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

[Paolo Bonzini]

On 25/11/2015 14:55, 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.  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.
> 
> Index: c-family/c-ubsan.c
> ===================================================================
> --- c-family/c-ubsan.c        (revision 230466)
> +++ c-family/c-ubsan.c        (working copy)
> @@ -128,7 +128,7 @@
>       (unsigned) x >> (uprecm1 - y)
>       if non-zero, is undefined.  */
>    if (code == LSHIFT_EXPR
> -      && !TYPE_UNSIGNED (type0)
> +      && !TYPE_OVERFLOW_WRAPS (type0)
>        && flag_isoc99)
>      {
>        tree x = fold_build2 (MINUS_EXPR, op1_utype, uprecm1,
> @@ -143,7 +143,7 @@
>       x < 0 || ((unsigned) x >> (uprecm1 - y))
>       if > 1, is undefined.  */
>    if (code == LSHIFT_EXPR
> -      && !TYPE_UNSIGNED (type0)
> +      && !TYPE_OVERFLOW_WRAPS (type0)
>        && (cxx_dialect >= cxx11))
>      {
>        tree x = fold_build2 (MINUS_EXPR, op1_utype, uprecm1,
> Index: testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c
> ===================================================================
> --- testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c        (revision 0)
> +++ testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c        (working copy)
> @@ -0,0 +1,9 @@
> +/* { dg-do run } */
> +/* { dg-options "-fsanitize=shift -fwrapv -w -std=c99" } */
> +
> +int
> +main (void)
> +{
> +  int a = -42;
> +  a << 1;
> +}
> Index: testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c
> ===================================================================
> --- testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c        (revision 0)
> +++ testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c        (working copy)
> @@ -0,0 +1,9 @@
> +/* { dg-do run } */
> +/* { dg-options "-fsanitize=shift -fwrapv -w -std=c99" } */
> +
> +int
> +main (void)
> +{
> +  int a = 1;
> +  a <<= 31;
> +}
> 

Ping?

Paolo