RFR: 8307352: AARCH64: Improve itable_stub

[Boris Ulasevich]

On Wed, 7 Jun 2023 13:42:44 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> This is a change for AARCH similar to https://github.com/openjdk/jdk/pull/13460 
>> 
>> The change replaces two separate iterations over the itable with a new algorithm consisting of two loops. First, we look for a match with resolved_klass, checking for a match with holder_klass along the way. Then we continue iterating (not starting over) the itable using the second loop, checking only for a match with holder_klass.
>> 
>> InterfaceCalls openjdk benchmark performance results on A53, A72, Neoverse N1 and V1 micro-architectures:
>> 
>> 
>> Cortex-A53 (Pi 3 Model B Rev 1.2)
>> 
>> test1stInt2Types    37.5      37.358     0.38
>> test1stInt3Types   160.166   148.04      8.19
>> test1stInt5Types   158.131   147.955     6.88
>> test2ndInt2Types    52.634    53.291    -1.23
>> test2ndInt3Types   201.39    181.603    10.90
>> test2ndInt5Types   195.722   176.707    10.76
>> testIfaceCall      157.453   140.498    12.07
>> testIfaceExtCall   175.46    154.351    13.68
>> testMonomorphic     32.052    32.039     0.04
>>                                  AVG:    6.85
>> 
>> Cortex-A72 (Pi 4 Model B Rev 1.2)
>> 
>> test1stInt2Types    27.4796   27.4738    0.02
>> test1stInt3Types    66.0085   64.9374    1.65
>> test1stInt5Types    67.9812   66.2316    2.64
>> test2ndInt2Types    32.0581   32.062    -0.01
>> test2ndInt3Types    68.2715   65.6643    3.97
>> test2ndInt5Types    68.1012   65.8024    3.49
>> testIfaceCall       64.0684   64.1811   -0.18
>> testIfaceExtCall    91.6226   81.5867   12.30
>> testMonomorphic     26.7161   26.7142    0.01
>>                                  AVG:    2.66
>> 
>> Neoverse N1 (m6g.metal)
>> 
>> test1stInt2Types     2.9104    2.9086    0.06
>> test1stInt3Types    10.9642   10.2909    6.54
>> test1stInt5Types    10.9607   10.2856    6.56
>> test2ndInt2Types     3.3410    3.3478   -0.20
>> test2ndInt3Types    12.3291   11.3089    9.02
>> test2ndInt5Types    12.328    11.2704    9.38
>> testIfaceCall       11.0598   10.3657    6.70
>> testIfaceExtCall    13.0692   11.2826   15.84
>> testMonomorphic      2.2354    2.2341    0.06
>>                                  AVG:    6.00
>> 
>> Neoverse V1 (c7g.2xlarge)
>> 
>> test1stInt2Types    2.2317     2.2320   -0.01
>> test1stInt3Types    6.6884     6.1911    8.03
>> test1stInt5Types    6.7334     6.2193    8.27
>> test2ndInt2Types    2.4002     2.4013   -0.04
>> test2ndInt3Types    7.9603     7.0372   13.12
>> test2ndInt5Types    7.9532     7.0474   12.85
>> testIfaceCall       6.7028     6.3272    5.94
>> testIfaceExtCall    8.3253     6.941...
>
> src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 1255:
> 
>> 1253:   //     }
>> 1254:   //   } while (temp_itbl_klass != 0);
>> 1255:   //   goto L_no_such_interface // Not found.
> 
> I don't think the pseudocode matches the assembly. This is more like
> 
> 
>   do {
>     temp_itbl_klass = *(scan_temp += scan_step);
>     if (holder_klass, temp_itbl_klass ) ...

Yes. Thanks!

> src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 1283:
> 
>> 1281:     bind(L_loop_scan_resolved_entry);
>> 1282:     cmp(holder_klass, temp_itbl_klass);
>> 1283:     csel(holder_offset, scan_temp, holder_offset, Assembler::EQ);
> 
> It's worth being cautious about conditional selects. In out-of-order machines they help when the outcome genuinely is unpredictable and the probability of each is close to 50-50. Benchmarks can fool us because they are often designed to be random, but real-world code rarely is.

Right. Thank you.
Besides micro-benchmarks I have seen good results on real applications.
I changed `csel` to branch. And it works even better. High noise makes it is hard to see the improvement, but it looks like this change makes us 0.25% faster on InterfaceCalls micro-benchmarks:

   // Loop: Look for resolved_class record in itable
+  //   L_save_holder_offset:
+  //     holder_offset = index;
   //   do {
-  //     index += step;
-  //     temp_itbl_klass = itable[index];
+  //     temp_itbl_klass = *(scan_temp += scan_step);
   //     if (temp_itbl_klass == holder_klass) {
-  //        holder_offset = index;
+  //        goto L_save_holder_offset;
   //     }
   //     if (temp_itbl_klass == resolved_klass) {
   //        goto L_resolved_found;  // Found!
   //     }
   //   } while (temp_itbl_klass != 0);
   //   goto L_no_such_interface // Not found.
   //
+  Label L_save_holder_offset;
+  bind(L_save_holder_offset);
+    mov(holder_offset, scan_temp);
+    // continue searching for resolved_found (checked above: resolved found != resolved class)
   Label L_loop_scan_resolved;
   bind(L_loop_scan_resolved);
     ldr(temp_itbl_klass, Address(pre(scan_temp, scan_step)));
     bind(L_loop_scan_resolved_entry);
     cmp(holder_klass, temp_itbl_klass);
-    csel(holder_offset, scan_temp, holder_offset, Assembler::EQ);
+    br(Assembler::EQ, L_save_holder_offset);
     cmp(resolved_klass, temp_itbl_klass);
     br(Assembler::EQ, L_resolved_found);
     cbnz(temp_itbl_klass, L_loop_scan_resolved);

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/13792#discussion_r1224339342
PR Review Comment: https://git.openjdk.org/jdk/pull/13792#discussion_r1224338913