[jdk11u-dev] RFR: 8267652: c2 loop unrolling by 8 results in reading memory past array

[Martin Doerr]

On Wed, 6 Oct 2021 18:05:52 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

> The backport can't be applied clean to jdk11 ( due to the miss of 8223347: Integration of Vector API (Incubator) )  and needed big rework.
> 
> The main idea behind the backport is to add new check to functions with VecS or VecD argument ( those who can read less than 16 bytes from memory) and not touch VecX/VecY/VecZ types.
> 
> With LoopMaxUnroll=8
> The problematic place before the patch :
> 
>   0x000000011dd55603: vmovq  0x10(%r10,%rsi,1),%xmm0  <---loading 8 bytes from memory
>   0x000000011dd5560a: vpxor  0x10(%r11,%rsi,1),%xmm0,%xmm0 <--- loading 16 bytes from memory, it's not right
>   0x000000011dd55611: vmovq  %xmm0,0x10(%r13,%rsi,1)  ;*bastore {reexecute=0 rethrow=0 return_oop=0}
>                                                 ; - repro::xor_array at 18 (line 12)
> 
> and after the patch:
> 
>   0x000000011f9a4d95: vmovq  0x10(%r11,%rsi,1),%xmm0
>   0x000000011f9a4d9c: vmovq  0x10(%r10,%rsi,1),%xmm1
>   0x000000011f9a4da3: vpxor  %xmm1,%xmm0,%xmm0
>   0x000000011f9a4da7: vmovq  %xmm0,0x10(%r13,%rsi,1)  ;*bastore {reexecute=0 rethrow=0 return_oop=0}
>                                                 ; - repro::xor_array at 18 (line 12)
> 
> 
> With LoopMaxUnroll=16 it's works the old way ( as expected), loading 16 bytes from memory with vmovdqu and vpxor
> 
>    0x000000011e2b8903: vmovdqu 0x10(%r10,%rsi,1),%xmm0
>   0x000000011e2b890a: vpxor  0x10(%r11,%rsi,1),%xmm0,%xmm0
>   0x000000011e2b8911: vmovdqu %xmm0,0x10(%r13,%rsi,1)  ;*bastore {reexecute=0 rethrow=0 return_oop=0}
> 
> Testing is pending.
> 
> Also fixed a typo at https://github.com/openjdk/jdk11u-dev/pull/488/files#diff-d6a3624f0f0af65a98a47378a5c146eed5016ca09b4de1acd0a3acc823242e82L9069

Marked as reviewed by mdoerr (Reviewer).

Test results are good. Hopefully, nobody will backport other nodes which require the addition.

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PR: https://git.openjdk.java.net/jdk11u-dev/pull/488