Revert "[llvm-exegesis] Add option to check the hardware support for a given feature before benchmarking."

This reverts commit 4fcd1a8e6528ca42fe656f2745e15d2b7f5de495 as
`llvm/test/tools/llvm-exegesis/X86/lbr/mov-add.s` failed on hosts
without LBR supported if the build has LIBPFM enabled. On that host,
`perf_event_open` fails with `EOPNOTSUPP` on LBR config. That change's
basic assumption

> If this is run on a non-supported hardware, it will produce all zeroes for latency.

could not stand as `perf_event_open` system call will fail if the
underlying hardware really don't have LBR supported.

test/tools/llvm-exegesis/X86/lbr/lit.local.cfg

@@ -19,9 +19,9 @@ else:
       try:
           with open(os.devnull, 'w') as quiet:
               check_llvm_exegesis_uops_result = subprocess.call(
-                [llvm_exegesis_exe, '-mode', 'uops', '-snippets-file', '/dev/null'], stdout=quiet, stderr=quiet)
+                [llvm_exegesis_exe, '-allowed-host-cpu', 'skylake', '-allowed-host-cpu', 'skylake-avx512', '-mode', 'uops', '-snippets-file', '/dev/null'], stdout=quiet, stderr=quiet)
               check_llvm_exegesis_latency_result = subprocess.call(
-                [llvm_exegesis_exe, '-mode', 'latency', '-snippets-file', '/dev/null'], stdout=quiet, stderr=quiet)
+                [llvm_exegesis_exe, '-allowed-host-cpu', 'skylake', '-allowed-host-cpu', 'skylake-avx512', '-mode', 'latency', '-snippets-file', '/dev/null'], stdout=quiet, stderr=quiet)
       except OSError:
           print('could not exec llvm-exegesis')
           config.unsupported = True

tools/llvm-exegesis/lib/Target.h

@@ -142,11 +142,6 @@ public:
     return {&Instr};
   }
 
-  // Checks hardware and software support for current benchmark mode.
-  // Returns an error if the target host does not have support to run the
-  // benchmark.
-  virtual Error checkFeatureSupport() const { return Error::success(); }
-
   // Creates a snippet generator for the given mode.
   std::unique_ptr<SnippetGenerator>
   createSnippetGenerator(InstructionBenchmark::ModeE Mode,

tools/llvm-exegesis/lib/X86/Target.cpp

@@ -674,23 +674,6 @@ private:
     return Arch == Triple::x86_64 || Arch == Triple::x86;
   }
 
-  Error checkFeatureSupport() const override {
-    // LBR is the only feature we conditionally support now.
-    // So if LBR is not requested, then we should be able to run the benchmarks.
-    if (LbrSamplingPeriod == 0)
-      return Error::success();
-
-#if defined(__linux__) && defined(HAVE_LIBPFM) &&                              \
-    defined(LIBPFM_HAS_FIELD_CYCLES)
-    // If the kernel supports it, the hardware still may not have it.
-    return X86LbrCounter::checkLbrSupport();
-#else
-    return llvm::make_error<llvm::StringError>(
-        "LBR not supported on this kernel and/or platform",
-        llvm::errc::not_supported);
-#endif
-  }
-
   static const unsigned kUnavailableRegisters[4];
 };
 

tools/llvm-exegesis/lib/X86/X86Counter.cpp

@@ -21,7 +21,6 @@
 #endif // HAVE_LIBPFM
 
 #include <atomic>
-#include <chrono>
 #include <cstddef>
 #include <cstdint>
 #include <limits>
@@ -36,8 +35,6 @@
 namespace llvm {
 namespace exegesis {
 
-// Number of entries in the LBR.
-static constexpr int kLbrEntries = 16;
 static constexpr size_t kBufferPages = 8;
 static const size_t kDataBufferSize = kBufferPages * getpagesize();
 
@@ -73,6 +70,7 @@ static void copyDataBuffer(void *MMappedBuffer, char *Buf, uint64_t Tail,
 static llvm::Error parseDataBuffer(const char *DataBuf, size_t DataSize,
                                    const void *From, const void *To,
                                    llvm::SmallVector<int64_t, 4> *CycleArray) {
+  assert(From != nullptr && To != nullptr);
   const char *DataPtr = DataBuf;
   while (DataPtr < DataBuf + DataSize) {
     struct perf_event_header Header;
@@ -151,47 +149,21 @@ void X86LbrCounter::start() {
   ioctl(FileDescriptor, PERF_EVENT_IOC_REFRESH, 1024 /* kMaxPollsPerFd */);
 }
 
