double condition checking in assembly

There is a way to express a range check like this using only a single conditional jump:

     sub  eax,  20
     cmp  eax,  80
     ja   END
     // do something
END: ret

This is a very common optimization trick when working with integer ranges. The initial subtract maps the range [20,100] to [0,80]; membership in that range is then be checked with a single unsigned comparison.

Note also that the same thing can be done in C:

unsigned int upperBound = 100;
unsigned int lowerBound = 20;
if (yourValue - lowerBound <= upperBound - lowerBound) {
    // do something
}

Depending on what syntax you're using, and assuming x is in the eax register, something like this:

cmp  eax, 20
jl   ELSE
cmp  eax, 100
jg   ELSE
#do something
jmp  END

ELSE:
#do else

END:

You can try compiling it from a higher level language (C/C++/ ...) with optimalisations at high (-O3 for gcc), and have a look at what the compiler generates (objdump). It should generate very efficient code.

double condition checking in assembly

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