Re: Subtraction + comparison in one asm instruction?

    Anton Ertl <anton@mips.complang.tuwien.ac.at> wrote:


> Do you value such an optimization more than being able to get the same
> results with and without optimization?


For those who write programs not based on non-standard 2s-complement
wraparound, yes.


> Also, do you want to be able to compile code into its intended
> equivalent that is non-standard, but nonetheless exists?


In this case, a compatibility compiler switch can been added.


> int n2;
> int olddiff = n1-nlimit;
> n2=n1+n;
> if ((olddiff^(olddiff+n))>=0 /* the limit is not crossed */
> || (olddiff^n)>=0 /* it is a wrap-around effect */) {
> ...


> You might be able to implement this in fully compliant ANSI C, but it
> probably will be longer, and probably will also run slower when
> compiled with gcc.


No, you can modify it to make it ISO C compliant, though still based
on non-portable assumptions (i.e. 2s-complement), and that should not
be slower if the compiler is smart enough (which seems to be the
case). Just change the condition to:


    (((unsigned) olddiff ^ ((unsigned) olddiff + (unsigned) n)) >> 31 == 0 ||
      ((unsigned) olddiff ^ (unsigned) n) >> 31 == 0)


i.e. cast all variables to unsigned and check the most significant
bit. I've written 31, assuming 32-bit int's, but in the real world,
you can find the correct value using INT_MAX. gcc 2.94.4 for the ARM
generates exactly the same assembly code as with your C source.


Vincent Lefèvre.

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