\section{Challenges and Solutions}
-This section will describe the challenges faced during each of the tasks and the
-solutions found for both the task itself and the challenges.
+This section will describe the challenges faced during the work I have performed
+and the solutions found for both the task itself and the challenges.
\subsection{What is MontiumC?}
-A critical question popped up during at the beginning of my internship: What is
+A critical question popped up at the beginning of my internship: What is
MontiumC? Previously, there was no real specification of MontiumC. There was
documentation about the functions that could be used, some examples and a lot of
personal knowledge in the heads of the Recore employees, but ultimately MontiumC
-was "whatever the compiler eats".
+was ``whatever the compiler eats''.
To be able to create a proper set of transformations, the constraints on the
input and output of that transformation process should be properly specified.
of) the specification, because they will not work reliably in all cases.
The best way to detect these cases is making the compiler check its input using
-an the specification. This way, any code operating outside of the specification
+the specification. This way, any code operating outside of the specification
can be detected automatically. Writing such checks has not happened yet, mainly
because the impact of the new hardware on MontiumC is not quite clear yet.
Existing transformations, on the other hand, might miss a few corner cases. When
writing the specification, a particular feature might appear supported by the
-compiler. On closer inspection, the transformation passes might miss some corner
+compiler. On closer inspection, the transformation passes might not do
+the right thing when faced with particular corner
cases, which either need to be fixed or put into the specification.
The best way to detect these cases is writing a lot of structured testing code,
features can be described and supported with relative ease, combining features
can easily lead to complex constructs which are hard to transform into supported
Montium IR. This became more of an issue after adding features to the base
-specification of MontiumC, though.
+specification of MontiumC, since the base specification only supports a
+few C features.
\subsubsection{What is wanted?}
-A completely different angle of looking at this, is from the requirements point
+A completely different angle of looking at this is from the requirements point
of view. What do we want MontiumC to support? This angle is even harder than the
previous one, since there are a lot of levels of requirements. Ideally, MontiumC
would not exist and our compiler would support the C language fully. However,
most efficient code. In the Montium case, a lot of things simply cannot be
mapped on the hardware at all.
-Considering that our ideal is not reachable (by far), every feature in our
-wanted MontiumC should be evaluated thoroughly for feasibility, both in hardware
+Considering that our ideal is not reachable (Though the new hardware might take
+us a lot closer), every feature
+considered for MontiumC was evaluated thoroughly for feasibility, both in hardware
and in the compiler. In practice, this meant that new language features would be
informally expressed and discussed, and only added to the specification after
being succesfully implemented. This is conforming to the incremental development
of MontiumC that was envisioned at the outset of its development.
+To get a feeling for what MontiumC looks like, consider the fragment in
+figure \ref{ExampleLow}. This is a piece of code that reads values from
+one memory, multiplies them by two, and writes them back to another
+memory. As you can see, this is an awful lot of code. This has two main
+reasons. First, looping and memory indexing is very explicit and takes a
+lot of instructions. Second, the code is effectively software pipelined
+to make it run more efficiently.
+
+In figure \ref{ExampleHigh} the same code is displayed, but this time
+using higer level C features (for loops, array indexing). This is the
+level of code we are trying to achieve, but we're not there yet. It
+should be noted that this is still not "normal" C, since we use the
+"imul" function instead of the normal * operator. However, since the
+Montium defines a lot of different operations, most of which have a
+number of options (saturation, truncation, post shifting, etc.) these
+cannot all be mapped onto normal C operators. By using a specific
+function call for each, we can still distinguish between all the
+different operations and add extra arguments where needed.
+
+\subsubsection{What do we have now?}
+The result of this work is a usable, but conservative, specification. It
+defines the subset of features that should certainly be supported. In practice,
+some other features will also work, but not reliably. Therefore, these are left
+out of the specification.
+
+It is not unlikely that the specification is still incorrect in a few places (or
+rather, that the code does not implement the specification properly). Since
+so far there has been not any automated checking of programs against the
+specification, these errors have not been uncovered. Once the new hardware is
+more clearly defined and the MontiumC specification is updated for it, this
+checking should be added so the specification and compiler can be better
+matched.
