[llvm-exegesis] Fix X86LbrCounter destructor to correctly unmap memory and not double-close fd (PR50620)

[RKSimon]

Summary

As was reported on PR50620, the X86LbrCounter destructor was double-closing the filedescriptor and not unmapping the buffer.

[oontvoo]

LGTM - Thanks!

[RKSimon]

Cheers - I've now confirmed that this works locally

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

 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();
 
+// First page is reserved for perf_event_mmap_page. Data buffer starts on
+// the next page, so we allocate one more page.
+static const size_t kMappedBufferSize = (kBufferPages + 1) * getpagesize();
+
 // Waits for the LBR perf events.
 static int pollLbrPerfEvent(const int FileDescriptor) {
   struct pollfd PollFd;
   PollFd.fd = FileDescriptor;
   PollFd.events = POLLIN;
   PollFd.revents = 0;
   return poll(&PollFd, 1 /* num of fds */, 10000 /* timeout in ms */);
 }
X86LbrPerfEvent::X86LbrPerfEvent(unsigned SamplingPeriod) {
   Attr->exclude_hv = 1;
   Attr->read_format = PERF_FORMAT_GROUP;
 
   FullQualifiedEventString = EventString;
 }
 
 X86LbrCounter::X86LbrCounter(pfm::PerfEvent &&NewEvent)
     : Counter(std::move(NewEvent)) {
-  // First page is reserved for perf_event_mmap_page. Data buffer starts on
-  // the next page, so we allocate one more page.
-  MMappedBuffer = mmap(nullptr, (kBufferPages + 1) * getpagesize(),
-                       PROT_READ | PROT_WRITE, MAP_SHARED, FileDescriptor, 0);
+  MMappedBuffer = mmap(nullptr, kMappedBufferSize, PROT_READ | PROT_WRITE,
+                       MAP_SHARED, FileDescriptor, 0);
   if (MMappedBuffer == MAP_FAILED)
     llvm::errs() << "Failed to mmap buffer.";
 }
 
-X86LbrCounter::~X86LbrCounter() { close(FileDescriptor); }
+X86LbrCounter::~X86LbrCounter() {
+  if (0 != munmap(MMappedBuffer, kMappedBufferSize))
+    llvm::errs() << "Failed to munmap buffer.";
+}
 
 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.