Fix util/benchmark build or less mainstream architectures, like Alpha, HPPA, IA64, ...

[rener]

Summary

Currently utils/benchmark/src/cycleclock.h timing defines error out for unsupported architectures. However, some Linux distributions like T2 (https://t2sde.org) still somewhat support, and even cross compile Alpha, HPPA, IA64, Motorola 68k, and llvm is needed for Mesa 3d, BPF, etc.

Simply fix the build by moving the MIPS gettimeofday fallback to the last else case to fix building for most non mainstream architecture where the C++ codebase otherwise just compiles fine, too ;-)

[arichardson]

Wouldn't clock_gettime(CLOCK_MONOTONIC) make more sense here?

[arichardson]

Or of course __builtin_readcyclecounter() for supported architectures, but it appears to be a no-op for MIPS that always returns zero.

[lebedev.ri]

Why not just disable building of benchmarks then?
I'm not sure silently producing guesstimates are a good choice here.
If disabling benchmark building is not an option for some reason,
i'd suggest to provide patches to add support for each of the arches in question.

[rener]

In D98427#2619772, @lebedev.ri wrote:

Why not just disable building of benchmarks then?
I'm not sure silently producing guesstimates are a good choice here.
If disabling benchmark building is not an option for some reason,
i'd suggest to provide patches to add support for each of the arches in question.

Well the fallback is already there, and silently used for older 32-bit ARM as well as MIPS. I'm just moving it to avoid build errors, so using that for the time being sounds good enough to me for the time being on this non primary support architectures, ...

[lebedev.ri]

This review seems to be stuck/dead, consider abandoning if no longer relevant.

llvm-12.0.0rc3.src/utils/benchmark/src/cycleclock.h

   struct timeval tv;
   gettimeofday(&tv, nullptr);
   return static_cast<int64_t>(tv.tv_sec) * 1000000 + tv.tv_usec;
-#elif defined(__mips__) || defined(__m68k__)
-  // mips apparently only allows rdtsc for superusers, so we fall
-  // back to gettimeofday.  It's possible clock_gettime would be better.
-  struct timeval tv;
-  gettimeofday(&tv, nullptr);
-  return static_cast<int64_t>(tv.tv_sec) * 1000000 + tv.tv_usec;
 #elif defined(__s390__) // Covers both s390 and s390x.
   // Return the CPU clock.
   uint64_t tsc;

   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
   // a fast implementation and use generic version if nothing better is available.
-#error You need to define CycleTimer for your OS and CPU
+  // MIPS apparently only allows rdtsc for superusers, so for it and other
+  // fall back to gettimeofday. It's possible clock_gettime would be better.
+  struct timeval tv;
+  gettimeofday(&tv, nullptr);
+  return static_cast<int64_t>(tv.tv_sec) * 1000000 + tv.tv_usec;
 #endif
 }
 }  // end namespace cycleclock