gcc inline asm / constraints / "r" relates to "l" like "g" to ...?

Question

I'm trying to compile this for an ARM Cortex-M0+ (STM32G030) with arm-none-eabi-gcc:

asm("ldrb %0, [%1, %2]\n" : "=l" (v) : "l" (R.a+(o&~31)) , "g" (o&31));

where the variable o might be known already at compile time or later at run time (then i would use the constraint "l", because it has to be a lo register (r0-r7)... i use those 16bit thumb instructions).

Is there a constraint, that forces gcc to use a lo register, if a value is unknown at compile time?

Answer 1

seems like, i can just give multiple constraints and gcc chooses the best one:

asm("ldrb %0, [%1, %2]\n" : "=l" (v) : "l" (R.a+(o&~31)) , "li" (o&31));

yay