Reland 293e8fa

    [llvm-exegesis] Disable the LBR check on AMD

    https://bugs.llvm.org/show_bug.cgi?id=48918

    The bug reported a hang (or very very slow runtime) on a Zen2. Unfortunately, we don't have the hardware right now to debug it and I was not able to reproduce the bug on a HSW.
    Theory we've got is that the lbr-checking code could be confused on AMD.

    Differential Revision: https://reviews.llvm.org/D97504

New change:
 - Surround usages of x86 helper in llvm-exegesis/X86/Target.cpp with ifdef
 - Fix bug which caused the caller of getVendorSignature to not have a copy of EAX that it expected.

llvm/include/llvm/Support/Host.h

@@ -65,6 +65,20 @@ namespace sys {
   StringRef getHostCPUNameForARM(StringRef ProcCpuinfoContent);
   StringRef getHostCPUNameForS390x(StringRef ProcCpuinfoContent);
   StringRef getHostCPUNameForBPF();
+
+  /// Helper functions to extract CPU details from CPUID on x86.
+  namespace x86 {
+  enum class VendorSignatures {
+    UNKNOWN,
+    GENUINE_INTEL,
+    AUTHENTIC_AMD,
+  };
+
+  /// Returns the host CPU's vendor.
+  /// MaxLeaf: if a non-nullptr pointer is specified, the EAX value will be
+  /// assigned to its pointee.
+  VendorSignatures getVendorSignature(unsigned *MaxLeaf = nullptr);
+  } // namespace x86
   }
 }
 }

llvm/lib/Support/Host.cpp

@@ -420,11 +420,6 @@ StringRef sys::detail::getHostCPUNameForBPF() {
 #if defined(__i386__) || defined(_M_IX86) || \
     defined(__x86_64__) || defined(_M_X64)
 
-enum VendorSignatures {
-  SIG_INTEL = 0x756e6547 /* Genu */,
-  SIG_AMD = 0x68747541 /* Auth */
-};
-
 // The check below for i386 was copied from clang's cpuid.h (__get_cpuid_max).
 // Check motivated by bug reports for OpenSSL crashing on CPUs without CPUID
 // support. Consequently, for i386, the presence of CPUID is checked first
@@ -498,6 +493,42 @@ static bool getX86CpuIDAndInfo(unsigned value, unsigned *rEAX, unsigned *rEBX,
 #endif
 }
 
+namespace llvm {
+namespace sys {
+namespace detail {
+namespace x86 {
+
+VendorSignatures getVendorSignature(unsigned *MaxLeaf) {
+  unsigned EAX = 0, EBX = 0, ECX = 0, EDX = 0;
+  if (MaxLeaf == nullptr)
+    MaxLeaf = &EAX;
+  else
+    *MaxLeaf = 0;
+
+  if (!isCpuIdSupported())
+    return VendorSignatures::UNKNOWN;
+
+  if (getX86CpuIDAndInfo(0, MaxLeaf, &EBX, &ECX, &EDX) || *MaxLeaf < 1)
+    return VendorSignatures::UNKNOWN;
+
+  // "Genu ineI ntel"
+  if (EBX == 0x756e6547 && ECX == 0x6c65746e && EDX == 0x49656e69)
+    return VendorSignatures::GENUINE_INTEL;
+
+  // "Auth enti cAMD"
+  if (EBX == 0x68747541 && ECX == 0x69746e65 && EDX == 0x444d4163)
+    return VendorSignatures::AUTHENTIC_AMD;
+
+  return VendorSignatures::UNKNOWN;
+}
+
+} // namespace x86
+} // namespace detail
+} // namespace sys
+} // namespace llvm
+
+using namespace llvm::sys::detail::x86;
+
 /// getX86CpuIDAndInfoEx - Execute the specified cpuid with subleaf and return
 /// the 4 values in the specified arguments.  If we can't run cpuid on the host,
 /// return true.
@@ -1095,14 +1126,12 @@ static void getAvailableFeatures(unsigned ECX, unsigned EDX, unsigned MaxLeaf,
 }
 
 StringRef sys::getHostCPUName() {
-  unsigned EAX = 0, EBX = 0, ECX = 0, EDX = 0;
-  unsigned MaxLeaf, Vendor;
-
-  if (!isCpuIdSupported())
+  unsigned MaxLeaf = 0;
+  const VendorSignatures Vendor = getVendorSignature(&MaxLeaf);
+  if (Vendor == VendorSignatures::UNKNOWN)
     return "generic";
 
-  if (getX86CpuIDAndInfo(0, &MaxLeaf, &Vendor, &ECX, &EDX) || MaxLeaf < 1)
-    return "generic";
+  unsigned EAX = 0, EBX = 0, ECX = 0, EDX = 0;
   getX86CpuIDAndInfo(0x1, &EAX, &EBX, &ECX, &EDX);
 
   unsigned Family = 0, Model = 0;
@@ -1117,10 +1146,10 @@ StringRef sys::getHostCPUName() {
 
   StringRef CPU;
 
-  if (Vendor == SIG_INTEL) {
+  if (Vendor == VendorSignatures::GENUINE_INTEL) {
     CPU = getIntelProcessorTypeAndSubtype(Family, Model, Features, &Type,
                                           &Subtype);
-  } else if (Vendor == SIG_AMD) {
+  } else if (Vendor == VendorSignatures::AUTHENTIC_AMD) {
     CPU = getAMDProcessorTypeAndSubtype(Family, Model, Features, &Type,
                                         &Subtype);
   }
@@ -1254,6 +1283,19 @@ StringRef sys::getHostCPUName() {
 }
 #else
 StringRef sys::getHostCPUName() { return "generic"; }
+namespace llvm {
+namespace sys {
+namespace detail {
+namespace x86 {
+
+VendorSignatures getVendorSignature(unsigned *MaxLeaf) {
+  return VendorSignatures::UNKNOWN;
+}
+
+} // namespace x86
+} // namespace detail
+} // namespace sys
+} // namespace llvm
 #endif
 
 #if defined(__linux__) && (defined(__i386__) || defined(__x86_64__))

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

@@ -22,6 +22,7 @@
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FormatVariadic.h"
+#include "llvm/Support/Host.h"
 
 #include <memory>
 #include <string>
@@ -742,13 +743,25 @@ class ExegesisX86Target : public ExegesisTarget {
 
 #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();
+      // FIXME: Fix this.
+      // https://bugs.llvm.org/show_bug.cgi?id=48918
+      // For now, only do the check if we see an Intel machine because
+      // the counter uses some intel-specific magic and it could
+      // be confuse and think an AMD machine actually has LBR support.
+#if defined(__i386__) || defined(_M_IX86) || defined(__x86_64__) ||            \
+    defined(_M_X64)
+    using namespace sys::detail::x86;
+
+    if (getVendorSignature() == VendorSignatures::GENUINE_INTEL)
+      // If the kernel supports it, the hardware still may not have it.
+      return X86LbrCounter::checkLbrSupport();
 #else
+    llvm_unreachable("Running X86 exegesis on non-X86 target");
+#endif
+#endif
     return llvm::make_error<llvm::StringError>(
         "LBR not supported on this kernel and/or platform",
         llvm::errc::not_supported);
-#endif
   }
 
   std::unique_ptr<SavedState> withSavedState() const override {