Re: Integer overflow checking

[deadalnix via Digitalmars-d]

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).