Difference between revisions of "GCC-SH4 tips"

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Global variables are slow - to retrieve the value, the SH4 typically must execute:
 
Global variables are slow - to retrieve the value, the SH4 typically must execute:
  
<pre>
+
<syntaxhighlight lang="asm">
 
mov.l L2,r1
 
mov.l L2,r1
 
mov.l @r1,r1
 
mov.l @r1,r1
</pre>
+
</syntaxhighlight>
  
 
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'''.<ref>''[https://gcc-renesas.com/manuals/SH-ABI-Specification.html]'', SH ABI(Application Binary Interface) for GCC</ref>
 
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'''.<ref>''[https://gcc-renesas.com/manuals/SH-ABI-Specification.html]'', SH ABI(Application Binary Interface) for GCC</ref>
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=== Integer ===
 
=== Integer ===
 
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)
 
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)
<pre>
+
<syntaxhighlight lang="asm">
 
int result = var1 >> 1; //is the same as var1 / 2 but way faster
 
int result = var1 >> 1; //is the same as var1 / 2 but way faster
 
int result = var1 >> 2; // = var1 / 4
 
int result = var1 >> 2; // = var1 / 4
 
int result = var1 >> 3; // = var1 / 8
 
int result = var1 >> 3; // = var1 / 8
</pre>
+
</syntaxhighlight>
  
 
=== Float ===
 
=== Float ===
 
If you're using float, transforming your division into a multiplication is also faster. Multiplication('''fmul''') only takes ~3 cycles.<ref name="sh4asm"/>
 
If you're using float, transforming your division into a multiplication is also faster. Multiplication('''fmul''') only takes ~3 cycles.<ref name="sh4asm"/>
<pre>
+
<syntaxhighlight lang="asm">
 
float result = var1 * 0.5; // same as / 2 but way faster
 
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 * 0.25; // same as / 4 but way faster
 
float result = var1 * 0.1; // same as / 10 but way faster
 
float result = var1 * 0.1; // same as / 10 but way faster
</pre>
+
</syntaxhighlight>
  
 
==References==
 
==References==
 
<references/>
 
<references/>

Latest revision as of 12:59, 30 May 2020

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)

Dan 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.

Division

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

Integer

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

Float

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

References

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