[llvm-dev] the root cause is CP

Peter Lawrence via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 22 19:06:06 PDT 2017

            Here is another example directly out of your paper that
is not addressed by the paper, and also shows that copy-propagation
of “undef” is the root cause of the problem.  This problem disappears 
if “k = undef” is not allowed to be copy-propagated to the two uses  
 “if (undef != 0)”   and   “t = 1 / undef” .

Peter Lawrence.

3.2 Hoisting operations past control-flow
Consider this example:
if (k != 0) {
  while (c) {
use(1 / k); }
Since 1/k is loop invariant, LLVM would like to hoist it out of the loop. Hoisting the division seems safe because the top-level if-statement ensures that division by zero will not happen. This gives:
if (k != 0) {
  int t = 1 / k;
  while (c) {
use(t); }
Now consider the case where k is undef. Since each use of undef can yield a different result, we can have the top-level if-condition being true and still divide by zero, when this could not have happened in the original program if the execution never reached the division (e.g., if c was false). Thus, this transformation is unsound. LLVM used to do it, but stopped after it was shown to lead to end-to-end miscompilation.(3)

(3.  https://bugs.llvm.org/show_bug.cgi?id=21412 )

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