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

[Hao Sun]

On Wed, 22 Jun 2022 03:46:41 GMT, Dean Long <dlong at openjdk.org> wrote:

>> The range for aarch64 vector right-shift is 1 to the element width.  This issue fixes the problem in the back-end.  There is a separate problem in the front-end that shift by 0 is not always optimized out.
>
> 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.

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