phd-thesis/manuscrit/20_foundations/10_cpu_arch.tex

\section{A dive into processors' microarchitecture}

A modern computer can roughly be broken down into a number of functional parts:
a processor, a general-purpose computation unit; accelerators, such
as GPUs, computation units specialized on specific tasks; memory, both volatile
but fast (RAM) and persistent but slower (SSD, HDD); hardware specialized for
interfacing, such as networks cards or USB controllers; power supplies,
responsible for providing smoothed, adequate electric power to the previous
components.

This manuscript will largely focus on the processor. While some of the
techniques described here might possibly be used for accelerators, we did not
experiment in this direction, nor are we aware of efforts in this direction.

\subsection{High-level abstraction of processors}

A processor, in its coarsest view, is simply a piece of hardware that can be
fed with a flow of instructions, which will, each after the other, modify the
machine's internal state.

The processor's state, the available instructions themselves and their effect
on the state are defined by an \emph{Instruction Set Architecture}, or ISA\@;
such as x86-64 or A64 (ARM's ISA). More generally, the ISA defines how software
will interact with a given processor, including the registers available to the
programmer, the instructions' semantics ---~broadly speaking, as these are
often informal~---, etc.  These instructions are represented, at a
human-readable level, by \emph{assembly code}, such as \lstxasm{add (\%rax),
\%rbx} in x86-64. Assembly code is then transcribed, or \emph{assembled}, to a
binary representation in order to be fed to the processor ---~for instance,
\lstxasm{0x480318} for the previous instruction. This instruction computes the
sum of the value held at memory address \reg{rax} and of the value \reg{rbx},
but it does not, strictly speaking, \emph{return} or \emph{produce} a result:
instead, its stores the result of the computation in register \reg{rbx},
altering the machine's state.

This state, generally, is composed of a small number of \emph{registers}, small
pieces of memory on which the processor can directly operate ---~to perform
arithmetic operations, index the main memory, etc. It is also composed of the
whole memory hierarchy, including the persistent memory, the main memory
(usually RAM) and the hierarchy of caches between the processor and the main
memory. This state can also be extended to encompass external effects, such as
networks communication, peripherals, etc.

The way an ISA is implemented, in order for the instructions to alter the state
as specified, is called a \emph{microarchitecture}. Many microarchitectures can
implement the same ISA, as it is the case for instance with the x86-64 ISA,
implemented both by Intel and AMD, each with multiple generations, which
translates into multiple microarchitectures. It is thus frequent for ISAs to
have many extensions, which each microarchitecture may or may not implement.

\subsection{Microarchitectures}

\begin{figure}
    \centering
    \includegraphics[width=0.9\textwidth]{cpu_big_picture.svg}
    \caption{Simplified and generalized global representation of a CPU
    microarchitecture}\label{fig:cpu_big_picture}
\end{figure}

While many different ISAs are available and used, and even many more
microarchitectures are industrially implemented and widely distributed, some
generalities still hold for the vast majority of processors found in commercial
or server-grade computers. Such a generic view is obviously an approximation
and will miss many details and specificities; it should, however, be sufficient
for the purposes of this manuscript.

A microarchitecture can be broken down into a few functional blocks, shown in
\autoref{fig:cpu_big_picture}, roughly amounting to a \emph{frontend}, a \emph{backend}, a
\emph{register file}, multiple \emph{data caches} and a \emph{retire buffer}.

\medskip{}

\paragraph{Frontend and backend.} The frontend is responsible for fetching the
flow of instruction bytes to be executed, break it down into operations
executable by the backend and issue them to execution units.  The backend, in
turn, is responsible for the actual computations made by the processor.

As such, the frontend can be seen as a manager for the backend: the latter
actually executes the work, while the former ensures that work is made
available to it, orchestrates its execution and scheduling, and ensures each
``worker'' in the backend is assigned tasks within its skill set.

\paragraph{Register file.} The register file holds the processor's registers,
small amounts of fast memory directly built into the processor's cores, on
which computations are made.

