Proof-read chapter 5 (staticdeps)
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@ -20,7 +20,7 @@ down into four categories:
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For instance, in the kernel presented in the introduction of this chapter, the
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first instruction (\lstxasm{add \%rax, \%rbx}) reads its first operand, the
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register \reg{rax}, and both reads and write its second operand \reg{rbx}. The
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register \reg{rax}, and both reads and writes its second operand \reg{rbx}. The
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second \lstxasm{add} has the same behaviour. Thus, as \reg{rbx} is written at
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line 1, and read at line 2, there is a read-after-write dependency between the
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two.
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@ -51,7 +51,7 @@ however, other channels.
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As we saw in the introduction to this chapter, as well as in the previous
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chapter, dependencies can also be \emph{memory-carried}, in more or less
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straightforward ways, such as in the examples from
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\autoref{lst:mem_carried_exn}, where the last line always depend on the first.
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\autoref{lst:mem_carried_exn}, where the last line always depends on the first.
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\begin{lstfloat}[h!]
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\begin{minipage}[t]{0.32\linewidth}
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@ -67,8 +67,8 @@ add -8(%rbx), %rcx\end{lstlisting}
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\end{minipage}\hfill
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\begin{minipage}[t]{0.32\linewidth}
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\begin{lstlisting}[language={[x86masm]Assembler}]
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lea 16(%rbx), %r10
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add %rax, (%rbx)
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lea 16(%rbx), %r10
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add -16(%r10), %rcx\end{lstlisting}
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\end{minipage}\hfill
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\caption{Examples of memory-carried dependencies.}\label{lst:mem_carried_exn}
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@ -94,27 +94,36 @@ with a large emphasis on memory-carried dependencies.
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\paragraph{Presence of loops.} The previous examples were all pieces of
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\emph{straight-line code} in which a dependency arose. However, many
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dependencies are actually \emph{loop-carried}, such as those in
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\autoref{lst:loop_carried_exn}.
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\autoref{lst:loop_carried_exn}. In \autoref{lst:loop_carried_exn:sumA}, line 2
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depends on the previous iteration's line 2 as \reg{r10} is read, then written
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back. In \autoref{lst:loop_carried_exn:quasiFibo}, line 3 depends on line 2 of
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the same iteration; but line 2 alsp depends on line 3 two iterations ago by
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reading \lstxasm{-16(\%rbx, \%r10)}.
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\begin{lstfloat}
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\begin{minipage}[t]{0.48\linewidth}
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\begin{lstlisting}[language={[x86masm]Assembler}]
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# Compute sum(A), %rax points to A
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\centering
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\begin{sublstfloat}[b]{0.48\linewidth}
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\begin{lstlisting}[language={[x86masm]Assembler}]
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loop:
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add (%rax), %r10
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add $8, %rax
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jmp loop
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\end{lstlisting}
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\end{minipage}\hfill
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\begin{minipage}[t]{0.48\linewidth}
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\begin{lstlisting}[language={[x86masm]Assembler}]
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# Compute B[i] = A[i] + B[i-2]
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\caption{Compute the sum of array \lstxasm{A}'s terms in \reg{r10}. \reg{rax} points to
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\lstxasm{A}.}\label{lst:loop_carried_exn:sumA}
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\end{sublstfloat}
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\hfill
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\begin{sublstfloat}[b]{0.48\linewidth}
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\begin{lstlisting}[language={[x86masm]Assembler}]
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loop:
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mov -16(%rbx, %r10), (%rbx, %r10)
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add (%rax, %r10), (%rbx, %r10)
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add $8, %r10
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jmp loop
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\end{lstlisting}
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\end{minipage}\hfill
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\caption{Examples of loop-carried dependencies.}\label{lst:loop_carried_exn}
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\caption{Compute \lstxasm{B[i] = A[i] + B[i-2]}. \reg{rax} points to
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\lstxasm{A}, \reg{rbx} points to \lstxasm{B}.}\label{lst:loop_carried_exn:quasiFibo}
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\end{sublstfloat}
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\caption{Examples of loop-carried dependencies.}\label{lst:loop_carried_exn}
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\end{lstfloat}
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@ -5,18 +5,20 @@ the actual data dependencies occurring throughout an execution. While such
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analyzers are often too slow to use in practice, they can be used as a baseline
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to evaluate static alternatives.
