exceptions and optimization

[Nicholas Wilson]

On Monday, 21 October 2019 at 20:12:19 UTC, Peter Jacobs wrote:
> Toward the end of Walter's recent talk, D at 20, he says 
> something to the effect that optimizations are disabled when 
> exceptions can be thrown.  We have a compressible flow solver 
> in which it is very convenient to be able to throw an exception 
> from deep within the code and catch it at a relatively high 
> level where we can partially recover and continue the 
> calculation.  Because our calculations can run for days across 
> hundreds of processors, we also care about letting the 
> optimizer do its best. In what parts of our program would the 
> optimizer be disabled because of the presence of the exception 
> and its handling code?  How do we tell?

It really comes into effect if you use:

a) structs with destructors
b) scope(exit) and scope(failure)

as these need to be run, regardless of the return mode of the 
function i.e. normally or via an exception.

Also if the function is nothrow (possibly inferred because its a 
template, n.b. also that nothrow function can throw Errors as 
these are considered terminal) then you should also be fine. The 
effect of exceptions on optimisation effect logic heavy code a 
lot more that math heavy code.

What kind of conditions are you wanting to throw exception on? 
infinities, NaNs, ill conditioning, something else?

As always the best way to check is to mark the function of 
interest, nothrow take a look at the disassembly and compare to 
without nothrow. You may also want to look to the optimisation 
summary that I _think_ you can get LDC to generate.