performance of the swtich/case statements

[Erich Dollansky]

Hi,

On Thu, 30 Oct 2014 12:24:33 +0800
bycn82 <bycn82 at gmail.com> wrote:

> Hi,
> According to my understanding in Java programming, the compiler will

aren't we talking about C here?

Erich


> automatically store the values into a table and jump to the correct
> one according to the value only when the condition values are in
> running number,
> 
> for example.
> 
> swtich(a){
> case 1:  code block 1
> case 2:  code block 2
> case 3:  code block 3
> case 4:  code block 4
> default: code block 5
> }
> 
> it will be handled by an array
> 1-->code block 1
> 2-->code block 2
> 3-->code block 3
> 4-->code block 4
> others-->code block 5
> 
> so when the value N is greater than  or lesser than 1, it will be
> directly jump to the "code block 5"
> otherwise, it will jump to N, because call the cases are nice in
> running numbers,
> 
> but when the cases are messy, it will by just like lots of if/else
> 
> 
> On Thu, Oct 30, 2014 at 6:30 AM, Erich Dollansky <
> erichsfreebsdlist at alogt.com> wrote:
> 
> > Hi,
> >
> > On Wed, 29 Oct 2014 22:39:34 +0800
> > "bycn82" <bycn82 at gmail.com> wrote:
> >
> > > It is using the switch/case statement to make the code clear in
> > > the
> > >
> > > I am not a C programmer, so I am not clear how the switch/case
> > > will be optimized by the compiler in FreeBSD. But I used to write
> > > a compiler by myself and I use a hash table to handle all the
> > > conditions in the case statements because my compiler don't care
> > > about performance!, But in C it is different, the case statement
> > > can only accept "int" values, so I don't think it will use hash
> > > or what , it should be directly use an array(), So whether it can
> > > be optimized it depends on the conditions in the switch/case
> > > statements, and I noticed that the cases statement in the 2 loops
> > > are not arranging the opcode in running number, so does the
> > > compiler smart enough to optimize it?
> > >
> > >
> > I did not check recently. It was already a long, long time ago, that
> > compilers checked the limits and used the values as an index into a
> > table to jump to the code. I hope that this did not get changed.
> >
> > With other words, the order in the code does not matter. The only
> > optimisation the compiler can do, is not to use a table if the
> > statement consists of a low number of entries only.
> >
> > Erich
> >