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Due to its complexity, \gus{} is, however, ill-suited for the implementation of
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a simple experiment if one is not already familiar with its codebase. For this
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reason, we implement \depsim{}, a dynamic analysis tool based on top of
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As it is a complex tool performing a wide range of analyses, \gus{} is,
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however, unnecessarily complex to simply serve as a baseline. For the same
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reason, it is also impractically slower than a simple dynamic analysis. For
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this reason, we implement \depsim{}, a dynamic analysis tool based on top of
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\valgrind{}, whose goal is to report dependencies encountered at runtime.
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\subsection{Valgrind}
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Most low-level developers and computer scientists know
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\valgrind{}~\cite{tool:valgrind} as a memory
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analysis tool, reporting bad memory accesses, memory leaks and the like.
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However, this is only the \texttt{memcheck} tool built into \valgrind{}, while the
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whole program is actually a binary instrumentation framework.
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\valgrind{}~\cite{tool:valgrind} as a memory analysis tool, reporting bad
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memory accesses, memory leaks and the like. However, this is only a small part
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of \valgrind{} ---~the \texttt{memcheck} tool. The whole program is actually a
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binary instrumentation framework, upon which the famous \texttt{memcheck} is
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built.
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\valgrind{} supports a wide variety of platforms, including x86-64 and
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ARM. However, at the time of the writing, it supports AVX2, but does not yet
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@ -41,7 +43,7 @@ for some assembly code, independently of the Valgrind framework.
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\subsection{Depsim}\label{ssec:depsim}
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The tool we write to extract runtime-gathered dependencies, \depsim{}, is
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The tool we wrote to extract runtime-gathered dependencies, \depsim{}, is
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able to extract dependencies through both registers, memory and temporary
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variables ---~in its intermediate representation, Valgrind keeps some values
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assigned to temporary variables in static single-assignment (SSA) form.
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@ -92,5 +94,5 @@ avoid excessive instrumentation slowdown.
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We further annotate every write to the shadow memory with the timestamp at
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which it occurred. Whenever a dependency should be added, we first check that
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the dependency has not expired ---~that is, that it is not older than a given
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threshold. This threshold is tunable for each run --~and may be set to infinity
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threshold. This threshold is tunable for each run ---~and may be set to infinity
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to keep every dependency.
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@ -1,28 +1,24 @@
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\section{Static dependencies detection}\label{ssec:staticdeps_detection}
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Depending on the type of dependencies considered, it is more or less difficult
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to statically detect them.
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Depending on their type, some dependencies are significantly harder to
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statically detect than others.
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\paragraph{Register-carried dependencies in straight-line code.} This case is
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the easiest to statically detect, and is most often supported by code analyzers
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---~for instance, \llvmmca{} supports it. The same strategy that was used to
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dynamically find dependencies in \autoref{ssec:depsim} can still be used: a
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shadow register file simply keeps track of which instruction last wrote each
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register.
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\textbf{Register-carried dependencies}, when in straight-line code, can be
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detected by keeping track of which instruction last wrote each register in a
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\emph{shadow register file}. This is most often supported by code analyzers
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---~for instance, \llvmmca{} and \uica{} support it.
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\paragraph{Register-carried, loop-carried dependencies.} Loop-carried
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dependencies can, to some extent, be detected the same way. As the basic block
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is always assumed to be the body of an infinite loop, a straight-line analysis
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can be performed on a duplicated kernel. This strategy is \eg{} adopted by
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\osaca{}~\cite{osaca2} (§II.D).
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Loop-carried dependencies can, to some extent, be detected the same way. As the
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basic block is always assumed to be the body of an infinite loop, a
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straight-line analysis can be performed on a duplicated kernel. This strategy
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is \eg{} adopted by \osaca{}~\cite{osaca2} (§II.D). When dealing only with
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register accesses, this strategy is always sufficient: as each iteration always
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executes the same basic block, it is not possible for an instruction to depend
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on another instruction two iterations earlier or more.
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When dealing only with register accesses, this
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strategy is always sufficient: as each iteration always executes the same basic
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block, it is not possible for an instruction to depend on another instruction
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two iterations earlier or more.
