RFR 8239090: Improve CPU feature support in VM_version

[Hohensee, Paul]

Slightly adjusted patch.

http://cr.openjdk.java.net/~phh/8239090/webrev.02/

Thanks,
Paul

On 9/3/20, 3:47 PM, "hotspot-compiler-dev on behalf of Hohensee, Paul" <hotspot-compiler-dev-retn at openjdk.java.net on behalf of hohensee at amazon.com> wrote:

    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
        >