[Exegesis] Add the ability to dry-run the measurement phase (#121991)

With the new benchmark phase, `dry-run-measurement`, llvm-exegesis can
run everything except the actual snippet execution. It is useful when we
want to test some parts of the code between the `assemble-measured-code`
and `measure` phase without actually running on native platforms.

Author: mshockwave

llvm/docs/CommandGuide/llvm-exegesis.rst

@@ -301,6 +301,7 @@ OPTIONS
   * ``prepare-and-assemble-snippet``: Same as ``prepare-snippet``, but also dumps an excerpt of the sequence (hex encoded).
   * ``assemble-measured-code``: Same as ``prepare-and-assemble-snippet``. but also creates the full sequence that can be dumped to a file using ``--dump-object-to-disk``.
   * ``measure``: Same as ``assemble-measured-code``, but also runs the measurement.
+  * ``dry-run-measurement``: Same as measure, but does not actually execute the snippet.
 
 .. option:: --x86-lbr-sample-period=<nBranches/sample>
 

llvm/test/tools/llvm-exegesis/dry-run-measurement.test

@@ -0,0 +1,11 @@
+# RUN: llvm-exegesis --mtriple=riscv64 --mcpu=sifive-p470 --mode=latency --opcode-name=ADD --use-dummy-perf-counters --benchmark-phase=dry-run-measurement | FileCheck %s
+# REQUIRES: riscv-registered-target
+
+# This test makes sure that llvm-exegesis doesn't execute "cross-compiled" snippets in the presence of
+# --dry-run-measurement. RISC-V was chosen simply because most of the time we run tests on X86 machines.
+
+# Should not contain misleading results.
+# CHECK: measurements:    []
+
+# Should not contain error messages like "snippet crashed while running: Segmentation fault".
+# CHECK: error:           ''

llvm/tools/llvm-exegesis/lib/BenchmarkResult.h

@@ -38,6 +38,7 @@ enum class BenchmarkPhaseSelectorE {
   PrepareAndAssembleSnippet,
   AssembleMeasuredCode,
   Measure,
+  DryRunMeasure,
 };
 
 enum class BenchmarkFilter { All, RegOnly, WithMem };

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

@@ -99,22 +99,25 @@ class InProcessFunctionExecutorImpl : public BenchmarkRunner::FunctionExecutor {
   static Expected<std::unique_ptr<InProcessFunctionExecutorImpl>>
   create(const LLVMState &State, object::OwningBinary<object::ObjectFile> Obj,
          BenchmarkRunner::ScratchSpace *Scratch,
-         std::optional<int> BenchmarkProcessCPU) {
+         std::optional<int> BenchmarkProcessCPU, bool DryRun) {
     Expected<ExecutableFunction> EF =
         ExecutableFunction::create(State.createTargetMachine(), std::move(Obj));
 
     if (!EF)
       return EF.takeError();
 
     return std::unique_ptr<InProcessFunctionExecutorImpl>(
-        new InProcessFunctionExecutorImpl(State, std::move(*EF), Scratch));
+        new InProcessFunctionExecutorImpl(State, std::move(*EF), Scratch,
+                                          DryRun));
   }
 
 private:
   InProcessFunctionExecutorImpl(const LLVMState &State,
                                 ExecutableFunction Function,
-                                BenchmarkRunner::ScratchSpace *Scratch)
-      : State(State), Function(std::move(Function)), Scratch(Scratch) {}
+                                BenchmarkRunner::ScratchSpace *Scratch,
+                                bool DryRun)
+      : State(State), Function(std::move(Function)), Scratch(Scratch),
+        DryRun(DryRun) {}
 
   static void accumulateCounterValues(const SmallVector<int64_t, 4> &NewValues,
                                       SmallVector<int64_t, 4> *Result) {
@@ -143,9 +146,14 @@ class InProcessFunctionExecutorImpl : public BenchmarkRunner::FunctionExecutor {
       CrashRecoveryContext CRC;
       CrashRecoveryContext::Enable();
       const bool Crashed = !CRC.RunSafely([this, Counter, ScratchPtr]() {
-        Counter->start();
-        this->Function(ScratchPtr);
-        Counter->stop();
+        if (DryRun) {
+          Counter->start();
+          Counter->stop();
+        } else {
+          Counter->start();
+          this->Function(ScratchPtr);
+          Counter->stop();
+        }
       });
       CrashRecoveryContext::Disable();
       PS.reset();
@@ -177,6 +185,7 @@ class InProcessFunctionExecutorImpl : public BenchmarkRunner::FunctionExecutor {
   const LLVMState &State;
   const ExecutableFunction Function;
   BenchmarkRunner::ScratchSpace *const Scratch;
+  bool DryRun = false;
 };
 
