RFR: 8261671: X86 I2L conversion can be skipped for certain masked positive values [v7]

[Marcus G K Williams]

On Mon, 8 Mar 2021 21:09:33 GMT, Nils Eliasson <neliasso at openjdk.org> wrote:

>> Marcus G K Williams has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix copyright formatting, log description
>>   
>>   Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>
>
> test/hotspot/jtreg/compiler/codegen/BMI2.java line 56:
> 
>> 54:             z = BMITests.bzhiI2L(ix, i);
>> 55: 
>> 56:             for (int i2 = 0; i2 < ITERATIONS; i2++) {
> 
> Why are these manually unrolled? They are the same but with different index?
> 
> I mean something like:
> 
>         for (int i = 0; i <= 15; i++) {
>             z = BMITests.bzhiI2L(ix, i);
> 
>             for (int i2 = 0; i2 < ITERATIONS; i2++) {
>                 long ii = BMITests.bzhiI2L(ix, i);
>                 if (ii != z) {
>                     throw new Error(returnBzhiI2LErrMessage (z, ii));
>                 }
>             }
>         }

I wasn't able to get the above to C2 compile separately. I wrote essentially the same but could not verify the compilation. I then unrolled from the for loop and could verify the bzhi instructions were generated. Repetition felt like a reasonable compromise for completeness. I'll take another look and see if I can verify in a for loop. Perhaps one of the options I was using prevented it from working.

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PR: https://git.openjdk.java.net/jdk/pull/2590