RFR: 8286125: C2: "bad AD file" with PopulateIndex on x86_64

[Vladimir Kozlov]

On Sat, 7 May 2022 13:23:54 GMT, Pengfei Li <pli at openjdk.org> wrote:

> A fuzzer test reports an assertion failure issue with PopulateIndexNode
> on x86_64. It can be reproduced by the new jtreg case inside this patch.
> Root cause is that C2 superword creates a PopulateIndexNode by mistake
> while vectorizing below loop.
> 
>     for (int i = 304; i > 15; i -= 3) {
>         int c = 16;
>         do {
>             for (int t = 1; t < 1; t++) {}
>             arr[c + 1] >>= i;
>         } while (--c > 0);
>     }
> 
> This is a corner loop case with redundant code inside. After several C2
> optimizations, the do-while loop inside is unrolled and then isomorphic
> right shift statements can be combined in the superword optimization.
> Since all shift counts are the same loop IV value `i`, superword should
> generate a RShiftCntVNode to create a vector of scalar replications of
> the loop IV. But after JDK-8280510, a PopulateIndexNode is generated by
> mistake because of the `opd == iv()` condition.
> 
> To fix this, we add a `have_same_inputs` condition here checking if all
> inputs at position `opd_idx` of nodes in the pack are the same. If true,
> C2 code should NOT run into this block to generate a PopulateIndexNode.
> Instead, it should run into the next block for scalar replications.
> 
> Additionally, only adding this condition here is still not good enough
> because it breaks the experimental post loop vectorization. As in post
> loops, all packs are singleton, i.e., `have_same_inputs` is always true.
> Hence, we also add a pack size check here to make post loop logic run
> into this block. It's safe to let it go because post loop never needs
> scalar replications of the loop IV - it never combines nodes in packs.
> 
> We also add two more assertions in the code.
> 
> Jtreg hotspot::hotspot_all_no_apps, jdk::tier1~3 and langtools::tier1
> are tested and no issue is found.

My tier1-4 testing passed clean.

-------------

Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/8587