[llvm-exegesis] Fix compilation with old libpfm versions.

[courbet]

Summary

Do not try include perfmon/perf_event.h when we are not sure that it
exists.

Fixes PR51017.

[oontvoo]

Thanks!

[oontvoo]

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

19–23

I'm not sure why this block isn't enough to prevent the inclusion of all libpfm stuff.
(AFAIK, LIBPFM_HAS_FIELD_CYCLES signals a subset of features whereas HAVE_LIBPFM tells you if the library exsits)

[courbet]

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

19–23

The issue is only "perfmon/perf_event.h", which is what LIBPFM_HAS_FIELD_CYCLES tests: https://reviews.llvm.org/D84076

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

 //===-- X86Counter.cpp ------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "X86Counter.h"
 
+#if defined(HAVE_LIBPFM) && defined(LIBPFM_HAS_FIELD_CYCLES)
+
 // FIXME: Use appropriate wrappers for poll.h and mman.h
 // to support Windows and remove this linux-only guard.
 #ifdef __linux__
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Errc.h"
 
 #ifdef HAVE_LIBPFM
 #include "perfmon/perf_event.h"
 #include "perfmon/pfmlib.h"
 #include "perfmon/pfmlib_perf_event.h"
 #endif // HAVE_LIBPFM
 
 #include <atomic>
 #include <chrono>
 #include <cstddef>
 #include <cstdint>
 #include <limits>
 #include <memory>
 #include <vector>
 
 #include <poll.h>
 #include <sys/mman.h>
 #include <unistd.h>
 
-#if defined(HAVE_LIBPFM) && defined(LIBPFM_HAS_FIELD_CYCLES)
 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();
 
llvm::Error X86LbrCounter::checkLbrSupport() {
 
   for (int I = 0;
        I < kLbrEntries || std::chrono::high_resolution_clock::now() < TimeLimit;
        ++I) {
     Sum += *P;
   }
 
   counter.stop();
+  (void)Sum;
 
   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();