Dirty Optimization Secrets (C for Playdate)
Detailed hardware-level optimization guide for the Playdate console covering instruction cache management, custom linker scripts, and running code from tightly-coupled memory (TCM).
read it Essential reading for systems and embedded developers looking for concrete, low-level ARM Cortex-M7 optimization strategies.
summary
Developing high-performance software like emulators for the Playdate requires bypassing standard compiler optimizations to deal with a fast CPU and a slow memory cache. The author shares advanced techniques, including shrinking core loops to fit a 4KB instruction cache, using custom linker maps to control function placement, and manually copying hot functions into ITCM/DTCM memory regions. These methods mitigate the performance lottery caused by cache line misalignments and branch predictor collisions.
key points
- Compile critical inner loops with -Os rather than -O3 to fit within the restrictive 4KB instruction cache on older Playdate models.
- Use custom linker scripts and section attributes to group core functions contiguously and avoid cache line boundary straddling.
- Manually copy performance-critical functions and data structures into tightly-coupled memory (ITCM/DTCM) at runtime for zero-wait execution.
who should read it
Embedded systems engineers, game developers, and low-level programmers working with resource-constrained ARM microcontrollers.
discussion warm
consensus
Commenters share nostalgia for old-school console programming tricks and relate the memory-vs-CPU bottlenecks to vintage hardware constraints.
pushback
A tangent thread criticizes the Playdate's hardware pricing and value proposition compared to modern Chinese devkits or handhelds.
That reminds me of some of the tricks I've used while tinkering with a voxel space rendering tech demo on the PlayStation.
The quirks of that console for this is that the CPU takes a 6 cycle stall on any read from main RAM (it's not even stall-on-use) and doesn't have a data cache. That would make it a very poor fit for this, so I had to improvise a bunch of tricks to reclaim performance:
- Made all of my hot loop fit within one function, so that it fits entirely in the 4 KiB direct-mapped instruction cache (so less than 1024 instructions)
- Wrote a C++ template that can trampoline a …
Compares the Playdate tricks to similar low-level hacks used to squeeze voxel rendering performance out of the original PlayStation.
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