Function calls

When calling a function or sub-routine, we need a way to get back to the caller when finished. Adding an L to the B or BR instructions turns them into a branch with link. This means that a return address is written into LR (X30) as part of the branch.

Note: The names LR and X30 are interchangeable. An assembler, such as GNU GAS or armclang, will accept both.

There is a specialist function return instruction, RET. This performs an indirect branch to the address in the link register. Together, this means that we get:

 

Note: The figure shows the function foo() written in GAS syntax assembler. The keyword .global exports the symbol and .type indicates that the exported symbol is a function.

Why do we need a special function return instruction? Functionally, BR LR would do the same job as RET. Using RET tells the processor that this is a function return. Most modern processors, and all Cortex-A processors, support branch prediction. Knowing
that this is a function return allows processors to more accurately predict the branch.

Branch predictors guess the direction the program flow will take across branches. The guess is used to decide what to load into a pipeline with instructions waiting to be processed. If the branch predictor guesses correctly, the pipeline has the correct instructions and the processor does not have to wait for instructions to be loaded from memory.

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