Truly @nogc Exceptions?

[Steven Schveighoffer]

On 9/20/18 1:58 PM, Adam D. Ruppe wrote:
> On Thursday, 20 September 2018 at 17:14:12 UTC, Steven Schveighoffer wrote:
>> I don't know how a performance problem can occur on an error being 
>> thrown anyway -- the process is about to end.
> 
> Walter's objection was code size - it would throw stuff out of cache 
> lines, even if it doesn't need to actually run.
> 
> So like this line:
> 
> int[] a;
> a = a[1 .. $];
> 
> With no bounds checking is just
> 
> inc a.ptr;
> 
> but with bounds checking it becomes something more like
> 
> mov ecx, a.length
> cmp ecx, 1 // in other words, if length >= offset
> jae proceed
> push line
> push file
> call d_arraybounds // throws the error
> proceed:
> inc a.ptr
> 
> 
> Now, what my patch did was just, right before push line, it inserted 
> "push length; push offset;". I believe this to be trivial since they are 
> already loaded in registers or immediates and thus just a couple bytes 
> for those instructions, but Walter (as I recall, this was a while ago 
> and I didn't look up his exact words when writing this) said even a 
> couple bytes are important for such a common operation as it throws off 
> the L1 caches. I never got around to actually measuring the performance 
> impact to prove one way or another.
> 
> But... even if that is a problem, dmd -O will usually rearrange that to 
> avoid the jump on the in-bounds case, and I'm sure ldc/gdc do too, 
> sooooo the extra pushes' instruction bytes are off the main execution 
> path anyway and thus shouldn't waste cache space.
> 
> 
> idk though. regardless, to me, the extra info is *well* worth the cost 
> anyway.

Sounds like a case of premature optimization at best.

Besides, if it is a performance issue, you aren't doing bounds checks on 
every slice/index anyway. I know in iopipe, to squeeze out every bit of 
performance, I avoid bounds checks when I know from previous asserts the 
bounds are correct.

-Steve