Re: [PATCH] PR tree-optimization/109417 - Check if dependency is valid before using in may_recompute_p.

[Jeff Law <jeffreyalaw@gmail.com>]

On 4/24/23 07:51, Andrew MacLeod wrote:

> 
> Its not a real cache..  its merely a statement shortcut in dependency 
> analysis to avoid re-parsing statements every time we look at them for 
> dependency analysis
> 
> It is not suppose to be used for anything other than dependency 
> checking.   ie, if an SSA_NAME changes, we can check if it matches 
> either of the 2 "cached" names on this DEF, and if so, we know this name 
> is stale.  we are never actually suppose to use the dependency cached 
> values to drive anything, merely respond to the question if either 
> matches a given name.   So it doesnt matter if the name here has been freed
OK.  I'll take your word for it.  Note that a free'd SSA_NAME may have 
an empty TREE_TYPE or an unexpected TREE_CHAIN field IIRC.  So you have 
to be a bit careful if you're going to allow them.

> 
> 
>> We never re-use SSA names from within the pass releasing it.  But if
>> the ranger cache
>> persists across passes this could be a problem.  See
> 
> 
> This particular valueswould never persist beyond a current pass.. its 
> just the dependency chains and they would get rebuilt every time because 
> the IL has changed.
Good.  THat would limit the concerns significantly.  I don't think we 
recycle names within a pass anymore (we used to within DOM due to the 
way threading worked eons ago, but we no longer take things out of SSA 
form to handle the CFG/SSA graph updates.  One could even argue we don't 
need to maintain the freelist and recycle names anymore.

Jeff