\paragraph{Data caches.} The cache hierarchy (usually L1, L2 and L3) caches
data rows from the main memory, whose access latency would slow computation
down by several orders of magnitude if it was accessed directly. Usually, the
L1 cache resides directly in the computation core, while the L2 and L3 caches
are shared between multiple cores.

\subsubsection{An instruction's walk through the processor}

Several CPU cycles may pass from the moment an instruction is first fetched by
the processor, until the time this instruction is considered completed and
discarded. Let us follow the path of one such instruction through the
processor.

\smallskip{}

The CPU frontend constantly fetches a flow of instruction bytes. This flow must
first be broken down into a sequence of instructions. While on some ISAs, each
instruction is made of a constant amount of bytes ---~\eg{} ARM~--, this is not
always the case: for instance, x84-64 instructions can be as short as one byte,
while the ISA only limits an instruction to 15 bytes. This task is performed
by the \emph{decoder}, which usually outputs a flow of \emph{micro-operations},
or \uops.

Some microarchitectures rely on complex decoding phases, first splitting
instructions into \emph{macro-operations}, to be split again into \uops{}
further down the line. Part of this decoding may also be cached, \eg{} to
optimize loop decoding, where the same sequence of instructions will be decoded
many times.

\smallskip{}

Microarchitectures typically implement more physical registers in their
register file than the ISA exposes to the programmer. The CPU takes advantage
of those additional registers by including a \emph{renamer} in the frontend, to
which the just-decoded operations are fed.
The renamer maps the ISA-defined registers used explicitly in instructions to
concrete registers in the register file. As long as enough concrete registers
are available, this phase eliminates certain categories of data dependencies;
this aspect is explored briefly below, and later in \autoref{chap:staticdeps}.

\smallskip{}

Depending on the microarchitecture, the decoded operations ---~be they macro-
or micro-operations at this stage~--- may undergo several more phases, specific
to each processor.

\smallskip{}

Typically, however, \uops{} will eventually be fed into a \emph{Reorder
Buffer}, or ROB\@. Today, most consumer- or server-grade CPUs are
\emph{out-of-order}, with effects detailed below; the reorder buffer makes this
possible. The \uops{} may wait for a few cycles in this reorder buffer, before
being pulled by the \emph{issuer}.

\smallskip{}

Finally, the \uops{} are \emph{issued} to the backend towards \emph{execution
ports}. Each port usually processes at most one \uop{} per CPU cycle, and acts
as a sort of gateway towards the actual execution units of the processor.

Each execution port may be (and usually is) connected to multiple different
execution units: for instance, Intel Skylake's port 6 is responsible for both
branch \uops{} and integer arithmetics; while ARM's Cortex A72 has a single
port for both memory loads and stores.

\smallskip{}

In most cases, execution units are \emph{fully pipelined}, meaning that while
processing a single \uop{} takes multiple cycles, the unit is able to start
processing a new \uop{} every cycle: multiple \uops{} are then being processed,
at different stages, during each cycle, akin to a factory's assembly line.

\smallskip{}

Finally, when a \uop{} has been entirely processed and exits its processing
unit's pipeline, it is committed to the \emph{retire buffer}, marking the
\uop{} as complete.

\subsubsection{Dependencies handling}

In this flow of \uops{}, some are dependent on the result computed by a
previous \uop{} ---~or, rather more precisely, await the change of state
induced by a previous \uop{}. If, for instance, two successive identical
\uops{} compute $\reg{r10} \gets \reg{r10} + \reg{r11}$, the second instance
must wait for the completion of the first one, as the value of \reg{r10} after
the execution of the latter is not known before its completion.

The \uops{} that depend on a previous \uop{} are not \emph{issued} until the
latter is marked as completed by entering the retire buffer\footnote{Some
    processors, however, introduce ``shortcuts'' when a \uop{} can yield a
    result before its full completion. In such cases, while the \uop{} depended
    on is not yet complete and retired, the dependant \uop{} can still be
issued.}.

