RFR 8239090: Improve CPU feature support in VM_version

[Hohensee, Paul]

Taking over from Eric...

Thank you for the review, Igor. I took a completely different (and very old approach), however, and defined a method Abstract_VM_Version:: insert_features_names() that iterates over the feature flags set. If a feature bit is on, it appends to an output buffer a corresponding name string from an array indexed by the bit number. I've implemented it only for x86: using the mechanism for other platforms can be follow-on RFEs. I'd greatly appreciate a review.

Webrev: http://cr.openjdk.java.net/~phh/8239090/webrev.00/

To add a feature bit, all one now has to do is add a CPU_ definition and corresponding name string in the FEATURES_NAMES macro.

I've also included a few small changes to the x86 implementation beyond the above.

1. Unified the previous two bitset definitions into a single Feature_Flag enum and made it a uint64_t.
2. supports_tscinv_bit() referenced the CPU_TSCINV bit, which was a bit misleading, so added a new CPU_TSCINV_BIT mask and used it instead.
3. Repurposed CPU_TSCINV for supports_tscinv(), which was a "composite" property, but is now computed once in feature_flags().
4. Made supports_clflushopt() and supports_clwb() common to both 32 and 64-bit rather than have 32-bit versions that always return 'false'. These bits are never set by the hardware on 32-bit, so no need for separate methods.
5. Renamed CPU_HV_PRESENT to CPU_HV to conform with the CPU_ bit naming scheme. "_PRESENT" is redundant and not used for any other CPU_ name, and the feature string uses "hv", not "hv_present". Added CPU_HV to vmStructs_x86.hpp and vmStructs_jvmci.cpp.

Tested using -Xlog:os+cpu on my macbook pro: the same feature string is returned after the patch as before it. Suggestions for how to more thoroughly test the patch are very welcome.

Thanks,
Paul

On 8/27/20, 6:22 PM, "hotspot-compiler-dev on behalf of Igor Veresov" <hotspot-compiler-dev-retn at openjdk.java.net on behalf of igor.veresov at oracle.com> wrote:

    You can actually make a constexpr array of feature objects and then use constexpr function with a loop to look it up. The c++ compiler will generate an O(1) table lookup for it.
    That would be a good way to get rid of the ugly macro (we allow c++14 now).

    For example foo() in this example:

    enum E { a, b, c };

    struct P {
      E _e; // key
      int _v; // value
      constexpr P(E e, int v) : _e(e), _v(v) { }
    };


    constexpr static P ps[3] = { P(a, 0xdead), P(b, 0xbeef), P(c, 0xf00d)};

    constexpr int match(E e) {
      for (const auto& p : ps) {
        if (p._e == e) {
          return p._v;
        }
      }
      return -1;
    }


    int foo(E e) {
      return match(e);
    }

    Will be compiled into:

    __Z3foo1E:                              ## @_Z3foo1E
            .cfi_startproc
    ## %bb.0:
            movl    $-1, %eax
            cmpl    $2, %edi
            ja      LBB0_2
    ## %bb.1:
            pushq   %rbp
            .cfi_def_cfa_offset 16
            .cfi_offset %rbp, -16
            movq    %rsp, %rbp
            .cfi_def_cfa_register %rbp
            movslq  %edi, %rax
            leaq    l_switch.table._Z3foo1E(%rip), %rcx
            movq    (%rcx,%rax,8), %rax
            movl    4(%rax), %eax
            popq    %rbp
    LBB0_2:
            retq
            .cfi_endproc
                                            ## -- End function
            .section        __TEXT,__const
            .p2align        4               ## @_ZL2ps
    __ZL2ps:
            .long   0                       ## 0x0
            .long   57005                   ## 0xdead
            .long   1                       ## 0x1
            .long   48879                   ## 0xbeef
            .long   2                       ## 0x2
            .long   61453                   ## 0xf00d

            .section        __DATA,__const
            .p2align        3               ## @switch.table._Z3foo1E
    l_switch.table._Z3foo1E:
            .quad   __ZL2ps
            .quad   __ZL2ps+8
            .quad   __ZL2ps+16


    igor


    > On Aug 27, 2020, at 11:08 AM, Eric, Chan <jingxinc at amazon.com> wrote:
    >
    > Hi,
    >
    > Requesting review for
    >
    > Webrev : http://cr.openjdk.java.net/~phh/8239090/webrev.00/
    > JBS : https://bugs.openjdk.java.net/browse/JDK-8239090
    >
    > Yesterday I sent a wrong one, so I send it again,
    > I improve the “get_processor_features” method by store every cpu features in an enum array so that we don’t have to count how many “%s” that need to added. I passed the tier1 test successfully.
    >
    > Regards,
    > Eric Chen
    >