RFR: 8347459: C2: missing transformation for chain of shifts/multiplications by constants [v5]

[Damon Fenacci]

On Thu, 27 Feb 2025 07:54:34 GMT, Marc Chevalier <duke at openjdk.org> wrote:

>> This collapses double shift lefts by constants in a single constant: (x << con1) << con2 => x << (con1 + con2). Care must be taken in the case con1 + con2 is bigger than the number of bits in the integer type. In this case, we must simplify to 0.
>> 
>> Moreover, the simplification logic of the sign extension trick had to be improved. For instance, we use `(x << 16) >> 16` to convert a 32 bits into a 16 bits integer, with sign extension. When storing this into a 16-bit field, this can be simplified into simple `x`. But in the case where `x` is itself a left-shift expression, say `y << 3`, this PR makes the IR looks like `(y << 19) >> 16` instead of the old `((y << 3) << 16) >> 16`. The former logic didn't handle the case where the left and the right shift have different magnitude. In this PR, I generalize this simplification to cases where the left shift has a larger magnitude than the right shift. This improvement was needed not to miss vectorization opportunities: without the simplification, we have a left shift and a right shift instead of a single left shift, which confuses the type inference.
>> 
>> This also works for multiplications by powers of 2 since they are already translated into shifts.
>> 
>> Thanks,
>> Marc
>
> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
> 
>   + comment on why not zerocon

I just left one more small comment. Otherwise it looks good to me. Thanks @marc-chevalier!

test/hotspot/jtreg/compiler/c2/irTests/LShiftINodeIdealizationTests.java line 178:

> 176: 
> 177:         short[] arr = new short[1];
> 178:         arr[0] = (short)1;

What do you think about using random short value here? (I was just thinking it might slightly increase the chances of spotting if something is wrong with the shifts...)

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Marked as reviewed by dfenacci (Committer).

PR Review: https://git.openjdk.org/jdk/pull/23728#pullrequestreview-2666681057
PR Review Comment: https://git.openjdk.org/jdk/pull/23728#discussion_r1984716376