Proposal for SentinelInputRange

[deadalnix]

On Thursday, 28 February 2013 at 17:13:36 UTC, Dmitry Olshansky 
wrote:
> No as a compiler will take it (or may depending on its brain) 
> that 0 is what you want to test *first*. It may speed things up 
> if branch is almost always taken but its not the case with 
> sentinel. Thus its jsut dead code that needs to be decoded, 
> evalutated and skipped (as predicated) *before* doing switch 
> jump.
>
> In fact some people avoid the overhead of switch by placing one 
> or two of highly-frequent branches with tests before the switch 
> (thus avoiding indirect branch it entails in these frequent 
> cases).

That won't work as expected with LLVM and full optimizations, as 
it will combine everything in a switch, unless you use branch 
weight, in which case it can do the reverse : extract common 
cases from the switch.

See : http://llvm.org/docs/BranchWeightMetadata.html

GCC does something similar.

This is another example of how people think they are doing high 
level assembly when in fact, the compiler is changing everything 
behind their back, and not doing at all what they think.