[llvm-dev] StringSwitch class

[Anupama Chandrasekhar via llvm-dev]

Got it. Thanks.

On Wed, Feb 10, 2016 at 9:59 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

> From a pure syntactic point of view, yes.
> But it is meaningless as you are trying to think about a higher-level
> language construct and redundant check while you should really have a look
> at the optimized code if you're concerned with performance.
>
> --
> Mehdi
>
> On Feb 10, 2016, at 9:57 AM, Anupama Chandrasekhar <
> anupama.lists at gmail.com> wrote:
>
> Sorry for belaboring on a possibly minor point here, but is my
> understanding correct that even assuming that the case function is always
> inlined so we don't have extra function call overhead, we have the
> redundant if (!Result) checks when we use StringSwitch as opposed to a
> bunch of if- elses.
>
> Thanks.
>
> On Mon, Feb 8, 2016 at 12:00 PM, Anupama Chandrasekhar <
> anupama.lists at gmail.com> wrote:
>
>> 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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