[libfuzzer] Fix UB when calculating Log(0) in StackDepthStepFunction().

[Dor1s]

Summary

__builtin_clz used for Log calculation returns an undefined result
when argument is 0. I noticed that issue when was testing some fuzzers:

/src/libfuzzer/FuzzerTracePC.h:282:33: runtime error: shift exponent 450349 is too large for 32-bit type 'uint32_t' (aka 'unsigned int')
  #0 0x43d83f in operator() /src/libfuzzer/FuzzerTracePC.h:283:33
  #1 0x43d83f in void fuzzer::TracePC::CollectFeatures<fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*)::$_1>(fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*)::$_1) const /src/libfuzzer/FuzzerTracePC.h:290
  #2 0x43cbd4 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) /src/libfuzzer/FuzzerLoop.cpp:445:7
  #3 0x43e5f1 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, fuzzer::fuzzer_allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) /src/libfuzzer/FuzzerLoop.cpp:706:5
  #4 0x43e9e1 in fuzzer::Fuzzer::Loop(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, fuzzer::fuzzer_allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) /src/libfuzzer/FuzzerLoop.cpp:739:3
  #5 0x432f8c in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:754:6
  #6 0x42ee18 in main /src/libfuzzer/FuzzerMain.cpp:20:10
  #7 0x7f17ffeb182f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
  #8 0x407838 in _start (/out/rotate_fuzzer+0x407838)

[kcc]

LGTM

[Dor1s]

lib/fuzzer/FuzzerTracePC.h

279

I guess we can do if (A < 8) return A here and remove another check from line #281.

[Dor1s]

lib/fuzzer/FuzzerTracePC.h

279

or maybe even 1 << 3 instead of 8. Either way, landing what you've approved. Thanks!

lib/fuzzer/FuzzerTracePC.h

if (UseValueProfile) {
     ValueProfileMap.ForEach([&](size_t Idx) {
       HandleFeature(FirstFeature + Idx);
     });
     FirstFeature += ValueProfileMap.SizeInBits();
   }
 
   // Step function, grows similar to 8 * Log_2(A).
   auto StackDepthStepFunction = [](uint32_t A) -> uint32_t {
+    if (!A) return A;
     uint32_t Log2 = Log(A);
     if (Log2 < 3) return A;
     Log2 -= 3;
     return (Log2 + 1) * 8 + ((A >> Log2) & 7);
   };
   assert(StackDepthStepFunction(1024) == 64);
   assert(StackDepthStepFunction(1024 * 4) == 80);
   assert(StackDepthStepFunction(1024 * 1024) == 144);