[llvm-dev] StringSwitch class

[Anupama Chandrasekhar via llvm-dev]

The point I was wondering about is, say in the example, say the input
string is "de"

int i = StringSwitch<int>("de")
.case("de", 1)
.case("fghi", 2)
.case("jkl", 3)
.default(-1);

will cause the 3 function calls to "Case()" and 1 to "Default()", Even if
the functions were inlined I would perform an if(!Result) check though
Result has been found, however if I were to code the same as:

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
}

I would be performing only one if check.

On Mon, Feb 8, 2016 at 10:05 AM, Chris Lattner <clattner at apple.com> wrote:

>
> > 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
>
>
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