RFR: 8347459: C2: missing transformation for chain of shifts/multiplications by constants [v9]

[Marc Chevalier]

On Mon, 10 Mar 2025 15:33:48 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Random testing, trying...
>
> src/hotspot/share/opto/memnode.cpp line 3549:
> 
>> 3547: // Let's also remember that conIL < 32 since (x << 33) is simplified into (x << 1)
>> 3548: // and (x << 31) << 2 is simplified into 0. This means that in any case, after the
>> 3549: // left shift, we always have at least one bit of the original v.
> 
> What does `original v` refer to? Is this the same as the `X` and the `valIn`?

rephrased

> src/hotspot/share/opto/memnode.cpp line 3577:
> 
>> 3575: // +------------------+---------+-----+
>> 3576: //  31               8 7       2 1   0
>> 3577: // The non-rejected bits are the 8 lower one of (v << conIL - conIR).
> 
> Suggestion:
> 
> // The non-rejected bits are the 8 lower ones of (v << conIL - conIR).

nice.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23728#discussion_r1988873142
PR Review Comment: https://git.openjdk.org/jdk/pull/23728#discussion_r1988872799