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

[Liu, Xin]

Hi, Vladimir, 

Thanks for the clarification. 
Oh, yes, it's theoretically possible, but it's tedious. I am wrong at that point.
I think I got your point. ControlIntrinsics will make developer's life easier. I will implement it. 

Thanks,
--lx


On 4/17/20, 6:46 PM, "Vladimir Kozlov" <vladimir.kozlov at oracle.com> wrote:

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    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:
    >
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    >
    >
    >
    >      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
    >
    >