GCC-SH4 tips

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These notes is mostly taken from the good advice of all the Dreamcast scholars (Ian Micheal, Moop, MrNeo, and more). The original documents can be found [here.](https://dreamcast.wiki/wiki/images/f/f6/Gcc_asm_sh4_tips.txt)

Megan Potter's GCC-SH4 tips

Use local variables

Global variables are slow - to retrieve the value, the SH4 typically must execute:

mov.l L2,r1
mov.l @r1,r1

Local variables are faster - it's stack-relative, and function parameters are even faster because the first four integers parameters are passed in R4-R7 and first eight floating-point parameters in FR4-FR11.[1]

Write small functions

We've noticed GCC generates very pessimal code when it starts to spill registers, so try to avoid doing too much in one function.

A function which exceeds more than about a hundred lines should be broken into smaller functions.

Use struct copies (instead of copying individual elements of a struct)

GCC and G++ generate code with weak scheduling when copying a struct by individual elements. GCC and G++ generate code with better instruction scheduling when copying a struct via struct assignment.


Using division on the Dreamcast is very costly. Division(fdiv) takes ~13 cycles.[2]


If you're using integer and want to divide by a power of two, you're better off using bit shifting. (Note: GCC does this conversion automatically, and has done so for at least 10 years now, so you don't need to worry about it)

int result = var1 >> 1; //is the same as var1 / 2 but way faster
int result = var1 >> 2; // = var1 / 4
int result = var1 >> 3; // = var1 / 8


If you're using float, transforming your division into a multiplication is also faster. Multiplication(fmul) only takes ~3 cycles.[2]

float result = var1 * 0.5; // same as / 2 but way faster
float result = var1 * 0.25; // same as / 4 but way faster
float result = var1 * 0.1; // same as / 10 but way faster


  1. [1], SH ABI(Application Binary Interface) for GCC
  2. 2.0 2.1 [2], SH4 Assembly Instructions