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How to get absolute value of a 32-bit signed integer as fast as possible?

Matic

Hi.

I wonder how to calculate absolute value of a 32-bit signed integer in C as fast as possible. I saw that there is a FPU instruction VABS.F32, which do that in one cycle (above the floats). I thought, if it is possible to use it also with integers (sign bit is in both cases MSB bit). Maybe in some way with inline or embedded assembly?

Or do you have any other advice how to get absolute value of an integer in C in the fastest way.

Not to forget - I am speaking regarding Cortex-M4 processor.

Thanks

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  • 0 in reply to daith

    I want to say something about the codes that you posted. However, I don't have enough time anymore so, in the meantime, I'll just reply about the last portion of your reply.

    but what an optimising compiler does with that - who knows except by looking at the output. Which I think is an exercise I'll leave to the reader ;-)

    Invoke common subexpression elimination replacing the code with

         tmp = src >> 31;

         dst = src ^ (tmp);

         dst -= tmp;

    .

    Well, C compilers for ARM architecture are aware that the shift is a "built-in" feature of EOR and SUB so at least we are assured that the code will not be transformed as above. The rest of the compiler's behavior remains an exercise for us.

Reply
  • 0 in reply to daith

    I want to say something about the codes that you posted. However, I don't have enough time anymore so, in the meantime, I'll just reply about the last portion of your reply.

    but what an optimising compiler does with that - who knows except by looking at the output. Which I think is an exercise I'll leave to the reader ;-)

    Invoke common subexpression elimination replacing the code with

         tmp = src >> 31;

         dst = src ^ (tmp);

         dst -= tmp;

    .

    Well, C compilers for ARM architecture are aware that the shift is a "built-in" feature of EOR and SUB so at least we are assured that the code will not be transformed as above. The rest of the compiler's behavior remains an exercise for us.

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