How to evaluate digitalWrite at compile time?

[danieleff]

If the parameters are constant, I want digitalWrite(pin, val) to evaluate it at compile time. Down to one instruction.
Currently pseudocode (the exact structure of pin_type is irrelevant):

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//variant.cpp
const pin_type variant_pins[] = {...}
//gpio.h
extern const pin_type variant_pins[];
inline void digitalWrite(pin, value) {...variant_pins[pin]...}
//x.ino
...gpio.h included...
digitalWrite(PA0, LOW);
//other.cpp
...gpio.h included...
digitalWrite(PA1, LOW);

Theoretically variant_pins[pin] could be evaluated at compile time (well, link time, but at compile time it could be a placeholder that the linker fills in), but it is not.

I could put the whole list into the header (as static local), which works, but then it is duplicated in every compilation unit when used.

I checked https://stackoverflow.com/q/5010216/834966 , https://stackoverflow.com/q/11403800/834966 , -fmerge-all-constants , -flto and god knows what already.
I always get either no compile time evaluation, or duplicated array.
Anybody knows some solution to this?

[ag123]

i'm not too sure if c++ templates could help or perhaps some kind of pointer style codes is needed ?
i'm not too sure how to use those

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template
function(T&a) {
 static_cast(a);
 }

stuff but i do see them in various c++ codes

[stevestrong]

I would define separate inline functions digitalSet(PIN) and digitalClear(PIN) something like this:

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inline void digitalSet(p) { PIN_MAP[p].dev->regs->BSRR = PIN_MAP[p].pin; }

[danieleff]

Well it did need some trickery, making pin names constexpr int-convertable class instead of enum when compiling for c++, but now this

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void loop() {
    digitalWrite(LED_BUILTIN, HIGH);
    digitalWrite(LED_BUILTIN, LOW);
    digitalWrite(LED_BUILTIN, HIGH);
    digitalWrite(LED_BUILTIN, LOW);
    digitalWrite(LED_BUILTIN, HIGH);
    digitalWrite(LED_BUILTIN, LOW);
}

compiles to

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08002188 :
 8002188:	2202      	movs	r2, #2
 800218a:	4b04      	ldr	r3, [pc, #16]	; (800219c )
 800218c:	611a      	str	r2, [r3, #16]
 800218e:	615a      	str	r2, [r3, #20]
 8002190:	611a      	str	r2, [r3, #16]
 8002192:	615a      	str	r2, [r3, #20]
 8002194:	611a      	str	r2, [r3, #16]
 8002196:	615a      	str	r2, [r3, #20]
 8002198:	4770      	bx	lr
 800219a:	bf00      	nop
 800219c:	40010c00 	.word	0x40010c00

which is good enough

[lacklustrlabs]

Care to show the code?
The closest I've come to that result is to define a set of large constexpr functions (autogenerated) that a template class can operate on.
https://gitlab.com/LacklustrLabs/lacklu ... abs_gpio.h

The results are kind of clumsy, what I could use is a constexpr conversion of PIN_MAP in the core.

[victor_pv]

It's likely used in the STM32Generic core that he maintains, the link should be here in the forum, in the cores section, first post from Roger.

[victor_pv]

It's likely used in the STM32Generic core that he maintains, the link should be here in the forum, in the cores section, first post from Roger.

Out of curisity I looked it up, is in this file, although there is another related, variant.h in each variant folder:
https://github.com/danieleff/STM32GENER ... pin_list.h

[dannyf]

If the parameters are constant, I want digitalWrite(pin, val) to evaluate it at compile time. Down to one instruction.

for what end?

presumably for efficiency. but that's not high on arduino's priority list: this thing is geared to people who want to write generic code with minimum efforts.

if you care about efficiency, you probably should have picked on some other tools.

trying to do what you are trying to do is like driving a pig through a square hole.

[ahull]

...
trying to do what you are trying to do is like driving a pig through a square hole.

That can work... if you choose the correct pig.