SPARC <was> Re: RFR 8151163 All Buffer implementations should leverage Unsafe unaligned accessors

[Paul Sandoz]

Hi John,

Thanks for the thoughts and advice.

At this point i plan to proceed with the current patch (i will add tests for buffer views), and then follow up more more analysis and recommendations for SPARC. The situation is a little murky after some further performance analysis, non-optimal loop unrolling and address calculations may be dominating. Without eyeballing generated code and shedding some light on this i am fumbling around in the crepuscule.

Paul.

> On 31 Mar 2016, at 08:33, John Rose <john.r.rose at oracle.com> wrote:
> 
> On Mar 30, 2016, at 2:36 AM, Paul Sandoz <paul.sandoz at oracle.com <mailto:paul.sandoz at oracle.com>> wrote:
>> 
>> When access is performed in loops this can cost, as the alignment checks are not hoisted out. Theoretically could for regular 2, 4, 8 strides through the buffer contents. For such cases alignment of the base address can be checked. Not sure how complicated that would be to support.
>> 
>> I lack knowledge of the SPARC instruction set to know if we could do something clever as an intrinsic.
> 
> A couple of partial thoughts:
> 
> If we had bitfield type inference, we would be able to deduce that the low bits of p and p+8 are the same.  Graal has this (because I gave them the formulae[1]).  C2 may be too brittle to add it into TypeInt.  Bitfield inference on expressions of the form p&7 and (p+8)&7 would allow commoning tests in an unrolled loop, hoisting the alignment logic to the top of the loop body, and (perhaps) through the phi to the loop head.
> 
> [1]: http://hg.openjdk.java.net/graal/graal-core/file/ea5cc66ec5f2/graal/com.oracle.graal.compiler.common/src/com/oracle/graal/compiler/common/type/IntegerStamp.java#l460 <http://hg.openjdk.java.net/graal/graal-core/file/ea5cc66ec5f2/graal/com.oracle.graal.compiler.common/src/com/oracle/graal/compiler/common/type/IntegerStamp.java#l460>
> 
> An intrinsic that would guide the JIT more explicitly would (I think) need an extra argument.  Something like:  getLongUnaligned(p, uo, ao), where uo and ao are both longs, but only ao is required to be naturally aligned ((ao&7)==0).  Yuck.  Maybe this could be pattern-matched in C2; it would be a kludge.
> 
> — John