RFR: 8364766: C2: Improve Value() of DivI and DivL for non-constant inputs [v2]

[Manuel Hässig]

On Sun, 10 Aug 2025 12:27:55 GMT, Tobias Hotz <duke at openjdk.org> wrote:

> The main reason for all the if cases is that min_int / (-1) is undefined behavior in C++, as it overflows. All code has to be careful that this special case can't happen in C++ code, and that's the main motivation behind all the ifs. I've added a comment that describes that.
Otherwise, you would be right: Redudant calculations are no problem, min and max would take care of that.

Then I would suggest restructuring the code to express that intent. Reading it currently, gives me the impression that you care about handling the four corners carefully when you really only care about the UB case. The following pseudocode uses ifs to only avoid the corner case. It's only slightly different from your version, but I find it a bit clearer to guess its intent. What do you think?


if i1.lo == MIN_INT && (i2.lo == -1 || i2.hi  == -1) {
    new_lo = MIN_INT
    if i1.hi == MIN_INT { // is_con()
        new_hi = MAX_INT // (MIN_INT + 1) / -1
        return (new_lo, new_hi) // This is already the entire domain, so we can return early
    }
    if i2.lo != i2.hi {
        corner i1.lo, (i2.lo == -1 ? i2.hi : i2.lo) // corner is just shorthand for setting new_lo and new_hi
    }
} else {
    // i1.lo > MIN_INT
    corner i1.lo, i2.lo
    corner i1.lo, i2.hi
}
// i1.hi > MIN_INT because of early return
corner i1.hi, i2.lo
corner i1.hi, i2.hi

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PR Comment: https://git.openjdk.org/jdk/pull/26143#issuecomment-3173647591