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Efficiently combining bytes into a long int

Robert Schmanski

Hi,

I'm having problems assembling a set of 4 bytes into a long int. My approach has been this:

    ulong temp;
    temp = (temp << SHIFT_BYTE) | getByte();
    temp = (temp << SHIFT_BYTE) | getByte();
    temp = (temp << SHIFT_BYTE) | getByte();
    temp = (temp << SHIFT_BYTE) | getByte();
    return temp;

But this results in a ton of calls to the long int library shift and logical or routines, which really aren't necessary. My quick and dirty solution has been to code the function in assembly, and just put the bytes in the appropriate "temp+n" bytes that make up temp. I've tried fooling around with unions, but haven't had any luck.

Thanks,
Bob

Parents
  • 0 in reply to Robert Schmanski

    "I guess I should have done a bit more research before posting"

    yes - it's been discussed on this forum many times before!

    This is all standard 'C' stuff - the only Keil-specific detail is the byte ordering within a long.

    Note that your original approach using shifts is independent of the underlying byte order and is, therefore, portable;
    The union approach relies upon the byte ordering and is, therefore, non-portable.

    With suitable conditional-compilation directives you can, of course, create a portable union implementation.

Reply
  • 0 in reply to Robert Schmanski

    "I guess I should have done a bit more research before posting"

    yes - it's been discussed on this forum many times before!

    This is all standard 'C' stuff - the only Keil-specific detail is the byte ordering within a long.

    Note that your original approach using shifts is independent of the underlying byte order and is, therefore, portable;
    The union approach relies upon the byte ordering and is, therefore, non-portable.

    With suitable conditional-compilation directives you can, of course, create a portable union implementation.

Children
  • 0 in reply to Andrew Neil

    I did read back a few months worth of forum discussion before posting, but I didn't search the knowledge base. Thanks for responding anyway.

    There's more Keil-specific stuff than just the byte ordering in a long. The original purpose of my post was to find out which C code the compiler converted into efficient assembly. An intelligent enough compiler should have produced the same code for both the union approach and the shift approach, in which case, I would have opted for the portable approach. In my pre-purchase testing of the Keil compiler, I had found that it generally was very good at efficient assembly code for constant math such as dividing or multiplying by constant that is a power of 2 (shifting). I guess I was just expecting too much. The Keil tools do generally do a very good job.

    Thanks,
    Bob