RFR 8239090: Improve CPU feature support in VM_version

[Hohensee, Paul]

Thank you for the review, Igor.

I did indeed define FEATURES_NAMES to be close to the flags enum definition. And, as a macro, it might be useful in some future context. I also defined four CPU_ enum values and strings per line so it’s easy to keep track of the correspondence.

Anyone else up for a review please? I expect I’ll have to turn this into a PR though.

Paul

From: Igor Veresov <igor.veresov at oracle.com>
Date: Friday, September 4, 2020 at 4:04 PM
To: "Hohensee, Paul" <hohensee at amazon.com>
Cc: "hotspot-compiler-dev at openjdk.java.net" <hotspot-compiler-dev at openjdk.java.net>
Subject: RE: RFR 8239090: Improve CPU feature support in VM_version

This looks good. Did you make FEATURES_NAMES a macro just so that it’s close to the flags enum?

igor





On Sep 4, 2020, at 10:39 AM, Hohensee, Paul <hohensee at amazon.com<mailto:hohensee at amazon.com>> wrote:

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