[llvm-exegesis][AArch64] Disable pauth and ldgm as unsupported instructions fixed (llvm#136868)

[llvm-exegesis][AArch64] Recommit: Disable pauth and ldgm as unsupported instructions.

Skipping AUT and LDGM opcode variants which currently throws "illegal
instruction".

Last pull request
[llvm#132346](llvm#132346) got reviewed
and merged but builder bot got failed. This was due to undefined
`PR_PAC_SET_ENABLED_KEYS` utilized were not defined in x86 arch,
resulting in build failure.

This is followup to merge the changes with following changes to fixup
the build failure.

Changes: 
- Fixed up the problem with arch specific check for `prctl` library
import
- Defining `PR_PAC_SET_ENABLED_KEYS` if undefined.

Author: lakshayk-nv

llvm/test/tools/llvm-exegesis/AArch64/skip_unsupported_instructions.s

@@ -0,0 +1,10 @@
+llvm/test/tools/llvm-exegesis/AArch64/skip_unsupported_instructions.s
+
+# REQUIRES: aarch64-registered-target
+
+# Check for skipping of illegal instruction errors (AUT and LDGM)
+# RUN: llvm-exegesis -mcpu=neoverse-v2 -mode=latency --opcode-name=AUTIA --benchmark-phase=assemble-measured-code 2>&1 | FileCheck %s --check-prefix=CHECK-AUTIA
+# CHECK-AUTIA-NOT: snippet crashed while running: Illegal instruction
+
+# RUN: llvm-exegesis -mcpu=neoverse-v2 -mode=latency --opcode-name=LDGM --benchmark-phase=assemble-measured-code 2>&1 | FileCheck %s --check-prefix=CHECK-LDGM
+# CHECK-LDGM: LDGM: Unsupported opcode: load tag multiple 

llvm/tools/llvm-exegesis/lib/AArch64/Target.cpp

@@ -9,12 +9,64 @@
 #include "AArch64.h"
 #include "AArch64RegisterInfo.h"
 
+#if defined(__aarch64__) && defined(__linux__)
+#include <linux/prctl.h> // For PR_PAC_* constants
+#include <sys/prctl.h>
+#ifndef PR_PAC_SET_ENABLED_KEYS
+#define PR_PAC_SET_ENABLED_KEYS 60
+#endif
+#ifndef PR_PAC_GET_ENABLED_KEYS
+#define PR_PAC_GET_ENABLED_KEYS 61
+#endif
+#endif
+
 #define GET_AVAILABLE_OPCODE_CHECKER
 #include "AArch64GenInstrInfo.inc"
 
 namespace llvm {
 namespace exegesis {
 
+bool isPointerAuth(unsigned Opcode) {
+  switch (Opcode) {
+  default:
+    return false;
+
+  // FIXME: Pointer Authentication instructions.
+  // We would like to measure these instructions, but they can behave
+  // differently on different platforms, and maybe the snippets need to look
+  // different for these instructions,
+  // Platform-specific handling:  On Linux, we disable authentication, may
+  // interfere with measurements. On non-Linux platforms, disable opcodes for
+  // now.
+  case AArch64::AUTDA:
+  case AArch64::AUTDB:
+  case AArch64::AUTDZA:
+  case AArch64::AUTDZB:
+  case AArch64::AUTIA:
+  case AArch64::AUTIA1716:
+  case AArch64::AUTIASP:
+  case AArch64::AUTIAZ:
+  case AArch64::AUTIB:
+  case AArch64::AUTIB1716:
+  case AArch64::AUTIBSP:
+  case AArch64::AUTIBZ:
+  case AArch64::AUTIZA:
+  case AArch64::AUTIZB:
+    return true;
+  }
+}
+
+bool isLoadTagMultiple(unsigned Opcode) {
+  switch (Opcode) {
+  default:
+    return false;
+
+  // Load tag multiple instruction
+  case AArch64::LDGM:
+    return true;
+  }
+}
+
 static unsigned getLoadImmediateOpcode(unsigned RegBitWidth) {
   switch (RegBitWidth) {
   case 32:
@@ -134,6 +186,35 @@ class ExegesisAArch64Target : public ExegesisTarget {
     // Function return is a pseudo-instruction that needs to be expanded
     PM.add(createAArch64ExpandPseudoPass());
   }
+
+  const char *getIgnoredOpcodeReasonOrNull(const LLVMState &State,
+                                           unsigned Opcode) const override {
+    if (const char *Reason =
+            ExegesisTarget::getIgnoredOpcodeReasonOrNull(State, Opcode))
+      return Reason;
+
+    if (isPointerAuth(Opcode)) {
+#if defined(__aarch64__) && defined(__linux__)
+      // Disable all PAC keys. Note that while we expect the measurements to
+      // be the same with PAC keys disabled, they could potentially be lower
+      // since authentication checks are bypassed.
+      if (prctl(PR_PAC_SET_ENABLED_KEYS,
+                PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY |
+                    PR_PAC_APDBKEY, // all keys
+                0,                  // disable all
+                0, 0) < 0) {
+        return "Failed to disable PAC keys";
+      }
+#else
+      return "Unsupported opcode: isPointerAuth";
+#endif
+    }
+
+    if (isLoadTagMultiple(Opcode))
+      return "Unsupported opcode: load tag multiple";
+
+    return nullptr;
+  }
 };
 
 } // namespace