Evaluation order of index expressions

[via Digitalmars-d]

On Monday, 25 May 2015 at 15:35:02 UTC, Jonathan M Davis wrote:
> would hope good code would avoid. But defining the order of 
> evaluation as left-to-right, doesn't make those problems go 
> away. At best, it makes them consistent, and that may be worth 
> it, but it's not a silver bullet. And it takes optimization 
> opportunities away from the compiler, since in many cases, it 
> can reorder how the expression is evaluated to make better use 
> of the registers and whatnot. So, forcing the order of

One of C's design mistakes is to make assignments expressions and 
not statements. Favouring terseness over correctness, an issue 
the C follow-up language Go partially recognized by turning "++" 
and "--" into statements.

I agree with you that if an expression depends on evaluation 
order it is most likely either buggy or prone to become buggy 
when the program is modified later on. So it is reasonable to 
define it as illegal and leave it to a sanitizer. (Of course, the 
exception is shortcut operators like "&&" and "||", which already 
are have strict evaluation order).

Being able to change evaluation order can provide optimization 
opportunities that cannot be fixed by having the compiler infer 
it due to aliasing issues. Not only due to registers, but also 
because of barriers, aliasing,  lookup-tables, sub-expressions 
etc…

The downside to not having a strict evaluation order is contexts 
where you want to use multiple generator calls in a single 
expression (like a numeric range or random number generator), so 
it goes against the whole "range iterators" approach which will 
lead to lengthy expressions that do contain side effects.

So essentially D does not have any other reasonable option than 
strict LTR evaluation, since it is making "gigantic" expressions 
with generators in them a  selling point.

You have to support what you market as a major feature whether 
that is done by having dedicated "range-operators" that have 
strict evaluation order or by making all expressions strict..