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

[Corey Ashford]

Michihiro Horie uploaded a new revision of the Base64 decodeBlock 
intrinsic API for me:

http://cr.openjdk.java.net/~mhorie/8248188/webrev.01/

It has the following changes with respect to the original one posted:

  * In the event of encountering a non-base64 character, instead of 
having a separate error code of -1, the intrinsic can now just return 
either 0, or the number of data bytes produced up to the point where the 
illegal base64 character was encountered.  This reduces the number of 
special cases, and also provides a way to speed up the process of 
finding the bad character by the slower, pure-Java algorithm.

  * The isMIME boolean is removed from the API for two reasons:
    - The current API is not sufficient to handle the isMIME case, 
because there isn't a strict relationship between the number of input 
bytes and the number of output bytes, because there can be an arbitrary 
number of non-base64 characters in the source.
    - If an intrinsic only implements the (isMIME == false) case as ours 
does, it will always return 0 bytes processed, which will slightly slow 
down the normal path of processing an (isMIME == true) instantiation.
    - We considered adding a separate hotspot candidate for the (isMIME 
== true) case, but since we don't have an intrinsic implementation to 
test that, we decided to leave it as a future optimization.

Comments and suggestions are welcome.  Thanks for your consideration.

- Corey

On 6/23/20 6:23 PM, Michihiro Horie wrote:
> 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
> 
> Inactive hide details for "Corey Ashford" ---2020/06/24 
> 09:40:10---Currently in java.util.Base64, there is a 
> HotSpotIntrinsicCa"Corey Ashford" ---2020/06/24 09:40:10---Currently in 
> java.util.Base64, there is a HotSpotIntrinsicCandidate and API for 
> encodeBlock, but no
> 
> 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]
> 
>