RFR: 8346664: C2: Optimize mask check with constant offset [v5]

[Quan Anh Mai]

On Fri, 24 Jan 2025 15:15:33 GMT, Matthias Ernst <duke at openjdk.org> wrote:

>> Matthias Ernst has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fully randomized
>
> src/hotspot/share/opto/mulnode.cpp line 2112:
> 
>> 2110: 
>> 2111:   jint zeros = AndIL_min_trailing_zeros(phase, expr, bt);
>> 2112:   return zeros > 0 && ((((jlong)1) << zeros) > mask_t->hi_as_long() && mask_t->lo_as_long() >= 0);
> 
> FWIW I came up with another way to formulate this. I think it makes even more clear how this is all about "expr is shifted to be completely left of the mask", i.e. we're comparing right-most value bit position to left-most mask bit position, all other bits are irrelevant.
> 
> This could also inform how to bias the random generator. Let me know what you think.
> 
> Suggestion:
> 
>   jint expr_trailing_zeros = AndIL_min_trailing_zeros(phase, expr, bt);
>   if (expr_trailing_zeros == 0) return false;
> 
>   const TypeInteger* mask_t = phase->type(mask)->isa_integer(bt);
>   if (mask_t == nullptr || mask_t->lo_as_long() <= 0) return false; // mask = 0 handled in MulNode::Value
>   jint mask_bit_width = BitsPerLong - count_leading_zeros(mask_t->hi_as_long());
> 
>   return expr_trailing_zeros >= mask_bit_width;

`mask_t->lo_as_long() == 0` does not imply `mask == 0`, though. Other than that I think it is a great suggestion.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22856#discussion_r1928917715