Re: [PATCH] Checking undefined_p before using the vr

[Andrew MacLeod <amacleod@redhat.com>]

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.

Andrew