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\smallskip
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\paragraph{Memory-carried dependencies in straight-line code.} Memory
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dependencies, however, are significantly harder to tackle. While basic
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\textbf{Memory-carried dependencies}, however, are significantly harder to tackle. While basic
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heuristics can handle some simple cases, in the general case two main
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difficulties arise:
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\begin{enumerate}[(i)]
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Tracking memory-carried dependencies is, to the best of our knowledge, not done
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in code analyzers, as our results in \autoref{chap:CesASMe} suggests.
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\paragraph{Loop-carried, memory-carried dependencies.} While the strategy
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previously used for register-carried dependencies is sufficient to detect
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loop-carried dependencies from one occurrence to the next one, it is not
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sufficient at all times when the dependencies tracked are memory-carried. For
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instance, in the second example from \autoref{lst:loop_carried_exn}, an
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instruction depends on another two iterations ago.
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\smallskip{}
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While the strategy previously used for register-carried dependencies is
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sufficient to detect loop-carried dependencies from one occurrence to the next
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one, it is not sufficient at all times when the dependencies tracked are
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memory-carried. For instance, in the second example from
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\autoref{lst:loop_carried_exn}, an instruction depends on another two
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iterations ago.
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Dependencies can reach arbitrarily old iterations of a loop: in this example,
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\lstxasm{-8192(\%rbx, \%r10)} may be used to reach 1\,024 iterations back.
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@ -55,7 +53,7 @@ necessarily \emph{relevant} from a performance analysis point of view. Indeed,
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if an instruction $i_2$ depends on a result previously produced by an
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instruction $i_1$, this dependency is only relevant if it is possible that
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$i_1$ is not yet completed when $i_2$ is considered for issuing ---~else, the
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result is already produced, and $i_2$ needs not wait to execute.
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result is already produced, and $i_2$ needs never wait to execute.
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The reorder buffer (ROB) of a CPU can be modelled as a sliding window of fixed
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size over \uops{}. In particular, if a \uop{} $\mu_1$ is not yet retired, the
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@ -7,11 +7,11 @@ performance}\label{ssec:staticdeps:rob_proof}
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for the subtrace $\left(I_r\right)_{p < r < p'}$.
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\end{definition}
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\begin{theorem}[Long distance dependencies]\label{thm.longdist}
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There exists $R \in \nat$, only dependent of microarchitectural parameters,
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such that the presence or absence of a dependency between two instructions
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that are separated by at least $R-1$ other \uops{} has no impact on the
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performance of this kernel.
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\begin{theorem}[Long distance dependencies]\label{thm.longdist} Given a kernel
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$\kerK$, there exists $R \in \nat$, only dependent of microarchitectural
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parameters, such that the presence or absence of a dependency between two
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instructions that are separated by at least $R-1$ other \uops{} has no
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impact on the performance of this kernel.
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More formally, let $\kerK$ be a kernel of $n$ instructions. Let $(I_p)_{p
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\in \nat}$ be the trace of $\kerK$'s instructions executed in a loop. For
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@ -29,7 +29,10 @@ the reorder buffer.
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It contains only decoded \uops{}.
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Let us denote $i_d$ the \uop{} at position $d$ in the reorder buffer.
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Assume $i_d$ just got decoded.
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We have that for every $q$ and $q'$ in $[0,R)$:
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\nopagebreak{}As the buffer is a circular FIFO, we have that for every $q$
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and $q'$ in
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$[0,R)$:
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\[
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(q-d-1) \% R<(q'-d-1) \% R\
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\iff \ i_q \text{ was decoded before } i_{q'}
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@ -45,7 +48,8 @@ buffer}:
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\begin{postulate}[Full reorder buffer]
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Let us denote by $i_d$ the \uop{} that just got decoded.
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The reorder buffer is said to be full if for $q=(d+1) \% R$, \uop{} $i_q$ is not retired yet.
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The reorder buffer is said to be \emph{full} if for $q=(d+1) \% R$, \uop{}
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$i_q$ is not retired yet.
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If the reorder buffer is full, then instruction decoding is stalled.
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\end{postulate}
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@ -58,10 +62,10 @@ To prove the theorem above, we need to state that any two in-flight \uops{} are
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For any instruction $I_p$, we denote as $Q_p$ the range of indices such that
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$(i_q)_{q\in Q_p}$ are the \uops{} obtained from the decoding of $I_p$.
