[llvm-dev] StringSwitch class

[Chris Lattner via llvm-dev]

> On Feb 5, 2016, at 4:42 PM, Mehdi Amini via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
>> On Feb 5, 2016, at 2:43 PM, Anupama Chandrasekhar via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> Hi:
>> 
>> I have a question about the llvm StringSwitch class. Why is this more efficient than comparing the hashes of the strings or just using a bunch of if statements.
> 
> I don't know about hashes performance (comparing a small string is probably more efficient than comparing hashes, and I'd be reluctant to collisions anyway). Now since you mentioned a "bunch of if statements" you can get very close.
> StringSwitch is caching the length of the string to compare and first checking the length before doing the actual comparison using memcmp. So the two constructs below it should be equivalent:
> 
> int i = StringSwitch<int>("abc").case("de", 1).case("fghi", 2).case("jkl", 3).default(-1);
> 
> and:
> 
> int i;
> const char *str = "abc";
> int len = strlen(str);
> if(len == 2 && std::memcmp(str, "de", 2) {
>   i = 1;
> } else if(len == 4 && std::memcmp(str, "fghi", 4) {
>   i = 2;
> } else if(len == 3 && std::memcmp(str, "jkl", 3) {
>   i = 3;
> } else {
>   i = -1
> }

Also, note that this sequence is specifically intended to be jump threadable and recognized by the compiler’s optimizer.  This allows the compiler to turn it into:


int len = strlen(str);
switch (len) {
case 2:
   if (std::memcmp(str, "de", 2)
     i = 1;
   else (std::memcmp(str, “qr", 2)
     i = 1;
   else
     i = -1;
  case 4:
..

Which isn’t optimal perhaps, but isn’t bad either.  The compiler should theoretically be able to do good things with back-to-back memcmps, but probably isn’t doing anything good there for long ones.  Short memcmps should be turned into a load+compare, meaning they become another switch.

-Chris