 #ifdef __linux__
@@ -664,21 +673,29 @@ Expected<std::unique_ptr<BenchmarkRunner::FunctionExecutor>>
 BenchmarkRunner::createFunctionExecutor(
     object::OwningBinary<object::ObjectFile> ObjectFile,
     const BenchmarkKey &Key, std::optional<int> BenchmarkProcessCPU) const {
+  bool DryRun =
+      BenchmarkPhaseSelector == BenchmarkPhaseSelectorE::DryRunMeasure;
+
   switch (ExecutionMode) {
   case ExecutionModeE::InProcess: {
     if (BenchmarkProcessCPU.has_value())
       return make_error<Failure>("The inprocess execution mode does not "
                                  "support benchmark core pinning.");
 
     auto InProcessExecutorOrErr = InProcessFunctionExecutorImpl::create(
-        State, std::move(ObjectFile), Scratch.get(), BenchmarkProcessCPU);
+        State, std::move(ObjectFile), Scratch.get(), BenchmarkProcessCPU,
+        DryRun);
     if (!InProcessExecutorOrErr)
       return InProcessExecutorOrErr.takeError();
 
     return std::move(*InProcessExecutorOrErr);
   }
   case ExecutionModeE::SubProcess: {
 #ifdef __linux__
+    if (DryRun)
+      return make_error<Failure>("The subprocess execution mode cannot "
+                                 "dry-run measurement at this moment.");
+
     auto SubProcessExecutorOrErr = SubProcessFunctionExecutorImpl::create(
         State, std::move(ObjectFile), Key, BenchmarkProcessCPU);
     if (!SubProcessExecutorOrErr)

llvm/tools/llvm-exegesis/lib/Target.cpp

@@ -98,7 +98,7 @@ ExegesisTarget::createBenchmarkRunner(
     return nullptr;
   case Benchmark::Latency:
   case Benchmark::InverseThroughput:
-    if (BenchmarkPhaseSelector == BenchmarkPhaseSelectorE::Measure &&
+    if (BenchmarkPhaseSelector >= BenchmarkPhaseSelectorE::Measure &&
         !PfmCounters.CycleCounter) {
       const char *ModeName = Mode == Benchmark::Latency
                                  ? "latency"
@@ -116,7 +116,7 @@ ExegesisTarget::createBenchmarkRunner(
         State, Mode, BenchmarkPhaseSelector, ResultAggMode, ExecutionMode,
         ValidationCounters, BenchmarkRepeatCount);
   case Benchmark::Uops:
-    if (BenchmarkPhaseSelector == BenchmarkPhaseSelectorE::Measure &&
+    if (BenchmarkPhaseSelector >= BenchmarkPhaseSelectorE::Measure &&
         !PfmCounters.UopsCounter && !PfmCounters.IssueCounters)
       return make_error<Failure>(
           "can't run 'uops' mode, sched model does not define uops or issue "

llvm/tools/llvm-exegesis/llvm-exegesis.cpp

@@ -132,7 +132,10 @@ static cl::opt<BenchmarkPhaseSelectorE> BenchmarkPhaseSelector(
         clEnumValN(
             BenchmarkPhaseSelectorE::Measure, "measure",
             "Same as prepare-measured-code, but also runs the measurement "
-            "(default)")),
+            "(default)"),
+        clEnumValN(
+            BenchmarkPhaseSelectorE::DryRunMeasure, "dry-run-measurement",
+            "Same as measure, but does not actually execute the snippet")),
     cl::init(BenchmarkPhaseSelectorE::Measure));
 
 static cl::opt<bool>
@@ -476,7 +479,7 @@ static void runBenchmarkConfigurations(
 }
 
 void benchmarkMain() {
-  if (BenchmarkPhaseSelector == BenchmarkPhaseSelectorE::Measure &&
+  if (BenchmarkPhaseSelector >= BenchmarkPhaseSelectorE::Measure &&
       !UseDummyPerfCounters) {
 #ifndef HAVE_LIBPFM
     ExitWithError(
@@ -501,7 +504,7 @@ void benchmarkMain() {
 
   // Preliminary check to ensure features needed for requested
   // benchmark mode are present on target CPU and/or OS.
-  if (BenchmarkPhaseSelector == BenchmarkPhaseSelectorE::Measure)
+  if (BenchmarkPhaseSelector >= BenchmarkPhaseSelectorE::Measure)
     ExitOnErr(State.getExegesisTarget().checkFeatureSupport());
 
   if (ExecutionMode == BenchmarkRunner::ExecutionModeE::SubProcess &&