RFR: ZGC: C2: Add MaxVectorSize support in C2 load barrier ad rules

[Rickard Bäckman]

Not a reviewer in zgc, but looks good to me.

/R

On 03/08, Nils Eliasson wrote:
> Hi,
> 
> This patch fixes failures when MaxVectorSize and the size of supported
> vectors doesn't correlate. It can be easily be reproduced with: "java
> -XX:+UseZGC -XX:MaxVectorSize=16" on any CPU with AVX2 support.
> 
> "assert((uint)_ideal_reg < (uint)_last_machine_leaf &&
> Type::mreg2type[_ideal_reg]) failed: in bounds)"
> 
> This bug is a big problem on many AMD systems (family < 0x17h) where
> MaxVectorSize is set default 16 even when larger vectors are supported.
> MaxVectorSize will limit the initialized vector types (TypeVect::VECTZ etc.)
> to those actually available. In the loadbarrier matcher we fail when we try
> to kill YMM but no YMM-type is initialized.
> 
> In this patch we instead have rules specialized on the number of vector regs
> that need to be killed, and only kill the lowest sub reg of the vector. This
> has the effect that the entire vector use is killed, but that only types
> that are guaranteed to be supported are available.
> 
> This patch was forgotten in a dev branch when we transfered to openJDK. It
> has been tested on the few different CPU familys available to me.
> 
> http://cr.openjdk.java.net/~neliasso/zgc/maxvectorsize
> 
> Regards,
> 
> Nils
>