navigator: Change IF statement to SWITCH statement

[muramura]

Solved Problem

Clarify each decision value.

re pr #23065

Solution

Change the IF statement to a SWITCH statement.

Changelog Entry

None

Alternatives

None

Test coverage

None

Context

None

[dayjaby]

With these kinds of changes, it's always nice to show or prove what exactly is getting better (or worse). Like did you consider comparing the resulting firmware sizes?

Checking the assembly results with some similar code (https://gcc.godbolt.org/z/EeW8Tzcb3) against a similar compiler (arm 9.3.0), I'd argue that the if-solution is actually better. It's not doing multiple comparisons in a row, but some highly efficient bit calculations whereas the switch-solution creates a large lookup table.

[muramura]

@dayjaby san.
In both the SWITCH and IF statements, the memory size was the same.
The SWITCH statement uses the same variable for multiple comparison values.
The IF statement allows for different conditions.
Before the change, the same variable was used repeatedly in the conditions.
This is not considered good coding style.

IPA, an organization in Japan, has published a business-oriented C++ embedded coding style guide.

AFTER:
[1294/1296] Linking CXX executable px4_fmu-v6xrt_default.elf
Memory region Used Size Region Size %age Used
flash: 2114556 B 3968 KB 52.04%
sram: 91756 B 1280 KB 7.00%
itcm: 209256 B 256 KB 79.82%
dtcm: 936 B 256 KB 0.36%

BEFORE:
[1294/1296] Linking CXX executable px4_fmu-v6xrt_default.elf
Memory region Used Size Region Size %age Used
flash: 2114556 B 3968 KB 52.04%
sram: 91756 B 1280 KB 7.00%
itcm: 209256 B 256 KB 79.82%
dtcm: 936 B 256 KB 0.36%