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

[Keeran Rothenfusser 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/perf_user_access", File);
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keeranroth wrote:

This is not going to be available in the future. Access to rdcycle, and allowing the kernel to configure userspace access, is deprecated, and will be removed.

Try something with rdtime instead of rdcycle

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