From a84ed32608424bfc9d941caf49c45b9f643120b4 Mon Sep 17 00:00:00 2001
From: Yann Herklotz <git@yannherklotz.com>
Date: Fri, 10 Sep 2021 18:28:46 +0100
Subject: Small fixes

---
 evaluation.tex | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

(limited to 'evaluation.tex')

diff --git a/evaluation.tex b/evaluation.tex
index 2a98aa7..a80ee79 100644
--- a/evaluation.tex
+++ b/evaluation.tex
@@ -169,7 +169,7 @@ Looking at a few benchmarks in particular in Fig.~\ref{fig:polybench-nodiv} for
 \subsection{RQ2: How Area-Efficient is \vericert{}-Generated Hardware?}
 
 The bottom graphs in both Fig.~\ref{fig:polybench-div} and Fig.~\ref{fig:polybench-nodiv} compare the resource utilisation of the \polybench{} programs generated by \vericert{} and \legup{} at various optimisation levels.
-By looking at the median, when division/modulo operations are enabled, we see that \vericert{} produces hardware that is about the same size as optimised \legup{}, whereas the unoptimised versions of \legup{} actually produce slightly smaller hardware.  This is because optimisations can often increase the size of the hardware to make it faster.  Especially in Fig.~\ref{fig:polybench-div}, there are a few benchmarks where the size of the \legup{} design is much smaller than that produced by \vericert{}.  This can mostly be explained because of resource sharing in LegUp.  Division/modulo operations need large circuits, and it is therefore usual to only have one circuit per design.  As \vericert{} uses the na\"ive implementation of division/modulo, there will be multiple circuits present in the design, which blows up the size.  Looking at Fig.~\ref{fig:polybench-nodiv}, one can see that without division, the size of \vericert{} designs are almost always around the same size as \legup{} designs, never being more than 2$\times$ larger, and sometimes even being smaller.  The similarity in area also shows that RAM is correctly being inferred by the synthesis tool, and is therefore not implemented as registers.
+By looking at the median, when division/modulo operations are enabled, we see that \vericert{} produces hardware that is about the same size as optimised \legup{}, whereas the unoptimised versions of \legup{} actually produce slightly smaller hardware.  This is because optimisations can often increase the size of the hardware to make it faster.  Especially in Fig.~\ref{fig:polybench-div}, there are a few benchmarks where the size of the \legup{} design is much smaller than that produced by \vericert{}.  This can mostly be explained because of resource sharing in LegUp.  Division/modulo operations need large circuits, and it is therefore usual to only have one circuit per design.  As \vericert{} uses the na\"ive implementation of division/modulo, there will be multiple circuits present in the design, which blows up the size.  Looking at Fig.~\ref{fig:polybench-nodiv}, one can see that without division, the size of \vericert{} designs are almost always around the same size as \legup{} designs, never being more than 2$\times$ larger, and sometimes even being smaller.  The similarity in area also shows that RAM is correctly being inferred by the synthesis tool, and is therefore not implemented using registers.
 
 \subsection{RQ3: How Quickly does \vericert{} Translate the C into Verilog?}
 
@@ -205,7 +205,7 @@ By looking at the median, when division/modulo operations are enabled, we see th
 {\it ``Beware of bugs in the above code; I have only proved it correct, not tried it.''} \par\hfill -- D. E. Knuth (1977)
 \end{quotation}
 
-To gain further confidence that the Verilog designs generated by \vericert{} are actually correct, and that the correctness theorem is indeed effective, we fuzzed \vericert{} using Csmith~\cite{yang11_findin_under_bugs_c_compil}. \citeauthor{yang11_findin_under_bugs_c_compil} previously used Csmith in an extensive fuzzing campaign on CompCert and found a handful of bugs in the unverified parts of that compiler, so it is natural to explore whether it can find bugs in \vericert{} too. \citet{herklotz21_empir_study_reliab_high_level_synth_tools} have recently used Csmith to fuzz other HLS tools including \legup{}, so we configured Csmith in a similar way. In addition to the features turned off by \citeauthor{du21_fuzzin_high_level_synth_tools}, we turned off the generation of global variables and non-32-bit operations. The generated designs were tested by simulating them and comparing the output value to the results of compiling the test-cases with GCC 10.3.0.
+To gain further confidence that the Verilog designs generated by \vericert{} are actually correct, and that the correctness theorem is indeed effective, we fuzzed \vericert{} using Csmith~\cite{yang11_findin_under_bugs_c_compil}. \citeauthor{yang11_findin_under_bugs_c_compil} previously used Csmith in an extensive fuzzing campaign on CompCert and found a handful of bugs in the unverified parts of that compiler, so it is natural to explore whether it can find bugs in \vericert{} too. \citet{herklotz21_empir_study_reliab_high_level_synth_tools} have recently used Csmith to fuzz other HLS tools including \legup{}, so we configured Csmith in a similar way. In addition to the features turned off by \citeauthor{herklotz21_empir_study_reliab_high_level_synth_tools}, we turned off the generation of global variables and non-32-bit operations. The generated designs were tested by simulating them and comparing the output value to the results of compiling the test-cases with GCC 10.3.0.
 
 The results of the fuzzing run are shown in Fig.~\ref{tab:fuzzing}.  Out of 155267 test-cases generated by Csmith, 26\% of them passed, meaning they compiled without error and resulted in the same final value as GCC. Most of the test-cases, 73.97\%, failed at compile time.  The most common reasons for this were unsigned comparisons between integers (\vericert{} requires them to be signed), and the presence of 8-bit operations (which \vericert{} does not support, and which we could not turn off due to a limitation in Csmith). % The latter programs are present because of a slight limitation with how Csmith can be configured, as it does not allow 8-bit operations to be turned off completely, and will get stuck when it generates the C code.
 Because the test-cases generated by Csmith could not be tailored exactly to the C fragment that \vericert{} supports, such a high compile-time failure rate is not unexpected. Finally, and most interestingly, there were a total of 39 run-time failures, which the correctness theorem should be proving impossible.  However, all 39 of these failures are due to a bug in the pretty-printing of the final Verilog code, where a logical negation (\texttt{!}) was accidentally used instead of a bitwise negation (\verb|~|).  Once this bug was fixed, all test-cases passed.
-- 
cgit 

