[llvm-commits] patch: preserve branch to unreachable until codegenprep time

[Nick Lewycky]

Duncan Sands wrote:
> Hi Nick Lewycky,
> 
>> This patch moves the code that optimizes "branch to unreachable" from 
>> SimplifyCFG to CodeGenPrep. Most of this patch is just updating the 
>> unit tests that use unreachable everywhere. I've done my best to 
>> preserve the intent of the tests.
> 
> so does this mean I can now teach LLVM about the ranges of Ada variables
> by doing something like (suppose the variable only takes values in 0..5)
> 
>   if (variable > 5)
>     unreachable;
> 
> and now LLVM passes will reason about it rather than eliminating it?

That's the idea, but we still have to make sure that the optimizers 
actually *do* that.

GVN should do some of that right off the bat but I'd appreciate some 
help in going through and finding cases where the optimizers could take 
advantage of it but don't -- and then in fixing those cases. :)

Nick