RFR[M]: 8151779: Some intrinsic flags could be replaced with one general flag

[Vladimir Kozlov]

I withdraw my suggestion about EnableIntrinsic from JDK-8151779 because ControlIntrinsics will provide such 
functionality and will replace existing DisableIntrinsic.

Note, we can start deprecating Use*Intrinsic flags (and DisableIntrinsic) later in other changes. You don't need to do 
everything at once. What we need now a mechanism to replace them.

On 4/16/20 11:58 PM, Liu, Xin wrote:
> Hi, Corey and Vladimir,
> 
> I recently go through vmSymbols.hpp/cpp. I think I understand your comments.
> Each UseXXXIntrinsics does control a bunch of intrinsics (plural). Thanks for the hint.
> 
> Even though I feel I know intrinsics mechanism of hotspot better, I still need a clarification of JDK- 8151779.
> 
> There're 321 intrinsics (https://chriswhocodes.com/hotspot_intrinsics_jdk15.html).
> If there's no any option, they are all available for compilers.  That makes sense because intrinsics are always beneficial.
> But there're reasons we need to disable a subset of them. A specific architecture may miss efficient instructions or fixed functions. Or simply because an intrinsic is buggy.
> 
> Currently, JDK provides developers 2 ways to control intrinsics. > 1. Some diagnostic options. Eg. InlineMathNatives, UseBase64Intrinsics.
> Developers can use one option to disable a group of intrinsics.  That is to say, it's a coarse-grained approach.
>   
> 2. DisableIntrinsic="a,b,c"
> By passing a string list of vmIntrinsics::IDs, it's capable of disabling any specified intrinsic.
> 
> But even putting above 2 approaches together, we still can't precisely control any intrinsic.

Yes, you are right. We seems are trying to put these 2 different ways into one flag which may be mistake.

-XX:ControlIntrinsic=-_updateBytesCRC32C,-_updateDirectByteBufferCRC32C is a similar to -XX:-UseCRC32CIntrinsics but it 
requires more detailed knowledge about intrinsics ids.

May be we can have 2nd flag, as you suggested -XX:UseIntrinsics=-AESCTR,+CRC32C, for such cases.

> If we want to enable an intrinsic which is under control of InlineMathNatives but keep others disable, it's impossible now.  [please correct if I am wrong here].

You can disable all other from 321 intrinsics with DisableIntrinsic flag which is very tedious I agree.

> I think that the motivation JDK-8151779 tried to solve.

The idea is that instead of flags we use to control particular intrinsics depending on CPU we will use vmIntrinsics::IDs 
or other tables as you showed in your changes. It will require changes in vm_version_<cpu> codes.

> 
> If we provide a new option EnableIntrinsic and put it least priority, then we can precisely control any intrinsic.
> Quote Vladimir Kozlov "DisableIntrinsic list prevails if an intrinsic is specified on both EnableIntrinsic and DisableIntrinsic."
> 
>   "-XX:ControlIntrinsic=+_dabs,-_fabs,-_getClass" looks more elegant, but it will confuse developers with DisableIntrinsic.
> If we decide to deprecate DisableIntrinsic, I think ControlIntrinsic may be a better option. Now I prefer to provide EnableIntrinsic for simplicity and symmetry.

I prefer to have one ControlIntrinsic flag and deprecate DisableIntrinsic. I don't think it is confusing.

Thanks,
Vladimir

> What do you think?
> 
> Thanks,
> --lx
> 
> 
> On 4/13/20, 1:47 PM, "hotspot-compiler-dev on behalf of Corey Ashford" <hotspot-compiler-dev-bounces at openjdk.java.net on behalf of cjashfor at linux.ibm.com> wrote:
> 
>      CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
>      On 4/13/20 10:33 AM, Liu, Xin wrote:
>      > Hi, compiler developers,
>      > I attempt to refactor UseXXXIntrinsics for JDK-8151779.  I think we still need to keep UseXXXIntrinsics options because many applications may be using them.
>      >
>      > My change provide 2 new features:
>      > 1) a shorthand to enable/disable intrinsics.
>      > A comma-separated string. Each one is an intrinsic. An optional tailing symbol + or '-' denotes enabling or disabling.
>      > If the tailing symbol is missing, it means enable.
>      > Eg. -XX:UseIntrinsics="AESCTR-,CRC32C+,CRC32-,MathExact"
>      > This jvm option will expand to multiple options -XX:-UseAESCTRIntrinsics, -XX:+UseCRC32CIntrinsics, -XX:-UseCRC32Intrinsics, -XX:UseMathExactIntrinsics
>      >
>      > 2) provide a set of macro to declare intrinsic options
>      > Developers declare once in intrinsics.hpp and macros will take care all other places.
>      > Here are example: https://cr.openjdk.java.net/~xliu/8151779/00/webrev/src/hotspot/share/compiler/intrinsics.hpp.html
>      > Ion Lam is overhauling jvm options.  I am thinking how to be consistent with his proposal.
>      >
> 
>      Great idea, though to be consistent with the original syntax, I think
>      the +/- should be in front of the name:
> 
>      -XX:UseIntrinsics=-AESCTR,+CRC32C,...
> 
> 
>      > I handle UseIntrinsics before VM_Version::initialize. It means that platform-specific initialization still has a chance to correct those options.
>      > If we do that after VM_Version::initialize,  some intrinsics may cause JVM crash.  Eg. +UseBase64Intrinsics on x86_64 Linux.
>      > Even though this behavior is same as -XX:+UseXXXIntrinsics, from user's perspective, it's not straightforward when JVM overrides what users specify implicitly. It's dilemma here,  stable jvm or fidelity of cmdline.  What do you think?
>      >
>      > Another problem is naming convention. Almost all intrinsics options use UseXXXIntrinsics. One exception is UseVectorizedMismatchIntrinsic.
>      > Personally, I think it should be "UseXXXIntrinsic" because one option is for one intrinsic, right?  Is it possible to change this name convention?
> 
>      Some (many?) intrinsic options turn on more than one .ad instruct
>      instrinsic, or library instrinsics at the same time.  I think that's why
>      the plural is there.  Also, consistently adding the plural allows you to
>      add more capabilities to a flag that initially only had one intrinsic
>      without changing the plurality (and thus backward compatibility).
> 
>      Regards,
> 
>      - Corey
> 
>