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digitalmars.D - Integer overflow checking

reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Interesting: http://blog.regehr.org/archives/1384 -- Andrei
Apr 12 2016
parent reply Walter Bright <newshound2 digitalmars.com> writes:
On 4/12/2016 5:06 AM, Andrei Alexandrescu wrote:

 Interesting: http://blog.regehr.org/archives/1384 -- Andrei


Curiously never mentioned is the following optimization:

     return a+b*2+27;

becomes:

     LEA EAX,27[ESI][EDI*2]

To overflow check:

     ADD EDI,EDI
     JO overflow
     ADD EDI,27
     JO overflow
     MOV EAX,ESI
     ADD EAX,EDI
     JO overflow

I don't see efficiency there, even with the JO's being free.
Apr 13 2016
parent reply deadalnix <deadalnix gmail.com> writes:
On Wednesday, 13 April 2016 at 22:13:27 UTC, Walter Bright wrote:

 On 4/12/2016 5:06 AM, Andrei Alexandrescu wrote:

 Interesting: http://blog.regehr.org/archives/1384 -- Andrei


 Curiously never mentioned is the following optimization:

     return a+b*2+27;

 becomes:

     LEA EAX,27[ESI][EDI*2]

 To overflow check:

     ADD EDI,EDI
     JO overflow
     ADD EDI,27
     JO overflow
     MOV EAX,ESI
     ADD EAX,EDI
     JO overflow

 I don't see efficiency there, even with the JO's being free.


It is clearly not as optimal, but still pretty good. The article 
doesn't pretend it all come for free, just that it comes for much 
cheaper than before.
Apr 13 2016
parent reply deadalnix <deadalnix gmail.com> writes:
On Thursday, 14 April 2016 at 02:55:01 UTC, deadalnix wrote:

 On Wednesday, 13 April 2016 at 22:13:27 UTC, Walter Bright 
 wrote:

 On 4/12/2016 5:06 AM, Andrei Alexandrescu wrote:

 Interesting: http://blog.regehr.org/archives/1384 -- Andrei


 Curiously never mentioned is the following optimization:

     return a+b*2+27;

 becomes:

     LEA EAX,27[ESI][EDI*2]

 To overflow check:

     ADD EDI,EDI
     JO overflow
     ADD EDI,27
     JO overflow
     MOV EAX,ESI
     ADD EAX,EDI
     JO overflow

 I don't see efficiency there, even with the JO's being free.


 It is clearly not as optimal, but still pretty good. The 
 article doesn't pretend it all come for free, just that it 
 comes for much cheaper than before.


Also, just checked, on sandy bridge, the LEA has 3clock latency 
(but start earlier in the pipeline) and the add 1, so it is not 
as bad as it looks (it is still bad).
Apr 13 2016
parent reply Walter Bright <newshound2 digitalmars.com> writes:
On 4/13/2016 8:00 PM, deadalnix wrote:

 Also, just checked, on sandy bridge, the LEA has 3clock latency (but start
 earlier in the pipeline) and the add 1, so it is not as bad as it looks (it is
 still bad).


The size is larger, too (not so cache friendly). Integer arithmetic is sort of 
the bread and butter of computer programs, how much slowdown will people accept?
Apr 13 2016
parent deadalnix <deadalnix gmail.com> writes:
On Thursday, 14 April 2016 at 04:52:12 UTC, Walter Bright wrote:

 On 4/13/2016 8:00 PM, deadalnix wrote:

 Also, just checked, on sandy bridge, the LEA has 3clock 
 latency (but start
 earlier in the pipeline) and the add 1, so it is not as bad as 
 it looks (it is
 still bad).


 The size is larger, too (not so cache friendly). Integer 
 arithmetic is sort of the bread and butter of computer 
 programs, how much slowdown will people accept?


Maybe not everywhere, but some security related code and/or tool 
like ubsan surely can benefit from this.
Apr 13 2016