RFR: 8347645: C2: XOR bounded value handling blocks constant folding [v33]

[Emanuel Peter]

On Tue, 25 Feb 2025 07:44:55 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Johannes Graham has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   update tests
>
> test/hotspot/jtreg/compiler/c2/irTests/XorINodeIdealizationTests.java line 334:
> 
>> 332:         var xor = (x & 0b111) ^ (y & 0b100);
>> 333:         return xor < 0b1000;
>> 334:     }
> 
> These are nice simple examples, and we should keep them. But I'm missing these cases with randomization.
> 
> `calc_xor_upper_bound_of_non_neg` basically has two input ranges `[0, hi_0]` and `[0, hi_1]`, and gives us a new range `[0,max]`.
> 
> You could produce random input ranges like this:
> 
> public void test(int x) {
>     int lo_x = con1;
>     int hi_x = con2;
>     x = x < lo_x ? lo_x : (x > hi_x ? hi_x : x);
>     // x clamped to [lo_x, hi_x]
>     int lo_y = con3;
>     int hi_y = con4;
>     y = y < lo_y ? lo_y : (y > hi_y ? hi_y : y);
>     // y clamped to [lo_y, hi_y]
>     int z = x ^ y;
>     // This should now have a new range, possibly some [0, max]
>     // Now let's test the range with some random if branches.
>     int sum = 0;
>     if (z > somecon1) { sum += 1; }
>     if (z > somecon2) { sum += 2; }
>     if (z > somecon3) { sum += 4; }
>     // maybe add a few more...
>     if (z > someconi) { sum += pow(2,i); }
>     return sum;
> }
> 
> The `sum` at the end gives you a summary over all the checks. If one wrongly constant folds, you'll be missing one of the power of 2 contributions to it, or have it wrongly added.
> Now you do this with an `x` and a `y`
> 
> Maybe that's a little over-engineered, but it would target the `calc_xor_upper_bound_of_non_neg` logic really well.
> What do you think?

FYI I'm working on fuzzing the compiler currently, so I'm thinking about how to write such tests more generally, that's why I took the time to come up with this. If you have better ideas, I'm more than happy to see them ;)

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PR Review Comment: https://git.openjdk.org/jdk/pull/23089#discussion_r1969164548