C Optimization tips

ZX80, ZX 81, ZX Spectrum, TS2068 and other clones
Post Reply
User avatar
Well known member
Posts: 193
Joined: Wed Nov 18, 2020 5:08 pm

C Optimization tips

Post by jorgegv »

Hi fellow coders,

I'd like to share with you my current RAGE1 optimization notes and tips. They are the product of the process to make the main RAGE1 code smaller so that a common code base fits in both the 48K and the 128K memory maps for RAGE1. Despite what it may seem, the 128K memory map is much more constrained with regards to the low memory area, and it was the real force driving the optimization effort durig the 0.4.0 development cycle.

You can find the original document here (it will be updatedwith newer tips in the future): OPTIMIZATIONS.md

So, on with the current tips:

Conditional compilation

Place your code between #ifdef's so that it can be conditionally compiled out if it is not needed. See CONDITIONAL-COMPILATION.md for details on how this is done in RAGE1 code.

Use only the memory needed and no more

Optimize memory usage in data structures, specially for the ones that are instantiated mutiple times, e.g. enemy data, tile position information, etc. A couple of bytes saved in a structure that is defined dozens of times quickly add up for substantial memory savings.

Only expand your data structures when/if it is needed.

Example 1

The data structure that holds the tile type for each of the 32x24 character screen positions initially reserved 1 byte per screen position, thus occupying 768 bytes of memory. The check for a tile type was just a matter of calculating (32 * row + col) and indexing on the tile type array to get the tile type for that position

But tiles can be only 3 different types so far, so we didn't need 1 full byte (8 bits) for the tile type. In fact, only 2 bits were needed per position, so we could even pack 4 positions per byte. This would bring down the memory size of the tile type array to 25% of its original size, that is, 192 instead of 768 bytes.

The price to pay for that bit-packing was to have slightly more complex functions for getting and setting the tile type at a given position. But the small slowdown due to these functions was not noticeable during gameplay, so the change was commited

Example 2

There were several struct definitions in which a 16-bit flags field had been included. In some structures it was needed, but in others it was included as a sort of "defensive" programming design ("we will probably need it in the future")

The reality was that those flags fields were heavily underutilized, and: a) when they were really used, only 1 or 2 bits were being really used; b) there were several instances that were not used at all; and c) those structures were instantiated all over the game. The net result was that we had empty or almost-empty 16-bit flags fields all over the place

Big savings were generated by just changing the flags fields to 8-bit, and removing the unneeded ones

Calculate only once, reuse the value

Remember that in 8-bit environments, with low memory and a CPU with a limited instruction set, even the smallest calculation in C quickly generates dozens if not hundreds of assembler instructions.

It is thus very convenient that you refactor calculations as much as possible, by using temporary variables to store calculated variables and using those values where needed in a given function.

Replace indirect pointer accesses with constants if possible

A simple C assignment as the following one:

Code: Select all

sp = all_sprite_graphics[ game_state.hero.num_graphic ];
Generates 20-30 assembly instructions just for getting the value of a pointer in an array. But if the game_state.hero.num_graphic does not change during the game and is known (or can be calculated) at compile time, the assigment can be replaced with code similar to:

Code: Select all

sp = all_sprite_graphics[ HERO_NUM_GRAPHIC ];
In this assignment, all data is known at compile time (all_sprite_graphics is at a concrete address), so it only generates 6-8 assembly instructions to get the value.

As in previous cases, if you have multiple similar assignments scattered in your code, the memory savings quickly add up.

Replace indirect pointer accesses with function calls

A similar case to the previous one: if you have a complex assignment, in which some values are extracted from an array, which are then used to index another array and extract more data, and so on... And if this assignment is done in several places in your code, your complex calculation is a great candidate for refactoring into its own auxiliary function

Remember that for an 8-bit CPU, a simple C indexed array calculation can very easily be "a complex calculation"

Also, keep in mind that on 8-bit CPUs, function calls are not as disruptive to the execution flow as on modern processors. In modern CPUs, we have speculative execution, pipelining, caches, etc. which greatly improve performance if your code runs "straight", but suffer very badly when the execution flow changes unexpectedly (functions calls, jumps, returns).

Old CPUs do not have those bells and whistles, and only suffer a small performance penalty when making a CALL instruction; this penalty can probably be well compensated by the memory savings due to the duplicated code being replaced with a function call.

Replace indirect pointer accesses with precalculated variables

This a corollary of the previous recommendations: calculate things just once and reuse the value. You should only recalculate a value when its inputs change.


The current screen pointer is heavily used in RAGE1 for accessing the screen assets, in a way similar to this:

Code: Select all

num_enemies = banked_assets->all_screens[ game_state.current_screen ].enemies.num_enemies;
The banked_assets->all_screens[ game_state.current_screen ] part of the assignment is constant when you stay in the same screen (which is most of the time), but this kind of assignment was being used all over the place in the main game loop.

We added an additional current_screen_ptr field to the main game_state structure to keep the precalculated value for a pointer to the current screen data. We only recalculated it in the same function where we switched screen (and so we kept the current_screen and current_screen_ptr variables in sync). We then replaced the accesses above with:

Code: Select all

num_enemies = game_state.current_screen_ptr->enemies.num_enemies;
This code, as it has been seen in a previous recommendation, has all symbols known at compile time, and thus generates much shorter code for accessing the value.

Always optimize the common case

As a general rule, always optimize things that are repeated, since they are the ones that will bring you the most benefit for the time you spend optimizing them:
  • If you are optimizing your code size, search for repeated code sequences in your sources, and optimize them with smaller ones
  • If you are optimizing for execution speed, search for the most repeated loops, and optimize the code inside those loops
  • Keep in mind that sometimes optimizing for speed and optimizing for code size will be in conflict and you'll have to reach a compromise betwen both
Post Reply