Garrit Franke
4 years ago
1 changed files with 75 additions and 0 deletions
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title: Booleans are wasted memory |
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date: "2020-11-06" |
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--- |
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A boolean is either `true` or `false`. That translates to 1 or 0. If you think that one bit is enough to store this information, you'd be wrong. |
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In order to keep the binary layout of a program simple and convenient, most languages store information in either 4 bit or 8 bit blocks. If you allocate a `bool` in C, you will get [an integer constant with a value of either 1 or 0](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stdbool.h.html), taking up 4 bits. |
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If you allocate a bool in Rust or (most) other languages that are based on LLVM, [it will take up 1 `i1`, or 8 bit of memory](https://llvm.org/docs/LangRef.html#simple-constants). |
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## There's a better way |
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If you find yourself having to store multiple boolean states somewhere, you might simply declare those booleans and call it a day: |
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```c |
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#include <stdbool.h> |
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#include <stdio.h> |
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int main() |
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{ |
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bool can_read = true; |
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bool can_write = false; |
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bool can_execute = true; |
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if (can_read) |
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printf("read bit set\n"); |
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if (can_write) |
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printf("write bit set\n"); |
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if (can_execute) |
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printf("execute bit set\n"); |
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// Output: |
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// read bit set |
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// write bit set |
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} |
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``` |
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An alternative approach to store boolean values is to share a "chunk" of bits with other values. This is usually done using bitwise operations: |
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```c |
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#include <stdbool.h> |
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#include <stdio.h> |
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// Define permissions |
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#define PERM_NONE 0b000 |
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#define PERM_READ 0b001 |
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#define PERM_WRITE 0b010 |
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#define PERM_EXECUTE 0b100 |
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#define PERM_ALL PERM_READ | PERM_WRITE | PERM_EXECUTE |
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int main() |
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{ |
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// Set permissions |
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int permissions = PERM_READ | PERM_WRITE; |
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if (permissions & PERM_READ) |
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printf("write bit set\n"); |
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if (permissions & PERM_WRITE) |
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printf("read bit set\n"); |
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if (permissions & PERM_EXECUTE) |
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printf("execute bit set\n"); |
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// Output: |
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// read bit set |
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// write bit set |
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} |
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``` |
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This example still wastes 1 bit since we only use 3 out of 4 possible bits of the integer type, but I'm sure you get the point. Whenever you find yourself needing multiple boolean values, think twice if you can use this pattern. |
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Microcontrollers have a very constrainted environment, therefore bitwise operations are essential in those scenarios. 7 wasted bytes are a lot if there are only 4 kb of total memory available. For larger systems we often forget about these constraints, until they add up. |
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## My Plea |
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* Be mindful about the software you create. |
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* Appreciate the resources at your disposal. |
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