4.9 Factors that influence function inlining

There are a number of factors that influence the linker inlines functions.

The following factors influence the way functions are inlined:

  • The linker handles only the simplest cases and does not inline any instructions that read or write to the PC because this depends on the location of the function.

  • If your image contains both ARM and Thumb code, functions that are called from the opposite state must be built for interworking. The linker can inline functions containing up to two 16-bit Thumb instructions. However, an ARM calling function can only inline functions containing either a single 16-bit encoded Thumb instruction or a 32-bit encoded Thumb instruction.

  • The action that the linker takes depends on the size of the function being called. The following table shows the state of both the calling function and the function being called:

    Table 4-1 Inlining small functions

    Calling function state Called function state Called function size
    ARM ARM 4 to 8 bytes
    ARM Thumb 2 to 6 bytes
    Thumb Thumb 2 to 6 bytes

    The linker can inline in different states if there is an equivalent instruction available. For example, if a Thumb instruction is adds r0, r0 then the linker can inline the equivalent ARM instruction. It is not possible to inline from ARM to Thumb because there is less chance of Thumb equivalent to an ARM instruction.

  • For a function to be inlined, the last instruction of the function must be either:

    MOV pc, lr

    or

    BX lr

    A function that consists only of a return sequence can be inlined as a NOP.

  • A conditional ARM instruction can only be inlined if either:

    • The condition on the BL matches the condition on the instruction being inlined. For example, BLEQ can only inline an instruction with a matching condition like ADDEQ.

    • The BL instruction or the instruction to be inlined is unconditional. An unconditional ARM BL can inline any conditional or unconditional instruction that satisfies all the other criteria. An instruction that cannot be conditionally executed cannot be inlined if the BL instruction is conditional.

  • A BL that is the last instruction of a Thumb If-Then (IT) block cannot inline a 16-bit encoded Thumb instruction or a 32-bit MRS, MSR, or CPS instruction. This is because the IT block changes the behavior of the instructions within its scope so inlining the instruction changes the behavior of the program.

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