[llvm-exegesis] Add error for subprocess and dummy perf counters

[aidengrossman]

Summary

When using the subprocess execution mode and dummy perf counters
currently, cryptic errors will be printed out related to not being able
to send the file descriptor through a socket. This patch prints out an
explicit message that this configuration isn't (currently) supported.

[aidengrossman]

The plan is to support this configuration at some point (probably soon) as it should be fairly useful in a downstream application I'm working on, but for now we should report a better error message.

[aidengrossman]

Bump on this when reviewers have a chance. Thanks!

llvm/test/tools/llvm-exegesis/X86/dummy-perf-counters-subprocess.s

+# REQUIRES: exegesis-can-measure-latency, x86_64-linux
+
+# RUN: not llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -snippets-file=%s -execution-mode=subprocess -use-dummy-perf-counters 2>&1 | FileCheck %s
+
+# CHECK: llvm-exegesis error: Dummy perf counters are not supported in the subprocess execution mode.
+
+mov $0, %rax

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

#endif
   const LLVMState State =
       ExitOnErr(LLVMState::Create(TripleName, MCPU, "", UseDummyPerfCounters));
 
   // Preliminary check to ensure features needed for requested
   // benchmark mode are present on target CPU and/or OS.
   if (BenchmarkPhaseSelector == BenchmarkPhaseSelectorE::Measure)
     ExitOnErr(State.getExegesisTarget().checkFeatureSupport());
 
+  if (ExecutionMode == BenchmarkRunner::ExecutionModeE::SubProcess &&
+      UseDummyPerfCounters)
+    ExitWithError("Dummy perf counters are not supported in the subprocess "
+                  "execution mode.");
+
   const std::unique_ptr<BenchmarkRunner> Runner =
       ExitOnErr(State.getExegesisTarget().createBenchmarkRunner(
           BenchmarkMode, State, BenchmarkPhaseSelector, ExecutionMode,
           ResultAggMode));
   if (!Runner) {
     ExitWithError("cannot create benchmark runner");
   }