[Bug 92929] Add a NIR pass to lower indirects to conditional load/store operations.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92929

            Bug ID: 92929
           Summary: Add a NIR pass to lower indirects to conditional
                    load/store operations.
           Product: Mesa
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: medium
         Component: glsl-compiler
          Assignee: idr at freedesktop.org
          Reporter: jason at jlekstrand.net
        QA Contact: intel-3d-bugs at lists.freedesktop.org

One of the things that NIR is currently missing is a pass to lower indirect
load/store operations to if-ladders.  For example, an indirect load on a
5-element array would be rewritten as

if (i < 2) {
    if (i < 1) {
        a = foo[0];
    } else {
        b = foo[1];
    }
    c = phi(a, b);
} else {
    if (i < 3) {
        d = foo[2];
    } else {
        if (i < 4) {
            e = foo[3];
        } else {
            f = foo[4];
        }
        g = phi(e, f);
    }
    h = phi(d, g);
}
result = phi(c, h);

where the if-ladder effectively does a binary search over the space of possible
indirect values.  Implementing this shouldn't be terribly hard from a compiler
theory perspective; it's mostly an exercise in recursion.  Lots of recursion...

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