RFR(S): 8248188: [PATCH] Add HotSpotIntrinsicCandidate and API for Base64 decoding

[Michihiro Horie]

Hi Corey,

Following is the issue I created.
https://bugs.openjdk.java.net/browse/JDK-8248188

I will upload a webrev when you're ready as we talked in private.

Best regards,
Michihiro



From:	"Corey Ashford" <cjashfor at linux.ibm.com>
To:	"hotspot-compiler-dev at openjdk.java.net"
            <hotspot-compiler-dev at openjdk.java.net>,
            "ppc-aix-port-dev at openjdk.java.net"
            <ppc-aix-port-dev at openjdk.java.net>
Cc:	Michihiro Horie/Japan/IBM at IBMJP, Kazunori
            Ogata/Japan/IBM at IBMJP, joserz at br.ibm.com
Date:	2020/06/24 09:40
Subject:	RFR(S): [PATCH] Add HotSpotIntrinsicCandidate and API for
            Base64 decoding



Currently in java.util.Base64, there is a HotSpotIntrinsicCandidate and
API for encodeBlock, but none for decoding.  This means that only
encoding gets acceleration from the underlying CPU's vector hardware.

I'd like to propose adding a new intrinsic for decodeBlock.  The
considerations I have for this new intrinsic's API:

  * Don't make any assumptions about the underlying capability of the
hardware.  For example, do not impose any specific block size granularity.

  * Don't assume the underlying intrinsic can handle isMIME or isURL
modes, but also let them decide if they will process the data regardless
of the settings of the two booleans.

  * Any remaining data that is not processed by the intrinsic will be
processed by the pure Java implementation.  This allows the intrinsic to
process whatever block sizes it's good at without the complexity of
handling the end fragments.

  * If any illegal character is discovered in the decoding process, the
intrinsic will simply return -1, instead of requiring it to throw a
proper exception from the context of the intrinsic.  In the event of
getting a -1 returned from the intrinsic, the Java Base64 library code
simply calls the pure Java implementation to have it find the error and
properly throw an exception.  This is a performance trade-off in the
case of an error (which I expect to be very rare).

  * One thought I have for a further optimization (not implemented in
the current patch), is that when the intrinsic decides not to process a
block because of some combination of isURL and isMIME settings it
doesn't handle, it could return extra bits in the return code, encoded
as a negative number.  For example:

Illegal_Base64_char   = 0b001;
isMIME_unsupported    = 0b010;
isURL_unsupported     = 0b100;

These can be OR'd together as needed and then negated (flip the sign).
The Base64 library code could then cache these flags, so it will know
not to call the intrinsic again when another decodeBlock is requested
but with an unsupported mode.  This will save the performance hit of
calling the intrinsic when it is guaranteed to fail.

I've tested the attached patch with an actual intrinsic coded up for
Power9/Power10, but those runtime intrinsics and arch-specific patches
aren't attached today.  I want to get some consensus on the
library-level intrinsic API first.

Also attached is a simple test case to test that the new intrinsic API
doesn't break anything.

I'm open to any comments about this.

Thanks for your consideration,

- Corey


Corey Ashford
IBM Systems, Linux Technology Center, OpenJDK team
cjashfor at us dot ibm dot com
[attachment "decodeBlock_api-20200623.patch" deleted by Michihiro
Horie/Japan/IBM] [attachment "TestBase64.java" deleted by Michihiro
Horie/Japan/IBM]

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