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Using the non-leaf function technique to implement leaf functions

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• Comment on the "saving return address on the stack" approach
  • Fact:

    The technique that we used to implement non-leaf methods can also be used to implement leaf methods (Because saving the return address on the stack will preserve the return address)

  • Example:

    public static void main(String[] args) { A( ); } public static void A( ) { ... B( ); // Calls B( ) ... } public static void B( ) { ... // Does not call any method/function ... }

    Although B( ) is a leaf method, we can use the non-leaf method implementation:

    main: mov r0, #4 mov r1, #4 bl A mov r2, #4 mov r3, #4 Stop: A: push {lr} // Save return address mov r0, #44 mov r1, #44 bl B // Overwrites lr, it's OK, we saved it !!! mov r2, #44 mov r3, #44 pop {pc} // A returns to main !!! B: push {lr} // Save return address mov r0, #4444 mov r1, #4444 pop {pc} // B returns to A !!!

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/8-sub/bl+pop2.s

    How to run the program:

    To compile:   as255 bl+pop2          To run: use EGTAPI