How to implement switch with distinct cases? - c

Question

Here in the followed program if last condition is true then unnecessarily we have to check all conditions before it.
Is there any possibility to implement switch case in the below program?
I've to convert very similar code to this into Arm assembly.
main()
{
int x;
if (x< 32768)
x<<15;
elseif(x<49152)
(x<<12)- 7;
elseif(x<53248)
(x<<11)- 13;
elseif(x<59392)
(x<<10)-27;
elseif(x<60928)
(x<<9)-61;
elseif(x<62208)
(x<<8)-139;
elseif(x<64128)
(x<<7)-225;
elseif(x<65088)
(x<<6)-414;
elseif(x<65344)
(x<<5)-801;
elseif(x<65488)
(x<<4)-1595;
elseif(x<65512)
(x<<3)-2592;
elseif(x<65524)
(x<<2)-4589;
elseif(x<65534)
(x<<1)-8586;
}
Hope someone will help me.

Answer 1

So first things first: are you concerned about performance? If so, do you have actual profiling data showing that this code is a hot-spot and nothing else shows up on the profile?
I doubt that. In fact, I am willing to bed that you haven't even benchmarked it. You are instead looking at code and trying to micro-optimize it.
If that's the case then the answer is simple: stop doing that. Write your code the way it makes sense to write it and focus on improving the algorithmic efficiency of your code. Let the compiler worry about optimizing things. If the performance proves inadequate then profile and focus on the results of the profiling. Almost always the answer to your performance problems will use: choose a better-performing algorithm. The answer will almost never be "tinker with an if statement".
Now, to answer your question: A switch isn't helpful in this scenario because there's no sane way to represent the concept x < 32768 in a case statement, short of writing one statement for every such value of x. Obviously this is neithe practical nor sane.
More importantly you seem to operate under the misconception that a switch would translate to fewer comparisons. It's possible in some rare cases for a compiler to be able to avoid comparisons, but most of the time a switch will mean as many comparisons as you have case statements. So if you need to check a variable against 10000 different possible values using a switch, you'll get 10000 comparisons.
In your case, you're checking for way more than 10,000 possible values, so the simple if construct combined with the "less than" operator makes a lot more sense and will be much more efficient than a switch.
You write that "Here in the followed program if last condition is true then unnecessarily we have to check all conditions before it." True, you do. You could rewrite it so that if the last condition were true you would only need two comparisons. But then you'd simply flip the problem on it's head: if x< 32768 you'd end up having to check all the other possible values so you'd be back where you started.
One possible solution would be to perform binary search. This would certainly qualify as an algorithmic improvement, but again without hard data that this is, indeed, a hotspot, this would be a rather silly exercise.
The bottom line is this: write correct code that is both easy to understand and easy to maintain and improve and don't worry about reordering if statements. The excellent answer by perreal shows a good example of simple and easy to understand and maintain code.
And on the topic of writing correct code, there's no such thing as elseif in C and C++. Which brings us to my last point: before micro-optimizing code at least try to run the compiler.

Answer 2

You can't do that with switches since you need to have constant values to compare to x, not boolean conditions. But you can use a struct like this:
struct {
int u_limit;
int shift_left;
int add;
} ranges[13] = { {32768, 15, 0}, {49152, 12, -7} /*, ...*/};
for (int i = 0; i < 13; i++) {
if (x < ranges[i].u_limit) {
x = x << ranges[i].shift_left + ranges[i].add; break;
}
}
Of course, you can replace the linear search with binary search for some speedup.