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HELIA Kernel SPEED vs SIZE

The HELIA backend has a build-time kernel profile that selects between latency-oriented and footprint-oriented implementations for selected kernels. This is separate from the overall build flavor (debug, release_with_logs, or release) and separate from compiler flags such as -O2 or -Os.

The Two Profiles

Profile Optimizes for Effect
SPEED Minimum inference latency Enables speed paths in HELIA kernels
SIZE Smaller HELIA kernel code paths Trades runtime for footprint where the kernel offers both paths

The default is SPEED, matching the historical Makefile/prebuilt behavior.

Trade-Offs

quadrantChart
    title SPEED vs SIZE
    x-axis "Smaller code" --> "Faster inference"
    y-axis "Lower power" --> "Higher throughput"
    SIZE: [0.25, 0.35]
    SPEED: [0.75, 0.75]
Metric SPEED SIZE
Inference latency ▼ Lower ▲ Higher
Flash usage ▲ Larger ▼ Smaller
RAM usage Kernel-dependent Kernel-dependent
Best for Real-time audio, always-on wake-word Battery-first, Flash-constrained

How to Select

cmake -S . -B build \
  -DHELIA_RT_ENABLE_HELIA=ON \
  -DHELIA_RT_GLOBAL_KERNEL_OPTIMIZE=SPEED

HELIA_RT_GLOBAL_KERNEL_OPTIMIZE accepts SPEED or SIZE.

Advanced source builds can override specific HELIA kernel families:

cmake -S . -B build \
  -DHELIA_RT_ENABLE_HELIA=ON \
  -DHELIA_RT_GLOBAL_KERNEL_OPTIMIZE=SIZE \
  -DHELIA_RT_CONV_OPT=SPEED \
  -DHELIA_RT_FC_OPT=SIZE

# SPEED
make ... OPTIMIZED_KERNEL_DIR=helia GLOBAL_KERNEL_OPTIMIZE=SPEED microlite

# SIZE
make ... OPTIMIZED_KERNEL_DIR=helia GLOBAL_KERNEL_OPTIMIZE=SIZE microlite

Per-family overrides are available through CONV_OPT and FC_OPT.

The HELIA backend is selected by default. Choose the kernel profile in prj.conf:

CONFIG_HELIA_RT=y
CONFIG_HELIA_RT_KERNEL_OPTIMIZE_SPEED=y
# or
CONFIG_HELIA_RT_KERNEL_OPTIMIZE_SIZE=y

Current prebuilt release bundles select archives by CPU, toolchain, and build flavor (debug, release_with_logs, or release). They do not publish a separate SPEED/SIZE kernel-profile axis. Use the source module path when you need to choose the HELIA kernel profile explicitly.

Kernel-Level Knobs

The HELIA backend exposes per-kernel optimization overrides in source builds:

# Override individual kernels (values: SPEED or SIZE)
CONV_OPT=SPEED
FC_OPT=SIZE

This lets you optimize latency-critical operators for SPEED while keeping less critical ones optimized for SIZE.

Next Steps