RFR: 8356813: Improve Mod(I|L)Node::Value [v7]

[Quan Anh Mai]

On Tue, 26 Aug 2025 12:46:31 GMT, Hannes Greule <hgreule at openjdk.org> wrote:

>> This change improves the precision of the `Mod(I|L)Node::Value()` functions.
>> 
>> I reordered the structure a bit. First, we handle constants, afterwards, we handle ranges. The bottom checks seem to be excessive (`Type::BOTTOM` is covered by using `isa_(int|long)()`, the local bottom is just the full range). Given we can even give reasonable bounds if only one input has any bounds, we don't want to return early.
>> The changes after that are commented. Please let me know if the explanations are good, or if you have any suggestions.
>> 
>> ### Monotonicity
>> 
>> Before, a 0 divisor resulted in `Type(Int|Long)::POS`. Initially I wanted to keep it this way, but that violates monotonicity during PhaseCCP. As an example, if we see a 0 divisor first and a 3 afterwards, we might try to go from `>=0` to `-2..2`, but the meet of these would be `>=-2` rather than `-2..2`. Using `Type(Int|Long)::ZERO` instead (zero is always in the resulting value if we cover a range).
>> 
>> ### Testing
>> 
>> I added tests for cases around the relevant bounds. I also ran tier1, tier2, and tier3 but didn't see any related failures after addressing the monotonicity problem described above (I'm having a few unrelated failures on my system currently, so separate testing would be appreciated in case I missed something).
>> 
>> Please review and let me know what you think.
>> 
>> ### Other
>> 
>> The `UMod(I|L)Node`s were adjusted to be more in line with its signed variants. This change diverges them again, but similar improvements could be made after #17508.
>> 
>> During experimenting with these changes, I stumbled upon a few things that aren't directly related to this change, but might be worth to further look into:
>> - If the divisor is a constant, we will directly replace the `Mod(I|L)Node` with more but less expensive nodes in `::Ideal()`. Type analysis for these nodes combined is less precise, means we miss potential cases were this would help e.g., removing range checks. Would it make sense to delay the replacement?
>> - To force non-negative ranges, I'm using `char`. I noticed that method parameters of sub-int integer types all fall back to `TypeInt::INT`. This seems to be an intentional change of https://github.com/openjdk/jdk/commit/200784d505dd98444c48c9ccb7f2e4df36dcbb6a. The bug report is private, so I can't really judge if that part is necessary, but it seems odd.
>
> Hannes Greule has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Nice consolidation also. I have only some small style suggestion.

src/hotspot/share/opto/divnode.cpp line 1220:

> 1218:   // Mod by zero?  Throw exception at runtime!
> 1219:   if (t2 == TypeInteger::zero(bt)) {
> 1220:     return TypeInt::TOP;

`TypeInt::TOP` is actually `Type::TOP`

src/hotspot/share/opto/divnode.cpp line 1225:

> 1223:   const TypeInteger* i1 = t1->isa_integer(bt);
> 1224:   const TypeInteger* i2 = t2->isa_integer(bt);
> 1225:   if (i1 == nullptr || i2 == nullptr) {

If they are not `TOP` here, `isa_integer` should never return `nullptr`, it's better to do an `assert` here.

src/hotspot/share/opto/divnode.cpp line 1269:

> 1267:     hi = MIN2(hi, i1->hi_as_long());
> 1268:   }
> 1269:   return TypeInteger::make(lo, hi, MAX2(i1->_widen,i2->_widen), bt);

Small style: space after comma.

-------------

Marked as reviewed by qamai (Committer).

PR Review: https://git.openjdk.org/jdk/pull/25254#pullrequestreview-3205479330
PR Review Comment: https://git.openjdk.org/jdk/pull/25254#discussion_r2336282089
PR Review Comment: https://git.openjdk.org/jdk/pull/25254#discussion_r2336297089
PR Review Comment: https://git.openjdk.org/jdk/pull/25254#discussion_r2336288184