Re: [PATCH] Checking undefined_p before using the vr

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi Andrew,

Thanks for your explain! And sorry for later reply.

Andrew MacLeod <amacleod@redhat.com> writes:

> On 9/14/23 22:07, Jiufu Guo wrote:
>>>
>>> undefined is a perfectly acceptable range.  It can be used to
>>> represent either values which has not been initialized, or more
>>> frequently it identifies values that cannot occur due to
>>> conflicting/unreachable code.  VARYING means it can be any range,
>>> UNDEFINED means this is unusable, so treat it accordingly.  Its
>>> propagated like any other range.
>> "undefined" means the ranger is unusable. So, for this ranger, it
>> seems only "undefined_p ()" can be checked, and it seems no other
>> functions of this ranger can be called.
>
> not at all. It means ranger has determined that there is no valid
> range for the item you are asking about probably due to conflicting
> conditions, which imparts important information about the range.. or
> lack of range :-)
>
> Quite frequently it means you are looking at a block of code that
> ranger knows is unreachable, but a pass of the compiler which removes
> such blocks has not been called yet.. so the awareness imparted is
> that there isn't much point in doing optimizations on it because its
> probably going to get thrown away by a following pass.
>
>>
>> I'm thinking that it may be ok to let "range_of_expr" return false
>> if the "vr" is "undefined_p".  I know this may change the meaning
>> of "range_of_expr" slightly :)
>
> No.  That would be like saying NULL is not a valid value for a
> pointer.  undefined_p has very specific meaning that we use.. it just
> has no type.

Oh, get it.:)

BR,
Jeff (Jiufu Guo)
>
> Andrew