Merging r366868:

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r366868 | rogfer01 | 2019-07-24 07:33:46 +0200 (Wed, 24 Jul 2019) | 6 lines

[RISCV] Implement benchmark::cycleclock::Now

This is a cherrypick of D64237 onto llvm/utils/benchmark and
libcxx/utils/google-benchmark.

Differential Revision: https://reviews.llvm.org/D65142
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git-svn-id: https://llvm.org/svn/llvm-project/libcxx/branches/release_90@367813 91177308-0d34-0410-b5e6-96231b3b80d8

utils/google-benchmark/README.LLVM

@@ -4,3 +4,9 @@ LLVM notes
 This directory contains the Google Benchmark source code with some unnecessary
 files removed. Note that this directory is under a different license than
 libc++.
+
+Changes:
+* https://github.com/google/benchmark/commit/4abdfbb802d1b514703223f5f852ce4a507d32d2
+  is applied on top of
+  https://github.com/google/benchmark/commit/4528c76b718acc9b57956f63069c699ae21edcab
+  to add RISC-V timer support.

utils/google-benchmark/src/cycleclock.h

@@ -164,6 +164,21 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
   uint64_t tsc;
   asm("stck %0" : "=Q"(tsc) : : "cc");
   return tsc;
+#elif defined(__riscv) // RISC-V
+  // Use RDCYCLE (and RDCYCLEH on riscv32)
+#if __riscv_xlen == 32
+  uint64_t cycles_low, cycles_hi0, cycles_hi1;
+  asm("rdcycleh %0" : "=r"(cycles_hi0));
+  asm("rdcycle %0" : "=r"(cycles_lo));
+  asm("rdcycleh %0" : "=r"(cycles_hi1));
+  // This matches the PowerPC overflow detection, above
+  cycles_lo &= -static_cast<int64_t>(cycles_hi0 == cycles_hi1);
+  return (cycles_hi1 << 32) | cycles_lo;
+#else
+  uint64_t cycles;
+  asm("rdcycle %0" : "=r"(cycles));
+  return cycles;
+#endif
 #else
 // The soft failover to a generic implementation is automatic only for ARM.
 // For other platforms the developer is expected to make an attempt to create