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Note that in practice, we may not have $\bigcup{}_p Q_p = [0, n)$, as \eg{}
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branch mispredictions may introduce unwanted \uops{} in the pipeline. However,
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as the worst case for the lemma below occurs when no such ``spurious'' \uops{}
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are present, we may safely ignore such occurrences.
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Note that in practice, it is possible that we do not have $\bigcup{}_p Q_p =
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[0, n)$, as \eg{} branch mispredictions may introduce unwanted \uops{} in the
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pipeline. However, as the worst case for the lemma below occurs when no such
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``spurious'' \uops{} are present, we may safely ignore such occurrences.
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\begin{lemma}[Distance of in-flight \uops{}]
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For any pair of instructions $(I_p,I_{p'})$, and two corresponding \uops{},
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@ -1,8 +1,9 @@
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\section{Staticdeps}
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The static analyzer we present, \staticdeps{}, only aims to tackle the
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difficulty~\ref{memcarried_difficulty_arith} mentioned above: tracking
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dependencies across arbitrarily complex pointer arithmetic.
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difficulty~(\ref{memcarried_difficulty_arith}) mentioned in
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\autoref{ssec:staticdeps_detection}: tracking dependencies across arbitrarily
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complex pointer arithmetic.
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To do so, \staticdeps{} works at the basic-block level, unrolled enough times
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to fill the reorder buffer as detailed above; this way, arbitrarily
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\caption{The \staticdeps{} algorithm}\label{alg:staticdeps}
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\end{algorithm}
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Instead, we use an heuristic based on random values. We consider the set $\calR
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Instead, we use a heuristic based on random values. We consider the set $\calR
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= \left\{0, 1, \ldots, 2^{64}-1\right\}$ of values representable by a 64-bits
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unsigned integer; we extend this set to $\bar\calR = \calR \cup \{\bot\}$,
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where $\bot$ denotes an invalid value. We then proceed as previously for
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\smallskip{}
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\Function{read\_memory}{address}
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\State{} \textbf{Assert} address \neq{} ⊥
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\State{} \textbf{Assert} address \neq{} $\bot$
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\If{address \not\in{} shadow\_memory}
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\State{} shadow\_memory[address] $\gets$ \Call{fresh}{}
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\EndIf{}
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\EndFunction{}
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\Function{read\_register}{register}
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\Comment{likewise, without dependency tracking}
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\State{} \ldots \Comment{Likewise, without dependency tracking}
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\EndFunction{}
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\smallskip{}
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\EndIf{}
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\State{} \Return{} \Call{read\_memory}{addr}
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\ElsIf{expr == IntegerArithmeticOp(operator, op1, …, opN)}
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\If{\Call{expr\_value}{op\_i} == ⊥ for any i}
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\State{} \Return{} ⊥
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\If{\Call{expr\_value}{op\_i} == $\bot$ for any i}
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\State{} \Return{} $\bot$
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\EndIf{}
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\State\Return{} semantics(operator)(\Comment{provided by Valgrind's Vex}
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\State\Return{} semantics(operator)(\Comment{Provided by Valgrind's Vex}
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\State{} \quad \Call{expr\_value}{op1}, …, \Call{expr\_value}{opN})
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\Else{}
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\Return{} ⊥
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\Return{} $\bot$
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\EndIf{}
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\EndFunction{}
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\State{} shadow\_register[reg] \gets{} \Call{expr\_value}{rhs}
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\ElsIf{lhs == Memory(addr\_expr)}
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\State{} addr \gets{} \Call{expr\_value}{addr\_expr}
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\State{} last\_wrote\_at[addr] \gets{}(cur\_iter, cur\_instruction)
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\State{} shadow\_memory[addr] <- \Call{expr\_value}{rhs}
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\State{} last\_wrote\_at[addr] \gets{} (cur\_iter, cur\_instruction)
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\State{} shadow\_memory[addr] \gets{} \Call{expr\_value}{rhs}
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\ElsIf{\ldots}
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\Comment{Etc.}
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\EndIf{}
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@ -220,8 +220,8 @@ analyzers.
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\subsection{Enriching \uica{}'s model}
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To estimate the real gain in performance debugging scenarios, however, we
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integrate \staticdeps{} into \uica{}.
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To estimate the real gain in performance debugging scenarios, we integrate
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\staticdeps{} into \uica{}.
