[llvm-dev] How to let LLVM handle undefined behavior more gracefully?

[Zhao, Weiming via llvm-dev]

Hi,

There are cases where LLVM is able to detect some UB but clang is not.

For example,

unsigned int foo(unsigned int x) {

   int ret = 0;
   for(int i = 0; i <= 32; ++i)
     ret += x >> i;
   return ret;

}

When the loop is unrolled, LLVM InstructionSimplify will catch it and
return a UNDEF value.
How can we let LLVM report some warning message to help developers
correct the error?
Or should we use similar behavior as GCC (e.g. x >> 32 returns 0)?

This can also saves compiler engineer's effort: users complain that it's
a compiler bug because their code works with GCC or older version of
LLVM (because the loop is not unrolled). And it's really hard to debug
such UB in some large code base.

Thanks,
Weiming

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