RFR: 8332442: C2: refactor Mod cases in Compile::final_graph_reshaping_main_switch() [v2]

[Roland Westrelin]

On Tue, 10 Sep 2024 14:49:39 GMT, Kangcheng Xu <kxu at openjdk.org> wrote:

>> test/hotspot/jtreg/compiler/c2/TestDivModNodes.java line 34:
>> 
>>> 32:  * @test
>>> 33:  * @summary Test DIV and MOD nodes are converted into DIVMOD where possible
>>> 34:  * @requires os.arch=="amd64" | os.arch=="x86_64"
>> 
>> So this can run on aarch64 now?
>
> I was under the impression aarch64 cannot calculate quotient and remainder with a single div instruction (unlike x86), and I assumed that's why existing tests on divmod doesn't include this arch. Looking at the source, only `src/hotspot/cpu/x86/x86_64.ad` and `s390.ad` seems to contain divmod related instructions.

`UseDivMod` is true on all platforms but if there's no hardware support for `Op_DivModI` then this code:

        } else {
          // replace a%b with a-((a/b)*b)
          Node* mult = new MulINode(d, d->in(2));
          Node* sub  = new SubINode(d->in(1), mult);
          n->subsume_by(sub, this);
        }

computes the `ModI` from the `DivI` result and removes the `DivI`. That's what you could match on aarch64.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20877#discussion_r1752155128