RFR: 8288445: AArch64: C2 compilation fails with guarantee(!true || (true && (shift != 0))) failed: impossible encoding [v3]

[Dean Long]

On Thu, 23 Jun 2022 00:27:45 GMT, Hao Sun <haosun at openjdk.org> wrote:

>> Dean Long has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   remove unneeded lines
>
> test/hotspot/jtreg/compiler/codegen/ShiftByZero.java line 42:
> 
>> 40:     public static void bMeth() {
>> 41:         int shift = i32[0];
>> 42:         for (int i8 = 279; i8 > 1; --i8) {
> 
> Why one loop is used here? I thought low 6 bits of shift can turn to be zeros in one single iteration. Is there anything I missed? Thanks.

This is to confuse the optimizer, so that "shift" is optimized to 0 only after loop vectorization.  Otherwise, the back-end may never see such a value.

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PR: https://git.openjdk.org/jdk19/pull/40