+
+\begin{figure}
+ \caption{Low level MontiumC example}
+\label{ExampleLow}
+\begin{verbatim}
+mem input;
+mem output;
+word factor;
+
+void run(void) {
+ factor = from_int(2);
+ input = alloc_mem(P0M0);
+ output = alloc_mem(P0M1);
+ set_base(input, 0);
+ set_offset(input, 0);
+ set_base(output, -1);
+ set_offset(output, -1);
+
+ next_cycle();
+ word in = read_mem(input);
+ word out = p0o0(imul(ra1(in), rc1(factor)))
+ add_offset(input, 1);
+ add_offset(output, 1);
+ init_loop(LC1, 8);
+ do {
+ write_mem(output, out);
+ in = read_mem(input);
+ out = p0m0(imul(ra1(in), rc1(factor)))
+ add_offset(input, 1);
+ add_offset(output, 1);
+ } while(loop_next(LC1));
+
+ write_mem(output, out);
+\end{verbatim}
+\end{figure}
+
+\begin{figure}
+ \caption{High level MontiumC example}
+ \label{ExampleHigh}
+\begin{verbatim}
+P0M0 int input[10];
+P0M1 int output[10];
+
+void run(void) {
+ for (int i=0; i<10; ++i)
+ output[i] = mul(input[i], 2);
+}
+\end{verbatim}
+\end{figure}
+
\subsection{Familiarizing with LLVM}
Since the frontend heavily relies on the LLVM project for doing it's work, one
-of the first challenges was to get myself familiar with LLVM. There are two main
+of the first challenges was to get myself familiar with LLVM. There were two main
aspects to this: Getting to know my way around the LLVM codebase and getting to
know the LLVM community.
-Since LLVM has a pretty large amount of documentation, I spent most of my first
+LLVM has a pretty large amount of documentation, I spent most of my first
weeks with reading tutorials and documents. Since there was already a (very
preliminary) version of the clang-based frontend, I also had some code to play
with.
which I can't effectively learn without actively working with it, etc.) I
started out with adapting the loop unrolling pass in LLVM to be better suited to
the Montium architecture. Eventually, this code didn't turn out to be
-immediately useful (it's still not included currently), but it proved very
-insightful as to how the LLVM framework is built and what its possibilities are.
+immediately useful because deciding when to unroll a loop and when not to turned
+out rather hard (it's still not included currently). Working with this pass did
+prove very insightful, however, as to how the LLVM framework is built and what its
+possibilities are.
Additionally, during my working with the code in this internship I also produced
a number of patches for LLVM, containing bugfixes, some cleanup and
A major challenge during my internship was to find the balance between doing
work specifically for Recore, and doing work that is useful for LLVM in general.
-Any tasks that are required by the Montium frontend and the LLVM changes they
-directly need are obvious. However, usually when making changes to the main LLVM
-code, just changing enough for Recore is not engough for LLVM. Since the LLVM
+Any changes that are directly required by the Montium frontend and the LLVM changes they
+need are obvious. However, usually when making changes to the main LLVM
+tree, just changing enough for Recore is not engough for LLVM. Since the LLVM
code must work on any program, not just MontiumC programs, extra changes are
-required (see also parapgrah \ref{StayingGeneric}. This is also an issue of
+required (see also parapgrah \ref{StayingGeneric}). This is also an issue of
building up credit within the LLVM community: The more you contribute to LLVM,
the more influence you have when things need changing.
decent balance, though it might have been tipped slighly in favour of the LLVM
project.
-\subsection{Coding}
-This section says something about the challenges encountered while actually
-writing code, if I can think of enough things to say here. So far, haven't
-thought of anything particularly interesting yet.
-
\subsection{Working together}
Since the compiler plays a fairly central role in the development process at
Recore, I have been cooperating with a number of different people, in different
On the other end, I have been in contact with the developer of the backend very
intensively, since most changes made to either component needed changes in the
other one as well. Compiler changes also require hardware support, so working
-with the hardware developers was not uncommon either. In practice, most of this
-communication went through the backend developer, except for the design
-discussion concerning the new Montium hardware design (also see section
-\ref{Pipelining} below).
+with the hardware developers was not uncommon either. In practice, most
+communication with the hardware developers went through the backend
+developer, except for the design discussion concerning the new Montium
+hardware design (also see section \ref{Pipelining} below).
