Bug ID: JDK-8349128 C2 SuperWord: generalize PopulateIndex patterns with constant vector

Type: Enhancement
Component: hotspot
Sub-Component: compiler
Affected Version: 25

Priority: P4
Status: Open
Resolution: Unresolved

Submitted: 2025-01-31
Updated: 2025-01-31

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tbdUnresolved

There are many cases where we would benefit from a constant vector.

One example we found here:
https://github.com/openjdk/jdk/pull/22856 / JDK-8346664

        for (int i = 0; i < count; i++) {
            dst[i] = src[i] * (i & 7);
        }

Because

    // Does not vectorize: due to sum-under-mask optimization.
    // (i+0) & 7, (i+1) & 7 ... (i+8) & 7 ....  -> PopulateIndex
    // becomes
    // (i+0) & 7, (i+1) & 7 ... (i+0) & 7 .... -> pattern broken

We could just introduce some constants vector, and add this instead of the iota_indices.

That would allow us to generalize it to any i+c0, i+c1, i+c2, ....