[15] RFR(M): 8240227: Loop predicates should be copied to unswitched loops

[Nils Eliasson]

Hi Christian,

I like the comprehensive tests you added.

Reviewed!

// Nils

On 2020-03-11 13:20, Christian Hagedorn wrote:
> Hi
>
> Please review the following patch:
> https://bugs.openjdk.java.net/browse/JDK-8240227
> http://cr.openjdk.java.net/~chagedorn/8240227/webrev.00/
>
> This is the same problem as for JDK-8203915 [1] but now with 
> additional loop unswitching which was not handled in the fix for 
> JDK-8203915.
>
> Background:
> The original idea of inserting skeleton predicates [2] for range check 
> predicates involving the loop induction variable was to copy those 
> concrete predicates down to the main loop when creating pre/main/post 
> loops. When unrolling the loop later, the limit check predicate for 
> the induction variable is updated to become stronger and cover all 
> accesses of the unrolled loop body. If a predicate for the main loop 
> later turns out to always fail due to type information, then the main 
> loop can be completely removed.
>
> If we did not do that (which exactly happens in the test case since we 
> do not copy the range check predicates to the unswitched loops which 
> are therefore not found when creating pre/main/post loops for the slow 
> and fast loop) then the C2 matcher can hit the bad AD file assert when 
> over-unrolling the main loop. In the test case some data paths die 
> because CastII nodes for an array load are replaced by TOP. This 
> happens after Opaque1 nodes are removed and the type information about 
> the maximum number of iterations for the pre-loop propagate to the 
> induction variable of the main loop. IGVN concludes that the induction 
> variable is out-of-bounds for some CastII nodes which are then 
> replaced by TOP. As a result, the graph contains vectorized results 
> with a TOP memory input which cannot be handled by the matcher.
>
> [3] shows the IR before doing the first IGVN iteration where 
> major_progress() is false (Opaque1 nodes can be removed). The 719 
> Opaque1 is removed and the type information of 1063 MinI [int:0..13] 
> propagates to the induction variable of the pre-loop 701 Phi 
> [int:5..13] (loop starts with j = 5, at most 8 iterations of the 
> pre-loop) which then propagates over 716 CastII [int:6..14] -> 1665 
> Phi [int:6..max-31] (32 iterations after unrolling) -> 1537 AddI 
> [int:30..max-7] (adds 24) which is then compared with 1027 CastII 
> [int:5..7] (iArr[i]). [int:5..7] does not contain [int:30..max-7] and 
> therefore 1027 CastII is replaced with TOP.
>
> The main loop is dead but is not removed since there is no range check 
> predicate for the main loop that would notice that some loads are 
> always out-of-bounds due to the type information. The fix for 
> JDK-8203915 avoids that (for loops without unswitching) by adding 
> range check predicates for the main loop which are then updated while 
> unrolling and later allow the main loop to be removed.
>
> The only thing missing is that the created range check predicates for 
> a loop to be unswitched are not cloned to both unswitched loop 
> versions. Therefore, the changes for [1] cannot be applied since it 
> does not find any predicates when creating pre/main/post loops for the 
> slow and fast loop. This fix addresses that and clones all range check 
> predicates that have at least one control edge to a data node which is 
> part of the loop to be unswitched. All control edges to data nodes 
> which are part of either the slow or fast loop are updated to the new 
> cloned predicates accordingly. All old range predicates of the 
> original loop to be unswitched that do not have any control edges to 
> data nodes can be removed.
>
> This patch is also a prerequisite for the fix of JDK-8237859 [4]. I 
> included some renaming for predicate related code that should make the 
> difference between the various predicate types and how they are used 
> clearer (skeleton, concrete, empty, updated...). I also included many 
> more unswitch tests (PartialPeelingUnswitch.java) that I originally 
> wrote as part of an RFE in progress [5] but also helped with testing 
> this fix.
>
> Thank you!
>
> Best regards,
> Christian
>
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8203915
> [2] https://bugs.openjdk.java.net/browse/JDK-8193130
> [3] 
> https://bugs.openjdk.java.net/secure/attachment/87257/IR_before_IGVN_major_progress_false.png
> [4] https://bugs.openjdk.java.net/browse/JDK-8237859
> [5] https://bugs.openjdk.java.net/browse/JDK-8236722