[DUG] In [set] efficiency

[Ross Levis]

So it's best to reduce the set size as much as possible.  Looking at my
code, I have some variables declared as Word (16-bit) with 65535 possible
values but most are Byte.

 

From: delphi-bounces at listserver.123.net.nz
[mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Peter Ingham
Sent: Wednesday, 8 June 2016 8:15 p.m.
To: delphi at listserver.123.net.nz
Subject: Re: [DUG] In [set] efficiency

 

(From memory & without firing up a compiler to verify) ...

Sets are implemented as bitmaps with one bit allocated to each possible
value of the set type (hence the limitations on the number of possible
elements in the set).

So if you have
type bytesset = [0..7];  // 8 possible values
var  a: bytesset ;
begin
  if a in [1,4] then

The set literal [1,4] ends up as a byte with the value $0A.
The test "a in [1,4]" is equivalent to "(a AND $0A) <> 0".

The generated code (for a set of this size) is an AND  followed by a
conditional jump.  Very efficient.

It gets a little nastier when the possible values gets larger as the size of
the set grows  (set of AnsiChar is 32 bytes; Set of Widechar exceeds the
permissible size of a set).

The precise code generated by case varies between jump tables and linear
comparisons.

Regards
  

On 8/06/2016 5:27 p.m., Steve Peacocke wrote:

If I remember correctly, the compiler changes both to array anyway so you
come out with exactly the same compiled code. 

 

Perhaps someone can confirm this or tell me how wrong I am?

Steve Peacocke 

+64 220 612-611

 


On 8/06/2016, at 4:56 PM, Ross Levis <ross at stationplaylist.com> wrote:

I'm wondering which is more efficient to process...

 

if (a=1) or (a=2) then ...

 

or

 

if a in [1,2] then ...

 

If the answer is the first method, does it make a difference if more numbers
are checked, eg. if a in [1..3,5] then

 

Cheers.

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