What is SIMD?
SIMD has a long and storied history in DSP. It stands for
single instruction, multiple data
and started to be used in DSPs in the 1980s to speed up processor hungry operations like
convolution, the FFT, and FIR filtering. The basic idea is that a single specialized register
can perform an arithmetic operation on multiple data in a single cycle. The width of these
special SIMD registers is hardware and instruction-set dependent. Common SIMD register widths
are 2-way, 4-way, and 8-way. (Ie, 2, 4, or 8 floats could be processed in a single cycle.)
In essence, SIMD creates another “tier” of parallelism in software design. While multithreading can be used for parallelism at the task level, SIMD allows for parallelism at the instruction level. If each parallel “task” is also configured to use SIMD instructions, you’ll really be playing with power!
Like writing code with parallel tasks, writing parallel-instruction friendly code requires special design considerations to be effective. Luckily, some of the significant design changes used to make SIMD-friendly code also make your code more cache friendly.
How do I use it?
Lazy Approach: Auto-Vectorization
One can take advantage of SIMD in many different ways. Sometimes you even get it for free when your compiler performs auto-vectorization. So if you write code that happens to be “embarrassingly parallel” at the instruction level, your compiler may actually generate SIMD instructions for you. This is a really nice concept when you aren’t actually relying on SIMD. Small, seemingly inconsequential code changes can cause the compiler to bail on trying to generate SIMD and there’s really no way to tell unless you inspect the assembly output each time you compile after a small change. And at that point, you might as well be more intentional about writing your algo with SIMD.
Usual Best Approach: Wrapper Libraries and Code Generation
Another approach to using SIMD is using SIMD wrapper libraries. This is great for more consistent, broad strokes, intentional SIMD code. Wrapper libraries can range from math libraries that just use SIMD under-the-hood to libraries that swap out your primitive data-types (ie,
float) with SIMD data types (
vfloat4) that you can use to write a
“scalar-looking” algorithm that will consistently compile to SIMD instructions.
This is great for broad-strokes speedup in an application. Some efficiency may be lost
by the wrapper library doing expensive SIMD loads/stores behind your back more often than
it needs to or dealing with any unaligned data you throw at it, but overall this is a
great practical approach to SIMD.
Worth also mentioning are SIMD-code-generation tools such as
ispc. These tools are pretty
neat – you write C-like code and the tools output SIMD instructions from them. Could be
useful, though I don’t know much about them.
Most Flexible Approach: Intrinsics (or Inline Assembly)
Here is the one I plan to discuss. SIMD boils down to platform specific code. At the end of the day, the most predictable and powerful way to write SIMD code is using those platform specific instructions or compiler intrinsics (thin wrappers over inline assembly). And since inline assembly is becoming deprecated in some compilers (cough MSVC), intrinsics seem to be the way to go.
With that said, intrinsics are imperfect in several ways:
- They obfuscate the heck out of your code (as you will see below)
- They make your code platform specific, so you’ll likely need to reach for
#ifdefs or template code
- They defeat many optimizations the compiler otherwise might do
And with regards to that last point… ALWAYS profile your code when writing SIMD intrinsics. Since you are opting out of compiler optimizations, your SIMD code can actually be slower than scalar code that “does the same thing” if you don’t make the right design choices. (But if you do make the correct decisions, it can be much faster.) Always be on the lookout for both performance and output regressions.
In short, intrinsics are the tool you should reach for when you may otherwise be considering
inline assembly. In audio, this is usually the innermost loop of an expensive DSP algorithm in
a code-base where SIMD wrappers were used everywhere else.
If you decide to use SIMD intrinsics, where do you start?
Raw intrinsics at the application level make sense when you have something like this:
Convolution was one of the original motivations for creating SIMD hardware and it remains a nice, clean example. Keep in mind that the actual code that you see (a nested loop with arithmetic) is not the only part of the equation that makes this a good candidate for SIMD. A key factor is the usual high-throughput requirement of real-world convolution algorithms.
When you start off with scalar code like this, it helps to take an iterative approach in the conversion to SIMD intrinsics.
First Iteration: A “Naive SIMD” Approach
Now before you start hacking the intrinsics into the algorithm, make sure to create a backup of your code. Making a branch in your VCS is a good idea, and you also may find it convenient to
#ifdef your old code above the new
code to reference and do quick checks for output regressions.
Once you have that, it’s time to just rework your inner loop algorithm to naively use instrinsic-instruction versions of your arithmetic operators. At this point the performance is not as important as coaxing out any caveats you may have not noticed at first glance that give you compiler errors or cause you to to reorganize buffers, etc.
In this case, the inner loop is made to look as much like the original arithmetic as possible. An unaligned load of the input is done on each inner loop, which is a big inefficiency, but don’t worry about that yet. Just try and do something that looks as similar to your original algo as possible and test for regressions!
This code also starts to do some of the re-jiggering of the buffers necessary when
converting to SIMD. The filter kernel is reversed and stored in SIMD registers before
the actual convolution takes place.
Second Iteration: Time To Obfuscate
Once you have a basic idea of the implications or restructuring your algorithm to use SIMD intrinsics, it is time to make some more invasive changes. Here, we are going to preprocess both the input signal and the filter kernel to load them into SIMD registers before the actual computation.
This will involve more complicated indexing in the inner loop to access the data in groups of four rather than one.
The point of this example is not to show an efficient convolution implementation. In fact, this implementation really sucks – it is way less readable and only about twice as fast on my machine as the naive implementation.
My point is describing the refactoring process: make your scalar transition to SIMD in several iterations rather than a single go. This particular code could use several more. :)
Other Design Considerations
Operate on aligned data
Mentioned above, loading aligned data into SIMD registers is very important. SSE intrinsics have unaligned versions of loads and stores, but they usually defeat the purpose of using intrinsics due to the glacially slow access.
And if you accidentally try to load unaligned data with the aligned intrinsics (usually the defaults), you are looking at undefined behavior, and usually a segfault.
One technique to remember with SIMD intrinsics is to do as much computation and SIMD sugar between loads and stores into SIMD registers. It can be difficult and involve reworking the logic of your algorithm signficantly, but try to call the
_mm_load family of intrinsics as little as possible.
Restructure “Array of Structures” to “Structure of Arrays”
This is one concept that will feel the most unnatural in a C-like language. We are trained in OOP that objects should represent a single-entity with various components that make it up. To work with many such objects, we store the objects in an array and process the objects one-by-one by indexing into that array. Here is a simple example of a complex number struct and a contrived operation where the real component is multiplied with the imaginary component:
Array of Structures
To make this contrived operation more SIMD friendly, we want to convert it to Structure of Arrays form. This means that we create a struct that contains all of the data we want to operate on in parallel and we index into each array in that struct to get an individual entity’s component value.
Structure of Arrays
This not only makes our code work with the grain of the SIMD registers, but also increases the cache coherency. The larger topic this relates to is data oriented design, a concept that emphasizes laying data out with knowledge of how it is actually processed.
This article showed some practical considerations when writing C++ code with SIMD intrinsics. They can be a powerful tool, but should be used only in small doses. Prefer SIMD wrapper libraries for the majority of a codebase that requires optimization and only fall down to instrinsics for small segments of performance-critical code that you have verified aren’t being vectorized perfectly by your wrapper library or compiler.