<p>I've read that switching activity and switching frequency (frequency of flipping bits) are significant contributors to power consumption in the circuitry commonly used in flip-flops and hardware registers.</p>
<p>What generalizable, systematic techniques are there to optimize a fixed-instruction-length instruction set's encoding to minimize power consumption caused by switching activity- particularly in the <a href="https://en.wikipedia.org/wiki/Processor_register" rel="noreferrer">processor registers</a>?</p>
<p>I realize there are lots of different ways to design an ISA (Ex. bits per register), and that not all ISAs are necessarily designed to run the same kinds of programs. That's why I say &quot;<em>generalizable</em>, systematic techniques&quot;.</p>
<p>(A really quick and dirty explanation of what switching / bit flipping has to do with power consumption would also be nice (but optional)- just as a refresher for the reader).</p>

<instruction-set-encoding>
How can I optimize for low processor-register-bit-flip-related power-consumption when designing my ISA's fixed-instruction-length instruction set?
CC BY-SA 4.0
2023-05-20T19:25:04.523Z
related on meta: [Are questions about the design of ISA instruction-set encodings on-topic?](https://languagedesign.meta.stackexchange.com/q/181/251)
starball
CC BY-SA 4.0
2023-06-27T09:34:12.497Z
I wonder if it really worth the effort, when you can get quadratic improvement in power consumption by just reducing clock rate - and some too creative ISA tinkering is likely to affect overall performance worse than reducing clock rate (while, say, increasing IPC and throughput in general, even at a price of more registers flipping in a deeper pipeline).
SK-logic
<p>I think the general approach I outlined in <a href="https://langdev.stackexchange.com/a/914/54">my answer to another question of yours</a> is relevant here as well. In summary, you can build a model of programs, such as a Markov <em>n</em>-gram model, showing which instructions are most likely to follow which other instructions.</p>
<p>From such a model, you can construct an encoding that minimises bit flips in the following way:</p>
<ol>
<li><p>View the model as a directed graph, where the vertices are instructions, and the edges are the transition probabilities between instructions.</p>
</li>
<li><p>Perform a weighted topological sort this graph, using a metric such as the mean incoming edge weight as the vertex weight. A graph constructed from an <em>n</em>-gram model will have <em>n</em> out-edges for every node, but the number of in-edges may vary.</p>
</li>
<li><p>Traverse the sorted instructions and assign them a <a href="https://en.wikipedia.org/wiki/Gray_code" rel="nofollow noreferrer">Gray code</a>. This arranges for the most probable sequences of instructions to flip the fewest bits.</p>
</li>
</ol>
<p>Supposing that each incoming instruction is registered while it’s being dispatched, this minimises the bit flips in that register. If instructions are fetched into an SRAM buffer from main memory, then you can test the SRAM unit for whether it draws more power to read bytes of mostly-clear or mostly-set bits. Then you can reduce power use by assigning the most frequent instructions to the <em>lowest</em>-numbered encodings, so that most program bits are 0, or likewise the <em>highest</em>-numbered for 1.</p>
<p>However, if your concern is minimising power draw due to bit flips, then rather than the instruction encoding, I think it will profit you more to focus your attention on how the ISA uses the memory bus. This is because it consists of many high-capacitance wires, and even with caching and data bus inversion, switching activity may be very high.</p>
<p>For example, consider the representations of values that are likely to be overwritten: if the program initialises a register or a block of memory with all-set bits, then you might suppose that these are sentinel values, which are very likely to be later overwritten with ordinary values, such as small integers. In that case, most of the bits will flip—but all of the expense of actually flipping them could be saved by storing zeroes instead, and recording in the register file or memory access logic that a certain register or a certain range of RAM is stored inverted, or more generally, in a different internal representation than the program sees.</p>

CC BY-SA 4.0
2023-07-11T02:37:58.547Z
2023-07-11T02:37:58.547Z
Jon Purdy
# How can I optimize for low processor-register-bit-flip-related power-consumption when designing my ISA's fixed-instruction-length instruction set?
Posted by: username_0 on 2023-05-20
I've read that switching activity and switching frequency (frequency of flipping bits) are significant contributors to power consumption in the circuitry commonly used in flip-flops and hardware registers.

What generalizable, systematic techniques are there to optimize a fixed-instruction-length instruction set's encoding to minimize power consumption caused by switching activity- particularly in the [processor registers](https://en.wikipedia.org/wiki/Processor_register)?

I realize there are lots of different ways to design an ISA (Ex. bits per register), and that not all ISAs are necessarily designed to run the same kinds of programs. That's why I say "*generalizable*, systematic techniques".

(A really quick and dirty explanation of what switching / bit flipping has to do with power consumption would also be nice (but optional)- just as a refresher for the reader).
**Tags:** instruction-set-encoding
> **Comments:**
> **username_0**: related on meta: [Are questions about the design of ISA instruction-set encodings on-topic?](https://languagedesign.meta.stackexchange.com/q/181/251)  
> **username_1**: I wonder if it really worth the effort, when you can get quadratic improvement in power consumption by just reducing clock rate - and some too creative ISA tinkering is likely to affect overall performance worse than reducing clock rate (while, say, increasing IPC and throughput in general, even at a price of more registers flipping in a deeper pipeline).  
---
## 1 answers
### Answer by username_2 on 2023-07-11. Score: 3
I think the general approach I outlined in [my answer to another question of yours](https://langdev.stackexchange.com/a/914/54) is relevant here as well. In summary, you can build a model of programs, such as a Markov *n*-gram model, showing which instructions are most likely to follow which other instructions.

From such a model, you can construct an encoding that minimises bit flips in the following way:

1. View the model as a directed graph, where the vertices are instructions, and the edges are the transition probabilities between instructions.
2. Perform a weighted topological sort this graph, using a metric such as the mean incoming edge weight as the vertex weight. A graph constructed from an *n*-gram model will have *n* out-edges for every node, but the number of in-edges may vary.
3. Traverse the sorted instructions and assign them a [Gray code](https://en.wikipedia.org/wiki/Gray_code). This arranges for the most probable sequences of instructions to flip the fewest bits.

Supposing that each incoming instruction is registered while it’s being dispatched, this minimises the bit flips in that register. If instructions are fetched into an SRAM buffer from main memory, then you can test the SRAM unit for whether it draws more power to read bytes of mostly-clear or mostly-set bits. Then you can reduce power use by assigning the most frequent instructions to the *lowest*-numbered encodings, so that most program bits are 0, or likewise the *highest*-numbered for 1.

However, if your concern is minimising power draw due to bit flips, then rather than the instruction encoding, I think it will profit you more to focus your attention on how the ISA uses the memory bus. This is because it consists of many high-capacitance wires, and even with caching and data bus inversion, switching activity may be very high.

For example, consider the representations of values that are likely to be overwritten: if the program initialises a register or a block of memory with all-set bits, then you might suppose that these are sentinel values, which are very likely to be later overwritten with ordinary values, such as small integers. In that case, most of the bits will flip—but all of the expense of actually flipping them could be saved by storing zeroes instead, and recording in the register file or memory access logic that a certain register or a certain range of RAM is stored inverted, or more generally, in a different internal representation than the program sees.
---
