RFR: 8284883: JVM crash: guarantee(sect->end() <= sect->limit()) failed: sanity on AVX512

[Vladimir Kozlov]

On Thu, 28 Apr 2022 19:42:05 GMT, Dean Long <dlong at openjdk.org> wrote:

> This fix prevents overflowing the C2 scratch buffer for large ClearArray operations.  I also noticed that when IdealizeClearArrayNode is turned off, the "is_large" flag on the ClearArray node was not set correctly, so I fixed that too.
> 
> I could use some help testing the x86_32 change.

I think we should fix code in `MacroAssembler::clear_mem()` to generate loop code (4 in loop and remaining after it) instead of line of instructions if more then 8 64-bytes move instructions are generated.
Even with 256 value, you suggested, there will be 32 instructions. Originally it was assumed that `!is_large()`  will be true for arrays with < InitArrayShortSize (64) so you will have only 8 instructions. But, as you said, InitArrayShortSize could be set to ridiculous value.
Also forcing use Mach instruction with value loaded into register may affect spilling in surrounding code.

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PR: https://git.openjdk.java.net/jdk/pull/8457