CR for RFR 8153998

Berg, Michael C michael.c.berg at
Wed Apr 13 21:35:39 UTC 2016

See below for context.


From: Christian Thalinger [mailto:christian.thalinger at]
Sent: Wednesday, April 13, 2016 2:08 PM
To: Berg, Michael C <michael.c.berg at>
Cc: hotspot-compiler-dev at
Subject: Re: CR for RFR 8153998

On Apr 12, 2016, at 8:26 PM, Berg, Michael C <michael.c.berg at<mailto:michael.c.berg at>> wrote:

Hi Folks,

I would like to contribute Programmable SIMD as implemented on multi-versioned post loops.  See: for the first half of the implementation.
This component delivers mask programmed post loops which execute in a single iteration in place of fixup scalar loops which used to take many iterations to complete work for user loops.
Currently I have enabled this optimization for x86 only, specifically for machines with masked data predication implemented as per fully enabled EVEX targets.  It delivers up to 2x performance and has been modeled over a large number of loop lengths and forms of loops.

This code was tested as follows (see jbs entry below):




+// Machine function Msk Node

+class MachMskNode : public MachIdealNode {
Does “Msk” mean mask?  Then we should call it MachMaskNode.

<MCB> Ok, that’s easy enough.

Also, I don’t quite understand why we have:

+instruct set_mask(rRegI dst, rRegI src) %{

+  predicate(VM_Version::supports_avx512vl());

+  match(Set dst (MaskCreateI src));

+  effect(TEMP dst);

+  format %{ "createmsk   $dst, $src" %}

+  ins_encode %{

+    __ createmsk($dst$$Register, $src$$Register);

+  %}

+  void MachMskNode::emit(CodeBuffer &cbuf, PhaseRegAlloc*) const {

+    MacroAssembler _masm(&cbuf);

+    __ restoremsk();

+  }

The reason: Currently k registers or mask registers are not allocated, meaning we have to treat their usage as side effects.
The case with set_mask is to take our remaining iterations as an index and provide a mask used in k1 that is applicable to its post loop.
The subsequent restore, preplaces the default value back into k1.  The set_mask rule is posed in such a way that it ensures that the side effect value will survive optimization.
The restore is fully a side effect with no produced definition in rule space as mask registers are not formal definitions.


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