Since computation units are pipelined, they reach their best efficiency only
when \uops{} can be fed to them in a constant flow.  Yet, as such, a dependency
may block the computation entirely until its dependent result is computed,
throttling down the CPU's performance.

The \emph{renamer} helps relieving this dependency pressure when the dependency
can be broken by simply renaming one of the registers. We detail this later on
\autoref{chap:staticdeps}, but such dependencies may be \eg{}
\emph{write-after-read}: if $\reg{r11} \gets \reg{r10}$ is followed by
$\reg{r10} \gets \reg{r12}$, then the latter must wait for the former's
completion, as it would else overwrite $\reg{r10}$, which is read by the
former. However, the second instruction may be \emph{renamed} to write to
$\reg{r10}_\text{alt}$ instead ---~also renaming every subsequent read to the same
value~---, thus avoiding the dependency.

\subsubsection{Out-of-order vs. in-order processors}

When computation is stalled by a dependency, it may however be possible to
issue immediately a \uop{} which comes later in the instruction stream, but
depends only on results already available.

For this reason, many processors are now \emph{out-of-order}, while processors
issuing \uops{} strictly in their original order are called \emph{in-order}.
Out-of-order microarchitectures feature a \emph{reorder buffer}, from which
instructions are picked to be issued. The reorder buffer acts as a sliding
window of microarchitecturally-fixed size over \uops{}, from which the oldest
\uop{} whose dependencies are satisfied will be executed. Thus, out-of-order
CPUs are only able to execute operations out of order as long as the
\uop{} to be executed is not too far ahead from the oldest \uop{} awaiting to
be issued ---~specifically, not more than the size of the reorder buffer ahead.

It is also important to note that out-of-order processors are only out-of-order
\emph{from a certain point on}: a substantial part of the processor's frontend
is typically still in-order.

\subsubsection{Hardware counters}

Many processors provide \emph{hardware counters}, to help (low-level)
programmers understand how their code is executed. The counters available
widely depend on each specific processor. The majority of processors, however,
offer counters to determine the number of elapsed cycles between two
instructions, as well as the number of retired instructions. Some processors
further offer counters for the number of cache misses and hits on the various
caches, or even the number of \uops{} executed on a specific port.

While access to these counters is vendor-dependant, abstraction layers are
available: for instance, the Linux kernel abstracts these counters through the
\perf{} interface, while \papi{} further attempts to unify similar counters
from different vendors under a common name.

\subsubsection{SIMD operations}

Processors operate at a given \emph{word size}, fixed by the ISA ---~typically
32 or 64 bits nowadays, even though embedded processors might operate at lower
word sizes.

Some instructions, however, operate on chunks of multiple words at once. These
instructions are called \emph{vector instructions}, or \emph{SIMD} for Single
Instruction, Multiple Data. A SIMD ``add'' instruction may, for instance, add
two chunks of 128 bits, which can for instance be treated each as four integers
of 32 bits bundled together, as illustrated in \autoref{fig:cpu_simd}.

\begin{figure}
    \centering
    \includegraphics[width=0.6\textwidth]{simd.svg}
    \caption{Example of SIMD add instruction on 128b}\label{fig:cpu_simd}
\end{figure}

Such instructions present clear efficiency advantages. If the processor is able
to handle one such instruction every cycle ---~even if it is pipelined for
multiple cycles~---, it multiplies by its number of vector elements the
processor's throughput, making it able to process \eg{} four add operations per
cycle instead of one, as long as the data is arranged in memory in an
appropriate way. Some processors, however, are not able to process the full
vector instruction at once, by lack of backend units ---~it may, for instance,
only process two 32-bits adds at once, making the processor able to execute
only one such instruction per two cycles. Even in this case, there are clear
efficiency benefits: while there is no real gain in the backend, the frontend
has only one instruction to decode, rename, etc., greatly alleviating frontend
pressure. \qtodo{Ref. to RISC-V vector extensions?}