[LLVMdev] Helping the optimizer along (__assume)
clattner at apple.com
Thu Oct 23 21:15:38 PDT 2008
On Oct 23, 2008, at 12:23 AM, Duncan Sands wrote:
>>> %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.
>> 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 :)
> The difference is that simplifycfg will remove the branch
> to %never if %never only contains unreachable. The role
> of the intrinsic call is to fool simplifycfg into not
> doing this!
Sounds like the "bug" is in simplifycfg then. The IR is already fully
capable of expressing this without a new builtin. Note that these
annotations would have to be removed *sometime* in the optimizer
pipeline, the policy decision is picking a place.
More information about the llvm-dev