[PATCH] D57647: [llvm-exegesis] Throughput support in analysis mode

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: courbet, gchatelet.
lebedev.ri added a project: LLVM.
Herald added a subscriber: tschuett.

D57000 <https://reviews.llvm.org/D57000> / PR37698 <https://bugs.llvm.org/show_bug.cgi?id=37698> added support for measuring of the inverse throughput.
But the support for the analysis was not added.
This attempts to fix that. (analysis done o bdver2 / piledriver)

First, small-scale experiment:

  $ ./bin/llvm-exegesis -num-repetitions=10000 -mode=inverse_throughput -opcode-name=BSF64rr
  Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-d0acdd.o
  ---
  mode:            inverse_throughput
  key:             
    instructions:    
      - 'BSF64rr RAX RDX'
    config:          ''
    register_initial_values: 
      - 'RDX=0x0'
  cpu_name:        bdver2
  llvm_triple:     x86_64-unknown-linux-gnu
  num_repetitions: 10000
  measurements:    
    - { key: inverse_throughput, value: 3.0278, per_snippet_value: 3.0278 }
  error:           ''
  info:            instruction has no tied variables picking Uses different from defs
  assembled_snippet: 48BA0000000000000000480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2480FBCC2C3
  ...

If we plug `bsfq	%r12, %r10` into llvm-mca:
https://godbolt.org/z/ZtOyhJ

  Dispatch Width:    4
  uOps Per Cycle:    3.00
  IPC:               0.50
  Block RThroughput: 2.0

So RThroughput mismatch exists.

Now, let's upscale and analyse:
F8207148: benchmarks-inverse_throughput.yaml <https://reviews.llvm.org/F8207148>
`$ ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html`:
F8207172: clusters.html <https://reviews.llvm.org/F8207172>
F8207197: Screenshot_20190203_133559.png <https://reviews.llvm.org/F8207197>
And if we now look at https://www.agner.org/optimize/instruction_tables.pdf,
`Reciprocal throughput` for `BSF r,r` is listed as `3`.
Yay?


Repository:
  rL LLVM

https://reviews.llvm.org/D57647

Files:
  tools/llvm-exegesis/lib/Analysis.cpp


Index: tools/llvm-exegesis/lib/Analysis.cpp
===================================================================
--- tools/llvm-exegesis/lib/Analysis.cpp
+++ tools/llvm-exegesis/lib/Analysis.cpp
@@ -316,6 +316,7 @@
         writeLatencySnippetHtml(OS, Point.Key.Instructions, *InstrInfo_);
         break;
       case InstructionBenchmark::Uops:
+      case InstructionBenchmark::InverseThroughput:
         writeUopsSnippetHtml(OS, Point.Key.Instructions, *InstrInfo_);
         break;
       default:
@@ -507,9 +508,14 @@
       }
       ClusterCenterPoint[I].PerInstructionValue = Representative[I].avg();
     }
+  } else if (Mode == InstructionBenchmark::InverseThroughput) {
+    for (int I = 0, E = Representative.size(); I < E; ++I) {
+      SchedClassPoint[I].PerInstructionValue =
+          MCSchedModel::getReciprocalThroughput(STI, *RSC.SCDesc);
+      ClusterCenterPoint[I].PerInstructionValue = Representative[I].min();
+    }
   } else {
-    llvm::errs() << "unimplemented measurement matching for mode " << Mode
-                 << "\n";
+    llvm_unreachable("unimplemented measurement matching mode");
     return false;
   }
   return Clustering.isNeighbour(ClusterCenterPoint, SchedClassPoint);
@@ -519,9 +525,9 @@
                                        llvm::raw_ostream &OS) const {
   OS << "<table class=\"sched-class-desc\">";
   OS << "<tr><th>Valid</th><th>Variant</th><th>NumMicroOps</th><th>Latency</"
-        "th><th>WriteProcRes</th><th title=\"This is the idealized unit "
-        "resource (port) pressure assuming ideal distribution\">Idealized "
-        "Resource Pressure</th></tr>";
+        "th><th>RThroughput</th><th>WriteProcRes</th><th title=\"This is the "
+        "idealized unit resource (port) pressure assuming ideal "
+        "distribution\">Idealized Resource Pressure</th></tr>";
   if (RSC.SCDesc->isValid()) {
     const auto &SM = SubtargetInfo_->getSchedModel();
     OS << "<tr><td>✔</td>";
@@ -540,6 +546,12 @@
       OS << "</li>";
     }
     OS << "</ul></td>";
+    // inverse throughput.
+    OS << "<td>";
+    writeMeasurementValue<kEscapeHtml>(
+        OS,
+        MCSchedModel::getReciprocalThroughput(*SubtargetInfo_, *RSC.SCDesc));
+    OS << "</td>";
     // WriteProcRes.
     OS << "<td><ul>";
     for (const auto &WPR : RSC.NonRedundantWriteProcRes) {


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