[jdk17] RFR: 8269580: assert(is_valid()) failed: invalid register (-1)

[Jatin Bhateja]

On Thu, 1 Jul 2021 08:45:00 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Hi @sviswa7 , in all the following code paths originating from rep_stos and rep_stos_evex instruction pattern EVEX encoded instructions are guarded by compile time checks, thus it should be safe to remove AVX512BW check from rep_stos and just enable it for UseAVX <= 2.
>> - fill64_avx:  Emits 64 byte mode if AVXThreshold=0 and UseAVX > 2 else it disintegrates to two 32 byte vex encoded moves.
>> - fill64_masked_avx : Is again guarded by AVX3Threshold = 0 and UseAVX > 2
>> - fill32_masked_avx : Guarded by UseAVX > 2 and AVX512VL == true.
>> 
>> This bug is related to clear operation over small non-constant length.
>
> @jatin-bhateja I see your point.  With your change, the predicates are enough to prevent the assert.  But I noticed an existing issue for both the small and large cases.   The predicates do not prevent some cases where a mask register will be reserved when it isn't really needed.  For example when UseFastStosb is TRUE or UseXMMForObjInit is FALSE.  It seems like a good followup RFE.

Hi @dean-long ,  yes extra opmask allocation will exist for non default control path, but its not a bug, and that's the penalty of packing mutiple flows in one pattern. We can do more cleanup in this flow as suggested.

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PR: https://git.openjdk.java.net/jdk17/pull/172