[PATCH] [Jump-Threading] Fixed jump threading hang issues (PR15386, PR15851)

[Rafael Espíndola]

On 10 June 2014 01:53, Dinesh Dwivedi <dinesh.d at samsung.com> wrote:
> Each pass iterates over all BBs of the functions and all updated BBs are added to
> 'UpdatedBB' so if first pass adds 3 BBs and second pass adds 2, there will be
> 5 BBs in the list and we will still continue with next pass.

Oh, sorry. I misread it as being the BB updated in this iteration.

So we are sure that a BB is never profitably updated more than once?

> Every change in BBs actually adds atleast one new block XXX.thread and till it finds
> some new block added or updated, it will do next iteration on all BBs of the function.
> and the condition, that will break this is when we are not adding any new block or not
> updatind any new BB.
>
> In attached test case, jump threading stuck between 'for.cond1' and 'for.body' block.
> Here is sample output for few iterations.
> After first iteration:
>   for.cond1:                                        ; preds = %for.body.thread, %for.body, %land.rhs
>     %div5 = phi i32 [ 0, %for.body.thread ], [ %div, %for.body ], [ %div, %land.rhs ]
>     %inc = add nsw i32 %div5, 1
>     %cmp = icmp slt i32 %inc, 2
>     br i1 %cmp, label %for.body, label %for.cond1.thread
>
>   for.cond1.thread:                                 ; preds = %for.cond1
>     br label %for.body.thread
>
>   for.body.thread:                                  ; preds = %for.cond1.thread
>     br label %for.cond1
>
>   for.body:                                         ; preds = %for.cond1
>     %i.02 = phi i32 [ %inc, %for.cond1 ]
>     %div = sdiv i32 %i.02, 2
>     %i.0.off = add i32 %i.02, 1
>     %0 = icmp ugt i32 %i.0.off, 2
>     br i1 %0, label %land.rhs, label %for.cond1
>
> 2nd:
>   for.cond1:                                        ; preds = %for.body.thread, %for.body, %land.rhs
>     %div5 = phi i32 [ %div, %for.body ], [ %div, %land.rhs ], [ 0, %for.body.thread ]
>     %inc = add nsw i32 %div5, 1
>     %cmp = icmp slt i32 %inc, 2
>     br i1 %cmp, label %for.body, label %for.cond1.thread6
>
>   for.cond1.thread6:                                ; preds = %for.cond1
>     br label %for.body.thread
>
>   for.body.thread:                                  ; preds = %for.cond1.thread6
>     br label %for.cond1
>
>   for.body:                                         ; preds = %for.cond1
>     %i.02 = phi i32 [ %inc, %for.cond1 ]
>     %div = sdiv i32 %i.02, 2
>     %i.0.off = add i32 %i.02, 1
>     %0 = icmp ugt i32 %i.0.off, 2
>     br i1 %0, label %land.rhs, label %for.cond1
>
> and 3rd:
>   for.cond1:                                        ; preds = %for.body.thread, %for.body, %land.rhs
>     %div5 = phi i32 [ %div, %for.body ], [ %div, %land.rhs ], [ 0, %for.body.thread ]
>     %inc = add nsw i32 %div5, 1
>     %cmp = icmp slt i32 %inc, 2
>     br i1 %cmp, label %for.body, label %for.cond1.thread
>
>   for.cond1.thread:                                 ; preds = %for.cond1
>     br label %for.body.thread
>
>   for.body.thread:                                  ; preds = %for.cond1.thread
>     br label %for.cond1
>
>   for.body:                                         ; preds = %for.cond1
>     %i.02 = phi i32 [ %inc, %for.cond1 ]
>     %div = sdiv i32 %i.02, 2
>     %i.0.off = add i32 %i.02, 1
>     %0 = icmp ugt i32 %i.0.off, 2
>     br i1 %0, label %land.rhs, label %for.cond1
>
> I will update test cases as you suggested. Actually I am not sure about test as
> it hangs with trunk and check-all never completes.

Yes, but the patch fixes that, right? If so it is probably a good idea
to make the test a bit stronger than just "it finishes".

Cheers,
Rafael