CODEWITHSUNDEEP
c++gccassemblyinline-assembly
Bob
Bob

Reputation: 4970

inside naked function - how to do simple assignment

This is the beginning of a function that already exists and works; the commented line is my addition and its purpose is to toggle a pin.

inline __attribute__((naked)) 
void CScheduler::SwapToThread(void* pNew, void* pPrev)
{   
    //*(volatile DWORD*)0x400FF08C = (1 << 14);
    if (pPrev != NULL)
    {
        if (pPrev == this) // Special case to save scheduler stack on startup
        {
            asm("mov lr,%0"::"p"(&CScheduler_Run_Exit));     // load r1 with schedulers End thread
            asm("orr lr, 1");

When I uncomment my addition, my hard fault handler executes. I get it has something to do with this being a naked function but I don't understand why a simple assignment causes a problem.

Two questions:

  1. Why does this line trigger the hard fault?
  2. How can I perform this assignment inside this function?

Upvotes: 0

Views: 413

Answers (1)

Peter Cordes
Peter Cordes

Reputation: 364458

It was only luck that your previous version of the function happened to work without crashing.

The only thing that can safely be put inside a naked function is a pure Basic Asm statement. https://gcc.gnu.org/onlinedocs/gcc/ARM-Function-Attributes.html. You can split it up into multiple Basic Asm statements, instead of asm("insn \n\t" / "insn2 \n\t" / ...);, but you have to write the entire function in asm yourself.

While using extended asm or a mixture of basic asm and C code may appear to work, they cannot be depended upon to work reliably and are not supported.

If you want to run C++ code from a naked function, you could call a regular function (or bl on ARM, jal on MIPS, etc.), following to the standard calling convention.

As for the specific reason in this case? Maybe creating that address in a register stepped on the function args, leading to the branches going wrong? Inspect the generated asm if you want, but it's 100% unsupported.

Or maybe it ended up using more registers, and since it's naked didn't properly save/restore call-preserved registers? I haven't looked at the code-gen myself for naked functions.

Are you sure this function needs to be naked? I guess that's because you manipulate lr to return to the new context.

If you don't want to just write more logic in asm, maybe have this function's caller do more work (and maybe pass it pointer and/or boolean args telling it more simply what it needs to do, so your inputs are already in registers, and you don't need to access globals).

Upvotes: 3

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