Choosing a SIMD Width
Match width to your hardware lanes.
Choosing a SIMD Width is a free Mojo Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Mojo Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Width Decides Throughput
The SIMD width is how many lanes you process per instruction. Picking it well is key to real performance.
Hardware Has Fixed Lanes
Your CPU has vector registers of a fixed size. A good SIMD width matches the lanes that hardware can run at once.
Too Narrow Wastes Power
A width of 1 ignores the CPU's vector units entirely. You leave most of the chip's throughput sitting idle.
var slow = SIMD[DType.float32, 1](2.0)Too Wide Spills Over
If a width is larger than the hardware supports, Mojo splits it into smaller chunks. That still works but adds overhead.
Let Mojo Tell You
Mojo can report the ideal width for a type on this machine through simdwidthof, so you do not have to guess.
from sys import simdwidthof
alias w = simdwidthof[DType.float32]()Use It as a Parameter
Because simdwidthof is known at compile time, you can feed it straight into a SIMD type as its width.
alias Vec = SIMD[DType.float32, simdwidthof[DType.float32]()]Width Depends on DType
Smaller elements fit more lanes in the same register. So float32 usually gets a bigger native width than float64.
Portable by Design
Using simdwidthof means your code adapts to each machine automatically. The same source stays fast and portable.
Multiples of Width Are Easy
Data that comes in clean multiples of your width vectorizes neatly, with no leftover lanes to handle as a remainder.
Mind the Leftovers
When a count is not a multiple of the width, a few tail elements remain. You finish those with a small scalar step.
Start with the Native Width
A safe default is simdwidthof for your DType. Measure later, but the native width is almost always a strong start.
Quick Check
You want a SIMD width that fits this CPU's float32 lanes without hardcoding a guess. What helps?
Recap
Match the SIMD width to the hardware: simdwidthof gives the native lane count per DType, and a scalar tail cleans up leftover elements. 🎯
Frequently asked questions
Is the “Choosing a SIMD Width” lesson free?
Yes — the full text of “Choosing a SIMD Width” is free to read here on the web, and the Mojo Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Mojo Academy course, upgrade to CoddyKit PRO.
What will I learn in “Choosing a SIMD Width”?
Match width to your hardware lanes. You practise Mojo Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Mojo Academy?
No prior experience is required. Mojo Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Choosing a SIMD Width” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Mojo Academy lesson?
Yes. Every Mojo Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Meet the SIMD Type
- Element-Wise SIMD Math
- Choosing a SIMD Width
- Vectorizing a Loop