RFR: 8366815: C2: Delay Mod/Div by constant transformation

[Manuel Hässig]

On Sun, 19 Oct 2025 15:46:06 GMT, Hannes Greule <hgreule at openjdk.org> wrote:

> The test cases show examples of code where `Value()` previously wasn't run because idealization took place before, resulting in less precise type analysis.
> 
> Please let me know what you think.

Thinking about it a bit more, I think your fix is too superficial. If the discovery of the constant is slightly delayed, nothing is folded again. Consider the followig program for an example:

class Test {
    static boolean test(int x, boolean flag) {
        Integer a;
        if (flag) {
            a = 171384;
        } else {
            a = 2902;
        }

        return x % a >= a;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 20000; i++) {
            if (test(i, false)) {
                throw new RuntimeException("wrong result");
            }
        }
    }
}


In my opinion, the benefits do not outweigh the drawbacks for this PR. A better solution would probably be to delay the expansion of the Mod and Div nodes to post-loop optimizations and extend Superword to expand Div/Mod nodes to shifts. However, this is quite a bit of complexity, which raises if this complexity is worth it (@eme64 probably has opinions and/or guidance on this).

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PR Comment: https://git.openjdk.org/jdk/pull/27886#issuecomment-3432972151