[LLVMdev] Helping the optimizer along (__assume)

Duncan Sands baldrick at free.fr
Wed Oct 22 22:45:26 PDT 2008

> 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
    call void @llvm.abort()

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.



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