[PATCH] D16890: Fix PR25339: ARM Constant Island

Weiming Zhao via llvm-commits llvm-commits at lists.llvm.org
Wed Mar 16 18:13:06 PDT 2016

weimingz added a comment.


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.




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