The Cortex-M7 processor from ARM introduces DSP instructions to boost digital signal processing performance. These instructions allow common DSP operations like FFTs and filters to be executed more efficiently. The key benefits of the Cortex-M7 DSP instructions are:
- Improved performance for DSP algorithms – DSP instructions execute in a single cycle allowing more operations per second.
- Reduced code size – DSP operations require fewer instructions compared to doing the same function without DSP instructions.
- Power efficiency – By reducing the number of instructions needed, DSP instructions require less power.
DSP Instruction Categories
The Cortex-M7 DSP instructions can be grouped into several categories:
These include single-cycle 16×16 bit multiplications with 32-bit results. This allows faster execution of multiply-accumulate (MAC) operations commonly used in DSP. Some instructions include:
- SMULBB – Signed multiply of two 8-bit values
- SMULBT – Signed multiply of one 8-bit and one 16-bit value
- SMLABB – Signed multiply-accumulate of two 8-bit values
- SMLABT – Signed multiply-accumulate of one 8-bit and one 16-bit value
Saturation arithmetic limits results to a defined range and is useful for avoiding overflow in DSP algorithms. Instructions include:
- SSAT – Signed saturate
- USAT – Unsigned saturate
- QADD – Saturating addition
- QDADD – Saturating double addition
These perform bitwise logical operations with saturation. For example:
- SSAX – Signed saturating add & extract
- USAX – Unsigned saturating add & extract
- USAD8 – Unsigned sum of absolute differences
Packing and Unpacking
Packing condenses data into smaller bit widths. Unpacking does the reverse. This assists with optimized data storage and transfers. Instructions include:
- PKHBT – Pack halfword (16 bits) to byte (8 bits)
- SXTB – Sign extend byte to halfword
- SXTH – Sign extend halfword to word
- UXTB – Zero extend byte to halfword
SIMD (single instruction, multiple data) performs the same operation on multiple values at once. For example:
- SADD8 – Add 8-bit values from two registers
- SADD16 – Add 16-bit values from two registers
- SEL – Select bytes from two registers
DSP Extension Instructions
In addition to the base DSP instructions, the Cortex-M7 includes optional DSP extension instructions for added performance boosts. These include:
- Dot product – Efficient vector dot product calculation
- Multiply with accumulate – Combined multiply and accumulate
- Multiply with subtract – Combined multiply and subtract
- Min/max – Get min or max of two values with a single instruction
- Bitfield – Extract and insert bitfields
- Bit counting – Population count and parity
DSP Instruction Latency and Throughput
Understanding the latency and throughput of instructions is key to maximizing DSP performance. Important notes:
- Most DSP instructions have 1 cycle latency allowing back-to-back operations.
- Pipelining enables 1 instruction per cycle throughput with no stalls.
- The Cortex-M7 has dual-issue capability to execute many instructions simultaneously.
- Certain instructions have multi-cycle latency and affect throughput if used incorrectly.
By scheduling instructions appropriately and maximizing parallel execution, the highest throughput can be achieved.
Coding Efficient DSP Algorithms
Here are some tips for coding algorithms to take advantage of the Cortex-M7 DSP instructions:
- Use single-cycle 16×16 bit multiplies instead of 32-bit for better performance.
- Minimize data movement – process data in-place where possible.
- Maximize dual-issued instructions by interleaving independent instructions.
- Unroll small loops to reduce overhead and maximize parallelism.
- Use SIMD instructions to exploit data level parallelism.
- Consider using DSP extension instructions like dot product.
- Use saturation arithmetic instead of branches to avoid stalls.
DSP Optimization with Intrinsics
Compiler intrinsics provide access to DSP instructions without needing to write assembly code. For example: float32_t sum; float32_t *inp, *coeff; // Use intrinsic for multiply-accumulate sum = __SMLAD(*inp++, *coeff++);
Intrinsics allow the compiler to schedule instructions for optimal performance. Key advantages are:
- Write efficient DSP code in C/C++ instead of assembly
- Compiler handles instruction scheduling and pipelining
- Avoid errors from hand-written assembly code
- Code is portable between different ARM cores
- Allows use of high-level language tools/debugging
ARM provides an extensive set of intrinsics for the Cortex-M7 DSP instructions. Using intrinsics combined with the coding techniques outlined earlier allows DSP algorithms to take full advantage of the processor’s capabilities.
Benchmarking is important to validate the performance gains from using DSP instructions and quantify any improvements. Some tips for effective benchmarking include:
- Use representative DSP functions like FFTs, filtering, matrix math etc.
- Compare with and without DSP-optimized implementations.
- Measure both execution time and number of cycles.
- Compare code size between versions.
- Use optimized compiler settings throughout testing.
- Perform statistics across many iterations for accuracy.
Measuring real-world throughput and efficiency helps choose the best optimizations for a particular application. The Cortex-M7 DSP instructions enable substantial gains but optimal coding is needed to maximize performance.
The Cortex-M7 DSP instructions provide significant benefits for digital signal processing performance compared to conventional microcontroller architectures. By leveraging single-cycle MAC operations, saturation arithmetic, SIMD and specialized DSP extensions, complex algorithms can be made faster and more power efficient. Combined with techniques like loop unrolling, pipelining and multi-issue execution, the DSP instructions enable Cortex-M7 to address demanding DSP applications. Intrinsic functions give access to these instructions in C/C++ without relying on assembly. Thorough benchmarking is key to validate and quantify the performance gains. With its DSP feature set, Cortex-M7 delivers outstanding DSP capabilities not previously possible in microcontroller-class devices.