On Mon, Sep 13, 2021 at 11:42:08AM +0100, Roger Sayle wrote:
> gcc/c-family/ChangeLog
> PR c/102245
> * c-common.c (c_common_truthvalue_conversion) [LSHIFT_EXPR]:
> Special case (optimize) shifts by zero.
>
> gcc/testsuite/ChangeLog
> PR c/102245
> * gcc.dg/Wint-in-bool-context-4.c: New test case.
>
> Roger
> --
>
> diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
> index 017e415..44b5fcc 100644
> --- a/gcc/c-family/c-common.c
> +++ b/gcc/c-family/c-common.c
> @@ -3541,6 +3541,10 @@ c_common_truthvalue_conversion (location_t location,
> tree expr)
> break;
>
> case LSHIFT_EXPR:
> + /* Treat shifts by zero as a special case. */
> + if (integer_zerop (TREE_OPERAND (expr, 1)))
> + return c_common_truthvalue_conversion (location,
> + TREE_OPERAND (expr, 0));
> /* We will only warn on signed shifts here, because the majority of
> false positive warnings happen in code where unsigned arithmetic
> was used in anticipation of a possible overflow.
> /* PR c/102245 */
> /* { dg-options "-Wint-in-bool-context" } */
> /* { dg-do compile } */
>
> _Bool test1(_Bool x)
> {
> return !(x << 0); /* { dg-bogus "boolean context" } */
> }
While this exact case is unlikely a misspelling of !(x < 0) as
no _Bool is less than zero and hopefully we get a warning for
!(x < 0), what about
_Bool test1a(int x)
{
return !(x << 0);
}
? I think there is a non-zero chance this was meant to be !(x < 0)
and the current
pr102245.c: In function ‘test1a’:
pr102245.c:3:14: warning: ‘<<’ in boolean context, did you mean ‘<’?
[-Wint-in-bool-context]
3 | return !(x << 0);
| ~~~^~~~~
warning seems to be useful.
Jakub
