[ub] ub due to left operand of shift

Chandler Carruth chandlerc at google.com
Thu Oct 24 23:46:06 CEST 2013


On Thu, Oct 24, 2013 at 2:16 PM, John Regehr <regehr at cs.utah.edu> wrote:

> one way to help resolve the issue would be to stress-test a few
> large applications compiled with integer undefined behavior checking and
> then take a close look at the resulting signed left-shift UBs.
>

FWIW, sorry I wasn't more explicit, I'm saying we have done this on at
least a few applications. Unfortunately, at the moment I just have my
memory. I can dig up data if it is necessary.

My memory is that LLVM was a mixture of two patterns followed by a long
tail of other stuff, and I have data that shows several large C++
applications at Google had similar distributions.

1) Code that *wanted* to be unsigned, but forgot that a literal '42' was
signed. We changed it to '42u' which was preferable on all fronts.
2) Bugs
3) Everything else

You can call #1 false positives, but the code reviewers for the patches to
projects based on this have consistently felt that this was a useful
fix/clarification of their intent in the code.

We did consider some examples of #3 to be unfortunate (requiring a cast,
etc), but they were quite rare relatively.

The other thing to consider is that the bugs we found in #2 were serious,
hard to find bugs that had gone unnoticed for a long time. Our developers
were very willing to pay the price of finding those bugs.
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