[UBSan] Followup for silence_unsigned_overflow flag to handle negate overflows.

[Dor1s]

Summary

That flag has been introduced in https://reviews.llvm.org/D48660 for
suppressing UIO error messages in an efficient way. The main motivation is to
be able to use UIO checks in builds used for fuzzing as it might provide an
interesting signal to a fuzzing engine such as libFuzzer.

See https://github.com/google/oss-fuzz/issues/910 for more information.

[Dor1s]

Wait a second, some unintentional changes sneaked in. I'll clean up in a minute.

[Dor1s]

Remove unintentional changes that were uploaded in the initial version

[Dor1s]

Ready for review now. Please take a look.

[morehouse]

Nice catch!

lib/ubsan/ubsan_handlers.cc

static void handleNegateOverflowImpl(OverflowData *Data, ValueHandle OldVal,
   SourceLocation Loc = Data->Loc.acquire();
   bool IsSigned = Data->Type.isSignedIntegerTy();
   ErrorType ET = IsSigned ? ErrorType::SignedIntegerOverflow
                           : ErrorType::UnsignedIntegerOverflow;
 
   if (ignoreReport(Loc, Opts, ET))
     return;
 
+  if (!IsSigned && flags()->silence_unsigned_overflow)
+    return;
+
   ScopedReport R(Opts, Loc, ET);
 
   if (IsSigned)
     Diag(Loc, DL_Error, ET,
          "negation of %0 cannot be represented in type %1; "
          "cast to an unsigned type to negate this value to itself")
         << Value(Data->Type, OldVal) << Data->Type;
   else

test/ubsan/TestCases/Integer/negate-overflow.cpp

 // RUN: %clangxx -fsanitize=signed-integer-overflow %s -o %t1 && %run %t1 2>&1 | FileCheck %s --check-prefix=CHECKS
 // RUN: %clangxx -fsanitize=unsigned-integer-overflow %s -o %t2 && %run %t2 2>&1 | FileCheck %s --check-prefix=CHECKU
+// RUN: %clangxx -fsanitize=unsigned-integer-overflow %s -o %t2 && %env_ubsan_opts=silence_unsigned_overflow=1 %run %t2 2>&1 | FileCheck %s --check-prefix=CHECKU-SILENT --allow-empty
 
 int main() {
   // CHECKS-NOT: runtime error
-  // CHECKU: negate-overflow.cpp:[[@LINE+2]]:3: runtime error: negation of 2147483648 cannot be represented in type 'unsigned int'
+  // CHECKU: negate-overflow.cpp:[[@LINE+3]]:3: runtime error: negation of 2147483648 cannot be represented in type 'unsigned int'
   // CHECKU-NOT: cast to an unsigned
+  // CHECKU-SILENT-NOT: runtime error
   -unsigned(-0x7fffffff - 1); // ok
   // CHECKS: negate-overflow.cpp:[[@LINE+2]]:3: runtime error: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself
   // CHECKU-NOT: runtime error
   -(-0x7fffffff - 1);
 
   return 0;
 }