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There is, however, a discrepancy between the two tools: while \staticdeps{}
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works at the assembly instruction level, \uica{} works at the \uop{} level. In
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@ -233,11 +233,11 @@ for the \staticdeps{} analysis.
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We bridge this gap in a conservative way: whenever two instructions $i_1, i_2$
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are found to be dependant, we add a dependency between each couple $\mu_1 \in
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i_1, \mu_2 \in i_2$. This approximation is thus pessimistic, and should predict
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execution times biased towards a slower computation kernel. A finer model, or a
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finer (conservative) filtering of which \uops{} must be considered dependent
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---~\eg{} a memory dependency can only come from a memory-related \uop{}~---
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may enhance the accuracy of our integration.
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i_1, \mu_2 \in i_2$. This approximation is thus largely pessimistic, and should
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predict execution times biased towards a slower computation kernel. A finer
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model, or a finer (conservative) filtering of which \uops{} must be considered
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dependent ---~\eg{} a memory dependency can only come from a memory-related
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\uop{}~--- may enhance the accuracy of our integration.
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\begin{table}
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\centering
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@ -17,8 +17,10 @@ context} pointed out in the previous chapter.
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\medskip{}
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Our evaluation of \staticdeps{} against a dynamic analysis baseline,
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\depsim{}, shows that it only finds about 60\,\% of the existing dependencies.
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We however enrich \uica{} with \staticdeps{}, and find that it performs on the
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\depsim{}, shows that it finds between 95\,\% and 98\,\% of the existing
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dependencies, depending on the metric used, giving us good confidence in the
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reliability of \staticdeps{}.
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We further enrich \uica{} with \staticdeps{}, and find that it performs on the
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full \cesasme{}'s dataset as well as \uica{} alone on the pruned dataset of
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\cesasme{}, removing memory-carried bottlenecks. From this, we conclude that
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\staticdeps{} is very successful at finding the data dependencies through
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@ -44,15 +44,15 @@ create a full analyzer implementing this idea, such as what we did with \palmed{
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for backend models, or such as \uica{}'s implementation, focusing on frontend
|
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analysis.
|
||||
|
||||
In hindsight, we advocate for the emergence of such a modular code analyzer.
|
||||
It would maybe not be as convenient or well-integrated as ``production-ready''
|
||||
code analyzers, such as \llvmmca{} ---~which is packaged for Debian. It could,
|
||||
however, greatly simplify the academic process of trying a new idea on any of
|
||||
the three main models, by decorrelating them. It would also ease the
|
||||
comparative evaluation of those ideas, while eliminating many of the discrepancies
|
||||
between experimental setups that make an actual comparison difficult ---~the
|
||||
reason that prompted us to make \cesasme{} in \autoref{chap:CesASMe}. Indeed,
|
||||
with such a modular tool, it would be easy to run the same experiment, in the
|
||||
same conditions, while only changing \eg{} the frontend model but keeping a
|
||||
well-tried backend model.
|
||||
In hindsight, we advocate for the emergence of such a modular code analyzer. It
|
||||
would maybe not be as convenient or well-integrated as ``production-ready''
|
||||
code analyzers, such as \llvmmca{} ---~which is officially packaged for Debian.
|
||||
It could, however, greatly simplify the academic process of trying a new idea
|
||||
on any of the three main models, by decorrelating them. It would also ease the
|
||||
comparative evaluation of those ideas, while eliminating many of the
|
||||
discrepancies between experimental setups that make an actual comparison
|
||||
difficult ---~the reason that prompted us to make \cesasme{} in
|
||||
\autoref{chap:CesASMe}. Indeed, with such a modular tool, it would be easy to
|
||||
run the same experiment, in the same conditions, while only changing \eg{} the
|
||||
frontend model but keeping a well-tried backend model.
|
||||
|
||||
|
|
|
@ -88,6 +88,7 @@
|
|||
\newfloat{lstfloat}{htbp}{lop}
|
||||
\floatname{lstfloat}{Listing}
|
||||
\def\lstfloatautorefname{Listing}
|
||||
\DeclareCaptionSubType{lstfloat}
|
||||
|
||||
\newfloat{algorithm}{htbp}{lop}
|
||||
\floatname{algorithm}{Algorithm}
|
||||
|
|
Loading…
Reference in a new issue