[jdk16] RFR: 8259775: [Vector API] Incorrect code-gen for VectorReinterpret operation [v5]

[Rickard Bäckman]

On Tue, 19 Jan 2021 03:57:16 GMT, Jie Fu <jiefu at openjdk.org> wrote:

>> Hi all,
>> 
>> The code-gen for VectorReinterpret may be wrong on x86.
>> 
>> Let's see the opto-assembly for the reproducer in the JBS, which was actually based on @XiaohongGong 's example in JDK-8259353 and many thanks to her.
>> 066     B7: #   out( N1 ) <- in( B6 )  Freq: 0.999994
>> 066     vector_reinterpret_expand XMM0,XMM0     !
>> 066     store_vector [R12 + R11 << 3 + #16] (compressed oop addressing),XMM0
>>  
>> Please note that the dst and src [1] share the same XMM0 register and movdqu [2] should be generated for this case.
>> But when dst == src, movdqu actually generates nothing [3], which leads to incorrect result;
>> 
>> For this case, movdqu should not be empty since the upper bits of dst should be zeroed.
>> The similar error also exists for vmovdqu [4].
>> 
>> I think we should also change movflt [5] to movss but I just can't understand why we have 4-byte vectors.
>> Isn't the shortest vectors 8-byte on x86?
>> 
>> Thanks.
>> Best regards,
>> Jie
>> 
>> [1] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/x86.ad#L3354
>> [2] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/x86.ad#L3364
>> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/macroAssembler_x86.cpp#L2490
>> [4] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/macroAssembler_x86.cpp#L2515
>> [5] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/x86.ad#L3379
>
> Jie Fu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Refine the test to always reproduce the bug

Marked as reviewed by rbackman (Reviewer).

Marked as reviewed by rbackman (Reviewer).

Marked as reviewed by rbackman (Reviewer).

Marked as reviewed by rbackman (Reviewer).

Marked as reviewed by rbackman (Reviewer).

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PR: https://git.openjdk.java.net/jdk16/pull/122