[llvm] [Exegesis][RISCV] Add RISCV support for llvm-exegesis (PR #89047)

[Yingwei Zheng via llvm-commits]

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@@ -0,0 +1,100 @@
+//===-- RISCVCounters.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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines RISC-V perf counters.
+//
+//===----------------------------------------------------------------------===//
+
+#include "RISCVCounters.h"
+
+namespace llvm {
+namespace exegesis {
+
+// This implementation of RISCV target for Exegesis doesn't use libpfm
+// and provides manual implementation of performance counters.
+
+inline uint64_t getRISCVCpuCyclesCount() {
+#if defined(__riscv) && defined(__linux__)
+#if __riscv_xlen == 32
+  uint32_t cycles_lo, cycles_hi0, cycles_hi1;
+  asm volatile("rdcycleh %0\n"
+               "rdcycle %1\n"
+               "rdcycleh %2\n"
+               "sub %0, %0, %2\n"
+               "seqz %0, %0\n"
+               "sub %0, zero, %0\n"
+               "and %1, %1, %0\n"
+               : "=r"(cycles_hi0), "=r"(cycles_lo), "=r"(cycles_hi1));
+  return static_cast<uint64_t>((static_cast<uint64_t>(cycles_hi1) << 32) |
+                               cycles_lo);
+#else
+  uint64_t cycles;
+  asm volatile("rdcycle %0" : "=r"(cycles));
+  return static_cast<uint64_t>(cycles);
+#endif
+#else
+  return 0;
+#endif
+}
+
+class RISCVCpuCyclesCounter : public pfm::CounterGroup {
+  uint64_t StartValue;
+  uint64_t EndValue;
+  uint64_t MeasurementCycles;
+
+public:
+  explicit RISCVCpuCyclesCounter(pfm::PerfEvent &&Event);
+
+  void start() override { StartValue = getRISCVCpuCyclesCount(); }
+
+  void stop() override { EndValue = getRISCVCpuCyclesCount(); }
+
+  Expected<llvm::SmallVector<int64_t, 4>>
+  readOrError(StringRef FunctionBytes) const override;
+};
+
+RISCVCpuCyclesCounter::RISCVCpuCyclesCounter(pfm::PerfEvent &&Event)
+    : CounterGroup(std::move(Event), {}) {
+#if defined(__riscv) && defined(__linux__)
+  char Value[2] = "0";
+  int File = 0;
+  std::error_code FileError =
+      sys::fs::openFileForRead("/proc/sys/kernel/watchdog", File);
----------------
dtcxzyw wrote:

```suggestion
      sys::fs::openFileForRead("/proc/sys/kernel/perf_user_access", File);
```

https://github.com/llvm/llvm-project/pull/89047