[Bug c/104711] New: Unnecessary -Wshift-negative-value warning

["arnd at linaro dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104711

            Bug ID: 104711
           Summary: Unnecessary -Wshift-negative-value warning
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arnd at linaro dot org
  Target Milestone: ---

During the discussion of increasing the C standard version of the Linux kernel
fro m gnu89 to gnu99 or higher, it turned out that gcc warns about code that
shifts negative signed integers [2].

This is undefined behavior in standard C99, but defined as a GNU extension in
GCC.[3]. This warning is enabled by default at the -Wextra level for C99/GNU99
or higher, but disabled for C89/GNU89. In clang, the warning is enabled by
default at the -Wall level but in turn disabled when building with -fwrapv or
-fno-strict-overflow (as the Linux kernel does).

It would be nice if future compiler releases could either demote the warning
from being enabled at -Wextra to -Wpedantic, or follow clang and disable it
when used with -fwrapv/-fno-strict-overflow.

[1] https://lore.kernel.org/lkml/20220227010956.GW614@gate.crashing.org/
[2] https://www.godbolt.org/z/s1TzxrGz4
[3] https://gcc.gnu.org/onlinedocs/gcc-11.2.0/gcc/Integers-implementation.html