Question

What can I do to improve large switches and if-elses speed manually? I will probably need some kind of hash or lookup table.

I’m working with gcc and C code, I doubt that gcc has any inbuilt optimizations for this.

Edit:
My switch code is what every switch looks like, do something based on if a particular int is some value.
My if-elses look like this:

if( !strcmp( "val1", str ) )
foo();
else if( !strcmp( "val2", str ) )
foo2();
...

I also have ifs that do this

if( struct.member1 != NULL )
foo();
if( struct.member2 != NULL )
foo2();

EDIT2:
Thank you everyone. I’m not sure which one I should pick as an answer, because a lot of these answers have valid points and valuable insights. Unfortunately, I have to pick just one. But thanks all!
In the end, using a perfect hash table seems the best way to get O(n) time on the access for both ifs and switches.

Answer 1

To use a hash table:

1. Pick a hash function. This one is a biggie. There are tradeoffs between speed, the quality of the hash, and the size of the output. Encryption algorithms can make good hash functions. The hash function performs some computation using all the bits of your input value to return some output value with a smaller number of bits.
2. So the hash function takes a string
   and returns an integer between 0 and
   N .
3. Now you can look up a pointer to a function in a table of size N.
4. Each entry in the table will be a linked list (or some other searchable data structure) because of the chance of collision, that is two strings that map to the same hash value.

E.g.

lets say hash(char*) returns a value between 0 and 3.
hash("val1") returns 2
hash("val2") returns 0
hash("val3") also returns 0
hash("val4") returns 1

Now your hash table looks something like:

table[0] ("val2",foo2) ("val3", foo3)
table[1] ("val4",foo4)
table[2] ("val1",foo1)
table[3] <empty>

I hope you can see how the cost of doing matching using a hash table is bound by the time it takes to calculate the hash function and the small time it takes to search the entry in the hash table. If the hash table is large enough most hash table entries would have very few items.