-llvm::Error X86LbrCounter::checkLbrSupport() {
-  // Do a sample read and check if the results contain non-zero values.
-
-  X86LbrCounter counter(X86LbrPerfEvent(123));
-  counter.start();
-
-  // Prevent the compiler from unrolling the loop and get rid of all the
-  // branches. We need at least 16 iterations.
-  int Sum = 0;
-  int V = 1;
-
-  volatile int *P = &V;
-  auto TimeLimit =
-      std::chrono::high_resolution_clock::now() + std::chrono::microseconds(5);
-
-  for (int I = 0;
-       I < kLbrEntries || std::chrono::high_resolution_clock::now() < TimeLimit;
-       ++I) {
-    Sum += *P;
-  }
-
-  counter.stop();
-
-  auto ResultOrError = counter.doReadCounter(nullptr, nullptr);
-  if (ResultOrError)
-    if (!ResultOrError.get().empty())
-      // If there is at least one non-zero entry, then LBR is supported.
-      for (const int64_t &Value : ResultOrError.get())
-        if (Value != 0)
-          return Error::success();
-
-  return llvm::make_error<llvm::StringError>(
-      "LBR format with cycles is not suppported on the host.",
-      llvm::errc::not_supported);
-}
-
 llvm::Expected<llvm::SmallVector<int64_t, 4>>
 X86LbrCounter::readOrError(StringRef FunctionBytes) const {
+  // The max number of time-outs/retries before we give up.
+  static constexpr int kMaxTimeouts = 160;
+
   // Disable the event before reading
   ioctl(FileDescriptor, PERF_EVENT_IOC_DISABLE, 0);
 
+  // Parses the LBR buffer and fills CycleArray with the sequence of cycle
+  // counts from the buffer.
+  llvm::SmallVector<int64_t, 4> CycleArray;
+  std::unique_ptr<char[]> DataBuf(new char[kDataBufferSize]);
+  int NumTimeouts = 0;
+  int PollResult = 0;
+
   // Find the boundary of the function so that we could filter the LBRs
   // to keep only the relevant records.
   if (FunctionBytes.empty())
@@ -200,21 +172,6 @@ X86LbrCounter::readOrError(StringRef FunctionBytes) const {
   const void *From = reinterpret_cast<const void *>(FunctionBytes.data());
   const void *To = reinterpret_cast<const void *>(FunctionBytes.data() +
                                                   FunctionBytes.size());
-  return doReadCounter(From, To);
-}
-
-llvm::Expected<llvm::SmallVector<int64_t, 4>>
-X86LbrCounter::doReadCounter(const void *From, const void *To) const {
-  // The max number of time-outs/retries before we give up.
-  static constexpr int kMaxTimeouts = 160;
-
-  // Parses the LBR buffer and fills CycleArray with the sequence of cycle
-  // counts from the buffer.
-  llvm::SmallVector<int64_t, 4> CycleArray;
-  auto DataBuf = std::make_unique<char[]>(kDataBufferSize);
-  int NumTimeouts = 0;
-  int PollResult = 0;
-
   while (PollResult <= 0) {
     PollResult = pollLbrPerfEvent(FileDescriptor);
     if (PollResult > 0)

tools/llvm-exegesis/lib/X86/X86Counter.h

@@ -33,8 +33,6 @@ public:
 
 class X86LbrCounter : public pfm::Counter {
 public:
-  static llvm::Error checkLbrSupport();
-
   explicit X86LbrCounter(pfm::PerfEvent &&Event);
 
   virtual ~X86LbrCounter();
@@ -45,9 +43,6 @@ public:
   readOrError(StringRef FunctionBytes) const override;
 
 private:
-  llvm::Expected<llvm::SmallVector<int64_t, 4>>
-  doReadCounter(const void *From, const void *To) const;
-
   void *MMappedBuffer = nullptr;
 };
 

tools/llvm-exegesis/llvm-exegesis.cpp

@@ -160,6 +160,12 @@ static cl::opt<std::string>
                                       cl::desc(""), cl::cat(AnalysisOptions),
                                       cl::init(""));
 
+static cl::list<std::string>
+    AllowedHostCpus("allowed-host-cpu",
+                    cl::desc("If specified, only run the benchmark if the host "
+                             "CPU matches the names"),
+                    cl::cat(Options), cl::ZeroOrMore);
+
 static cl::opt<bool> AnalysisDisplayUnstableOpcodes(
     "analysis-display-unstable-clusters",
     cl::desc("if there is more than one benchmark for an opcode, said "
@@ -296,9 +302,12 @@ void benchmarkMain() {
 
   const LLVMState State(CpuName);
 
-  // Preliminary check to ensure features needed for requested
-  // benchmark mode are present on target CPU and/or OS.
-  ExitOnErr(State.getExegesisTarget().checkFeatureSupport());
+  llvm::StringRef ActualCpu = State.getTargetMachine().getTargetCPU();
+  for (auto Begin = AllowedHostCpus.begin(); Begin != AllowedHostCpus.end();
+       ++Begin) {
+    if (ActualCpu != *Begin)
+      ExitWithError(llvm::Twine("Unexpected host CPU ").concat(ActualCpu));
+  }
 
   const std::unique_ptr<BenchmarkRunner> Runner =
       ExitOnErr(State.getExegesisTarget().createBenchmarkRunner(