SimpleScalar is a collection of computer architecture simulators useful for
uniprocessor research and education. I (Todd Austin) created the SimpleScalar
tools during my PhD studies at University of Wisconsin, Madison. Near the end
of my PhD, I released the tools under an open source license with the help of
Doug Burger.

I started developing SimpleScalar after I moved from Programming Language
research to Computer Architecture research in the summer of 1993. My
inspiration for SimpleScalar came in part from working with Jim Larus, who was
a strong proponent of sharing research infrastructure with other researchers.
This may sound unremarkable today, but at that time I received much advice
from others suggesting that I would be giving away the crown jewels if
I decided to open source my research simulator.

The other inspiration for SimpleScalar came from Manoj Franklin and Dionisios
Pnevmatikatos. At the time, they were working on the Multiscalar simulator,
which was a large microarchitecture project at University of Wisconsin,
Madison. The simulator they were using was very powerful because it utilized
execution-driven simulation, which means that the simulator actually executed
the instructions that were being simulated. This, too, may seem quite
unremarkable, but at the time nearly all simulators were trace-driven. They
would open a file of instructions, captured from a hardware execution trace,
and then estimate the performance of the trace on the new microarchitecture.
This approach had limitations that were holding back innovation in the
computer architecture field. For example, there is no misspeculation path in
an instruction trace and typically no data values. I was inspired by the
execution-driven approach used in the Multiscalar simulator, but it was too
closely tied to the Multiscalar architecture, which I wasn't working on. What
I needed was something a little more simple than the Multiscalar simulator,
hence the project named "SimpleScalar".

Guided by my programming experience while working at Xerox, where Butler
Lampson famously said, "Be prepared to throw one away". I ended up throwing
away two SimpleScalar implementations. The first version wasn't very
presentable or flexible, but it got the job done for the early part of my PhD
research. Mid-1994, I decided to reimplement SimpleScalar in its entirety,
adopting a much more modular software architecture, and also adopting the ISA
definition abstractions that are still used today.  (The SimpleScalar
simulators have no idea which instruction set they are using, all those
details are in the machine-specific DEF files.) In the third rebuild, I added
debugging and the EIO tracing capabilities. External I/O (EOI) traces were
a mechanism used to share benchmarks without having to build and package the
benchmarks for sharing. These traces are still used today for testing the
simulator builds.

In 1996 with the help of Doug Burger, an open-source version of SimpleScalar
was released. This was an important event in my career, since I learned much
from supporting the open-source version of SimpleScalar. First, I was
reassured to learn that releasing your research infrastructure is actually
a good thing that renders many personal and professional benefits. As
SimpleScalar became more popular, it became clear to me that it was helping to
steer the course of computer architecture research. I recall one time
attending the MICRO conference and scanning the proceedings to learn that
nearly half of the papers in the conference were using SimpleScalar. In fact,
a very prominent computer architect once approached me and told me that
SimpleScalar had single-handedly set back progress in multiprocessor research
by nearly a decade, simply because it was "too easy to do uniprocessor
microarchitecture research with SimpleScalar". Fortunately, today there is
a rich array of multiprocessor simulators that researchers can choose from.

Second, I learned (and this may just be my own experience) that nearly all of
the feedback I received from SimpleScalar users was negative. In the nearly
thirty years that I have been interacting with SimpleScalar users, nearly
always they are expressing their frustration at using my tools. This motivated
me to improve the design of SimpleScalar, but these efforts never changed the
overwhelmingly negative aspects of users' feedback. I found this aspect of my
experience with SimpleScalar to be the most challenging to deal with. This
experience, in part, taught me to not rely on the approval and accolades of
others, but rather to take satisfaction in teaching well, mentoring well, and
setting and pursuing research goals that have measurable impact on the field.
As of March 2023, over 7000 published research papers have used SimpleScalar
as their evaluation infrastructure!

  Todd Austin
  todd.m.austin@gmail.com
  March 2023