[PATCH] D97854: [RFC][nsan] A Floating-point numerical sanitizer.

[Clement Courbet via Phabricator via cfe-commits]

courbet added a comment.

When bootstrapping LLVM with nsan, there are only a few issues.

Several of them stem from using `double` to measure elapsed time in seconds: We measure start time, end time, and subtract them. The resulting error depends on the arbitrary magnitude of the time since epoch, so as time passes the error will increase. This is especially visible when we measure short intervals of time (e.g. a few microseconds, which are small compared to the time since epoch).

For example one test has more than 2% error:

  WARNING: NumericalStabilitySanitizer: inconsistent shadow results while checking store to address 0x4b87860
  double       precision  (native): dec: 0.00000858306884765625  hex: 0x1.20000000000000000000p-17
  __float128   precision  (shadow): dec: 0.00000880600000000000  hex: 0x9.3bd7b64e9fe4fc000000p-20
  shadow truncated to double      : dec: 0.00000880600000000000  hex: 0x1.277af6c9d3fca0000000p-17
  Relative error: 2.53158247040370201937% (2^47 epsilons)
  Absolute error: 0x1.debdb274ff27e0000000p-23
  (131595325226954 ULPs == 14.1 digits == 46.9 bits)
      #0 0x119db71 in llvm::TimeRecord::operator-=(llvm::TimeRecord const&) [...]/llvm/llvm-project/llvm/include/llvm/Support/Timer.h:63:14
      #1 0x119db71 in llvm::Timer::stopTimer() [...]/llvm/llvm-project/llvm/lib/Support/Timer.cpp:176:8
      #2 0x108b1d2 in llvm::TimePassesHandler::stopTimer(llvm::StringRef) [...]/llvm/llvm-project/llvm/lib/IR/PassTimingInfo.cpp:248:14
      #3 0x108b1d2 in llvm::TimePassesHandler::runAfterPass(llvm::StringRef) [...]/llvm/llvm-project/llvm/lib/IR/PassTimingInfo.cpp:267:3
      #4 0x108e159 in llvm::TimePassesHandler::registerCallbacks(llvm::PassInstrumentationCallbacks&)::$_2::operator()(llvm::StringRef, llvm::Any, llvm::PreservedAnalyses const&) const [...]/llvm/llvm-project/llvm/lib/IR/PassTimingInfo.cpp:281:15
      #5 0x108e159 in void llvm::detail::UniqueFunctionBase<void, llvm::StringRef, llvm::Any, llvm::PreservedAnalyses const&>::CallImpl<llvm::TimePassesHandler::registerCallbacks(llvm::PassInstrumentationCallbacks&)::$_2>(void*, llvm::StringRef, llvm::Any&, llvm::PreservedAnalyses const&) [...]/llvm/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:204:12
      #6 0xa4f826 in llvm::unique_function<void (llvm::StringRef, llvm::Any, llvm::PreservedAnalyses const&)>::operator()(llvm::StringRef, llvm::Any, llvm::PreservedAnalyses const&) [...]/llvm/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:366:12
      #7 0xa4f826 in void llvm::PassInstrumentation::runAfterPass<llvm::Module, (anonymous namespace)::MyPass2>((anonymous namespace)::MyPass2 const&, llvm::Module const&, llvm::PreservedAnalyses const&) const [...]/llvm/llvm-project/llvm/include/llvm/IR/PassInstrumentation.h:227:9
      #8 0xa4f826 in (anonymous namespace)::TimePassesTest_CustomOut_Test::TestBody() [...]/llvm/llvm-project/llvm/unittests/IR/TimePassesTest.cpp:137:6
  ...


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