Thoughts about D

[Adam D. Ruppe]

On Thursday, 30 November 2017 at 16:12:04 UTC, H. S. Teoh wrote:
> Can you elaborate?

As I understand it, DWARF exception handling even on the 
non-exceptional case, is a bit more expensive than our old way. 
Not hugely expensive but for dmd where we count milliseconds it 
might add up.

> Which brings us to the implementation of assert() itself. What 
> about it makes it so big? I suspect most of the bloat comes 
> from throwing AssertError, which pulls in the stack-unwinding 
> code, which, if my memory is still up to date, suffers from 
> performance issues where it tries to construct the stacktrace 
> regardless of whether or not the catch block actually wants the 
> stacktrace.

That's false, I changed that many years ago myself, unless the 
DWARF change involved that too, but I don't think so.

What happens is the exception constructor walks the stack and 
copies the addresses to a local, static buffer. This is very fast 
- just walking a linked list and copying some void* into a 
void*[64] or whatever - and little code.

The expensive part of formatting it to a string, actually looking 
up the debug info, parsing out the addresses, etc., is done 
lazily when it is printed, which occurs only on demand or right 
before the program terminates.