assert semantic change proposal

[Andrei Alexandrescu via Digitalmars-d]

On 8/3/14, 6:59 PM, David Bregman wrote:
> w.r.t the one question about performance justification: I'm not
> necessarily asking for research papers and measurements, but based on
> these threads I'm not aware that there is any justification at all. For
> all I know this is all based on a wild guess that it will help
> performance "a lot", like someone who optimizes without profiling first.
> That certainly isn't enough to justify code breakage and massive UB
> injection, is it? I hope we can agree on that much at least!

I think at this point (without more data) a bit of trust in one's 
experience would be needed. I've worked on performance on and off for 
years, and so has Walter. We have plenty of war stories that inform our 
expertise in the matter, including weird stuff like "swap these two enum 
values and you'll get a massive performance regressions although code is 
correct either way".

I draw from numerous concrete cases that the right/wrong optimization at 
the right/wrong place may as well be the difference between winning and 
losing. Consider the recent php engine that gets within 20% of hhvm; 
heck, I know where to go to make hhvm 20% slower with 50 lines of code 
(compare at 2M+). Conversely, gaining those 20% were months multiplied 
by Facebook's best engineers.

Efficiency is hard to come by and easy to waste. I consider Walter's 
take on "assert" a modern, refreshing take on an old pattern that nicely 
preserves its spirit, and a good opportunity and differential advantage 
for D. If anything, these long threads have strengthened that belief. It 
has also clarified to me that:

(a) We must make sure we don't transform @safe code into unsafe code; in 
the first approximation that may simply mean assert() has no special 
meaning in release mode. Also bounds checking would need to probably be 
not elided by assert. I consider these challenging but in good, gainful 
ways.

(b) Deployment of optimizations must be carefully staggered and documented.


Andrei