Re: [PATCH v2] Do not sanitize left shifts for -fwrapv (PR68418)

[Jeff Law <law@redhat.com>]

On 12/09/2015 10:08 AM, Paolo Bonzini wrote:
> Left shifts into the sign bit is a kind of overflow, and the
> standard chooses to treat left shifts of negative values the
> same way.
>
> However, the -fwrapv option modifies the language to one where
> integers are defined as two's complement---which also defines
> entirely the behavior of shifts.  Disable sanitization of left
> shifts when -fwrapv is in effect, using the same logic as
> instrument_si_overflow.  The same change was proposed
> for LLVM at https://llvm.org/bugs/show_bug.cgi?id=25552.
>
> Bootstrapped/regtested x86_64-pc-linux-gnu.  Ok for trunk, and for
> GCC 5 branch after 5.3 is released?
>
> Thanks,
>
> Paolo
>
> gcc:
> 	PR sanitizer/68418
> 	* c-family/c-ubsan.c (ubsan_instrument_shift): Disable
> 	sanitization of left shifts for wrapping signed types as well.
>
> gcc/testsuite:
> 	PR sanitizer/68418
> 	* gcc.dg/ubsan/c99-wrapv-shift-1.c,
> 	gcc.dg/ubsan/c99-wrapv-shift-2.c: New testcases.
Thanks for the pointers to the earlier code that constrains the types.

FWIW Jan Beulich is twiddling the code leading to the 
ubsan_instrument_shift call.  In fact, your change may make Jan's change 
safe :-)


OK for the trunk.

Thanks,
Jeff