In addition, discussions regarding design issues at various levels often happen
-out in the open, which easily invites people with an opinion about something to
-chime in, without having to gather people around for every discussion that
-happens. This allows for very quick feedback on ideas from people in all areas
+out in the open, which invites people with an opinion about something to
+chime in, without the overhead of planning a seperate meeting for each
+topic. This allows for very quick feedback on ideas from people in all areas
in an efficient way.
\subsection{Staying generic}
\label{StayingGeneric}
The toughest challenge by far was to fit the features and changes required by
Recore into code that is maintained by the LLVM project. The main problem here
-is clash of goals: The LLVM project aims for code that performs well for their
+is a clash of goals: The LLVM project aims for code that performs well for their
supported architectures, while Recore aims for code that performs well (or more
-often, can compile MontiumC at all) for the Montium architecture.
+often, can compile at all) for the Montium architecture.
In general, these problems mainly occur because MontiumC and in particular
MontiumIR poses a number of constraints that are not present in other
In some cases, these extra constraints can be formulated in a generic way, such
that the LLVM code can handle architectures with or without the constraint.
+Examples include providing Montium specific parameters through LLVM's
+``\verb TargetData '' class and modifications to the loop unroller to support
+custom policies.
+
In a lot of cases, however, the constraint is so Montium specific that changing
-LLVM to support it is not feasible. In a few cases, this meant that the backend
-was changed to support more features of the LLVM IR. In other cases, a
-Recore-specific transformations was added to solve these problems. In a few more
-cases, the problems are still unresolved, effectively resulting in additional
-constraints on the MontiumC language.
+LLVM to support it is not feasible.
+
+In a few of these cases, this meant that the backend was changed to support more
+features of the LLVM IR. An example of this is finding datapath constants (which
+are the result of a function in MontiumC, so hard to track by LLVM).
+
+In other cases, Recore-specific transformations were added to solve these
+problems. Examples of these are a transformation that removes all global
+variables and passes them as arguments and return values instead, a
+transformation that forcibly inlines all functions marked as ``\verb inline ''
+and a transformation that limits variable lifetimes by introducing extra phi
+nodes.
+
+In a few more cases, the problems are still unresolved, effectively resulting in
+additional constraints on the MontiumC language. Examples of these are
+preventing instructions from being moved out of if/else blocks (which is
+perfectly fine from an LLVM IR standpoint, but does not take into account the
+extra meaning that an if statement has in MontiumIR) and removal of unused bits
+from a constant (which could introduce more different constants than the Montium
+has registers for them).
+
\subsection{New hardware design}
\label{Pipelining}
\subsubsection{Tradeoffs}
In general, the most important tradeoff seems to be between flexibility and
everything else (code size, performance, complexity, hardware area). This
-flexibility is expressed in which instructions are possible, which connections
-are present, how big register files are, etc.
+flexibility is defined in terms of possible instructions, present
+connections, register files sizes, etc.
An important reason to be flexible is for programmability. If the hardware is
regular, making a compiler that produces optimal code gets a lot easier.
area and complexity by making the hardware less flexible. Exactly for this
reason, it is important to develop hardware and supporting software in parallel,
instead of using the hardware first, software later approach used with the
-initial Montium (TODO: Is this true?). This allows for a much better balanced
-and usable design, without any unused extras.
+initial Montium. This allows for a much better balanced and usable design,
+without any unused extras.
\subsubsection{Inner loop pipelining}
When trying to improve runtime performance, the main focus is on
optimizing loops, and inner loops (loops that contain no other loops) in
-particular. Since the inner loop is executed the most often, it is the most
-efficient to optimize the inner loop. Also, the inner loop is also the piece of
-code that can most optimally use the parellel processing power of the Montium,
-because it can be software pipelined.
+particular. Since inner loops are executed the most often (compared to
+other loops and code), it is the most
+efficient to optimize inner loops. Also, inner loops can most optimally
+use the parellel processing power of the Montium, because they can be
+software pipelined.
Software pipelining means that the compiler will emit code that performs
operations that belong in different iterations of the original loop during the
projects / matthijs / projects / internship.git / blobdiff