kaminsknator kaminsknator - 26 days ago 11
C Question

::"r" vs :"=r" assembly clarification

I'm trying to understand the syntax of writing in assembly to first write the code correctly and second efficiently.
in this example it shows the example using

"=r"


asm volatile ("MRS %0, PMUSERENR_EL0\n": "=r"(value));


this reads the value of the register and stores it in the value variable. The other example uses
::"r"


asm volatile ("MSR PMUSERENR_EL0, %0\n":: "r"(value));


This writes the value variable to the PMUSERENR_ELO register. Here is another example of it:How to measure program execution time in ARM Cortex-A8 processor?.

When I attempt to compile a simple test code with the two above commands i get the error:
:9:2: error: output operand constraint lacks '='
If I add the "=" and remove one ":" it will compile but when I test it, it simply says
Illegal instruction


If someone could please explain the difference that would be helpful numerous assembly tutorials show the same format but no explanation. Its on a 64 bit arm platform if that offers any insight. Thanks.

Answer

Found the answer in this book:https://books.google.com/books?id=Bl0_hWV20TAC&pg=PA370&lpg=PA370&dq=two+colon+%22::%22+in+assembly+language&source=bl&ots=Ha8EkNUJee&sig=p6v6iPOCYjBJdmBBVx2ZdxoRT2I&hl=en&sa=X&ved=0ahUKEwj8xcOElYnMAhVQ_WMKHR-JC7IQ6AEIRjAH#v=onepage&q=two%20colon%20%22%3A%3A%22%20in%20assembly%20language&f=false

If no output values are associated with the assembly code, the section must be blank, but two colons must still separate the assembly code from the input operands.

The why is because that's the standard. One colon for outputs and two for inputs.