RFR: 8364766: C2: Improve Value() of DivI and DivL for non-constant inputs [v6]

[Johannes Graham]

On Sat, 23 Aug 2025 09:05:10 GMT, Tobias Hotz <duke at openjdk.org> wrote:

>> This PR improves the value of interger division nodes.
>> Currently, we only emit a good type if either input is constant. But we can also cover the generic case. It does that by finding the four corners of the division. This is guranteed to find the extrema that we can use for min/max. Some special logic is required for MIN_INT / -1, though, as this is a special case
>> We also need some special logic to handle ranges that cross zero, but in this case, we just need to check for the negative and positive range once.
>> This also cleans up and unifies the code paths for DivINode and DivLNode.
>> I've added some tests to validate the optimization. Without the changes, some of these tests fail.
>
> Tobias Hotz has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove too strict assert from old code path

test/hotspot/jtreg/compiler/c2/irTests/IntegerDivValueTests.java line 49:

> 47:     public int testIntConstantFolding() {
> 48:         // All constants available during parsing
> 49:         return 50 / 25;

This will be constant-folded by javac, so won't exercise c2

test/hotspot/jtreg/compiler/c2/irTests/IntegerDivValueTests.java line 56:

> 54:     public int testIntConstantFoldingSpecialCase() {
> 55:         // All constants available during parsing
> 56:         return Integer.MIN_VALUE / -1;

This will also be folded by javac

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PR Review Comment: https://git.openjdk.org/jdk/pull/26143#discussion_r2298271180
PR Review Comment: https://git.openjdk.org/jdk/pull/26143#discussion_r2298272480