[LLVMdev] Helping the optimizer along (__assume)

[Chris Lattner]

On Oct 22, 2008, at 10:45 PM, Duncan Sands wrote:

>> Can't you implement __builtin_assume(cond) to codegen to something  
>> like:
>>
>> 	%cond = i1 ...
>> 	br i1 %cond, label %always, label %never
>> never:
>> 	unreachable
>> always:
>
> The code generators will remove the branch to %never.
> I already tried this :)  What would work is to define
> an llvm.abort intrinsic, and do:
>
> 	%cond = i1 ...
> 	br i1 %cond, label %always, label %never
> never:
>    call void @llvm.abort()
> 	unreachable
>
> At codegen time @llvm.abort() can be lowered to
> nothing at all.  I'm not saying that this is my
> favorite solution, but it is simple.

How is this different than just branching to unreachable?  Branching  
to unreachable says that "this condition is true or else the program  
has undefined behavior".  This means that the condition must be true :)

-Chris