line_density}
+\end{figure}
+
+Since evaluating an \lstc{.eh_frame} FDE entry is, as seen in the previous
+section, roughly linear in time in its rows number, we must wonder what is the
+distribution of FDE rows count. The histogram in
+Figure~\ref{fig:fde_line_density} was generated on a random sample of around
+2000 ELF files present on an ArchLinux system.
+
+Most of the FDEs seem to be quite small, which only reflects that most
+functions found in the wild are relatively small and do not particularly
+allocate many times on the stack. Yet, the median value is at $8$ rows per FDE,
+and the average is at $9.7$, which is already not that fast to unwind. Values
+up to $50$ are not that uncommon, given some commonly used functions have such
+large FDEs, and often end up in the call stack.
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{Unwinding state-of-the-art}
@@ -312,7 +352,7 @@ the relevant FDE from its start, until it finds the row it was seeking.
 \section{DWARF semantics}\label{sec:semantics}
 
 We will now define semantics covering most of the operations used for
-CFI\footnote{To be defined elsewhere in the report} described in the DWARF
+CFI\todo{To be defined elsewhere in the report} described in the DWARF
 standard~\cite{dwarf5std}, with the exception of DWARF expressions. These are
 not exhaustively treated because they are quite rich and would take a lot of
 time and space to formalize, and in the meantime are only seldom used (see the
@@ -375,7 +415,7 @@ operand~--- are irrelevant and will be eluded.
         row. This is \emph{not implemented in this semantics} for simplicity
         and brevity (we would have to introduce CIE (preamble) and FDE (body)
         independently). This is also not much used in actual ELF
-        files\footnote{TODO: refer to stats}.
+        files\todo{refer to stats}.
     \item{} \dwcfa{remember\_state()}~:
         push the state of all the registers of this row on an implicit stack
     \item{} \dwcfa{restore\_state()}~:
@@ -715,7 +755,7 @@ the original program size.
 \todo{more in-depth analysis?}
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsection{Space optimization}
+\subsection{Space optimization}\label{ssec:space_optim}
 
 A lot of small space optimizations, such as filtering out empty FDEs, merging
 together the rows that are equivalent on all the registers kept, etc.\ were
@@ -923,7 +963,46 @@ seconds (using only a single core).
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{Measured compactness}\label{ssec:results_size}
-\todo{}
+
+A first measure of compactness was made in this report for one of the earliest
+working versions in Table~\ref{table:basic_eh_elf_space}.
+
+The same data, generated for the latest version of \ehelfs, can be seen in
+Table~\ref{table:bench_space}.
+
+The effect of the outlining mentioned in Section~\ref{ssec:space_optim} is
+particularly visible in this table: \prog{hackbench} has a significantly bigger
+growth than the other shared objects. This is because \prog{hackbench} has a
+way smaller \lstc{.eh_frame}, thus, the outlined data is reused only a few
+times, compared to \eg{} \prog{libc}, in which the outlined data is reused a
+lot.
+
+\begin{table}[h]
+    \centering
+    \begin{tabular}{r r r r r r}
+        \toprule
+        \thead{Shared object} & \thead{Original \\ program size}
+            & \thead{Original \\ \lstc{.eh\_frame}}
+            & \thead{Generated \\ \ehelf{} \lstc{.text}}
+            & \thead{\% of original \\ program size}
+            & \thead{Growth \\ factor} \\
+        \midrule
+            libc-2.27.so
+                & 1.4 MiB & 130.1 KiB & 313.2 KiB & 21.88 & 2.41 \\
+            libpthread-2.27.so
+                & 58.1 KiB & 11.6 KiB & 25.4 KiB & 43.71 & 2.19 \\
+            ld-2.27.so
+                & 129.6 KiB & 9.6 KiB & 28.6 KiB & 22.09 & 2.97 \\
+            hackbench
+                & 2.9 KiB & 568.0 B & 2.8 KiB & 93.87 & 4.99 \\
+            Total
+                & 1.6 MiB & 151.8 KiB & 370.0 KiB & 22.81 & 2.44 \\
+        \bottomrule
+    \end{tabular}
+
+    \caption{\ehelfs{} space usage}\label{table:bench_space}
+\end{table}
+
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \subsection{Instructions coverage}
@@ -937,6 +1016,7 @@ seconds (using only a single core).
 \printbibliography{}
 
 %% License notice %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\vfill
 \hfill \begin{minipage}{0.7\textwidth}
     \begin{flushright}
         \itshape{} \small{}

shared/report.bib

@@ -76,3 +76,14 @@
   bibsource = {dblp computer science bibliography, https://dblp.org}
 }
 
+@article{kell2016libcrunch,
+  title={Dynamically diagnosing type errors in unsafe code},
+  author={Kell, Stephen},
+  journal={ACM SIGPLAN Notices},
+  volume={51},
+  number={10},
+  pages={800--819},
+  year={2016},
+  publisher={ACM}
+}
+