Re: Fix PR 50565 (offsetof-type expressions in static initializers)

[Richard Guenther <richard dot guenther at gmail dot com>]

On Tue, Oct 11, 2011 at 5:32 PM, Joseph S. Myers
<joseph@codesourcery.com> wrote:
> This patch fixes PR 50565, a failure to accept certain offsetof-type
> expressions in static initializers introduced by my constant
> expressions changes. ?(These expressions are permitted but not
> required by ISO C to be accepted; the intent of my constant
> expressions model is that they should be valid in GNU C.)
>
> The problem comes down to an expression with the difference of two
> pointers being cast to int on a 64-bit system, resulting in
> convert_to_integer moving the conversions inside the subtraction.
> (These optimizations at conversion time should really be done later as
> a part of folding, or even later than that, rather than
> unconditionally in convert_to_*, but that's another issue.) ?So when
> the expression reaches c_fully_fold it is a difference of narrowed
> pointers being folded, which the compiler cannot optimize as it can a
> difference of unnarrowed pointers with the same base object. ?Before
> the introduction of c_fully_fold the difference would have been folded
> when built and so the narrowing of operands would never have been
> applied to it.
>
> This patch disables the narrowing in the case of pointer subtraction,
> as it doesn't seem particularly likely to be useful there and is known
> to prevent this folding required for these initializers to be
> accepted.
>
> Bootstrapped with no regressions on x86_64-unknown-linux-gnu. ?OK to
> commit?

Ok.

Thanks,
Richard.

> 2011-10-11 ?Joseph Myers ?<joseph@codesourcery.com>
>
> ? ? ? ?PR c/50565
> ? ? ? ?* convert.c (convert_to_integer): Do not narrow operands of
> ? ? ? ?pointer subtraction.
>
> testsuite:
> 2011-10-11 ?Joseph Myers ?<joseph@codesourcery.com>
>
> ? ? ? ?PR c/50565
> ? ? ? ?* gcc.c-torture/compile/pr50565-1.c,
> ? ? ? ?gcc.c-torture/compile/pr50565-2.c: New tests.
>
> Index: gcc/testsuite/gcc.c-torture/compile/pr50565-1.c
> ===================================================================
> --- gcc/testsuite/gcc.c-torture/compile/pr50565-1.c ? ? (revision 0)
> +++ gcc/testsuite/gcc.c-torture/compile/pr50565-1.c ? ? (revision 0)
> @@ -0,0 +1,4 @@
> +struct s { char p[2]; };
> +static struct s v;
> +const int o0 = (int) ((void *) &v.p[0] - (void *) &v) + 0U;
> +const int o1 = (int) ((void *) &v.p[0] - (void *) &v) + 1U;
> Index: gcc/testsuite/gcc.c-torture/compile/pr50565-2.c
> ===================================================================
> --- gcc/testsuite/gcc.c-torture/compile/pr50565-2.c ? ? (revision 0)
> +++ gcc/testsuite/gcc.c-torture/compile/pr50565-2.c ? ? (revision 0)
> @@ -0,0 +1,4 @@
> +struct s { char p[2]; };
> +static struct s v;
> +const int o0 = (int) ((void *) &v.p[0] - (void *) &v) + 0;
> +const int o1 = (int) ((void *) &v.p[0] - (void *) &v) + 1;
> Index: gcc/convert.c
> ===================================================================
> --- gcc/convert.c ? ? ? (revision 179754)
> +++ gcc/convert.c ? ? ? (working copy)
> @@ -745,6 +745,15 @@ convert_to_integer (tree type, tree expr
> ? ? ? ? ? ?tree arg0 = get_unwidened (TREE_OPERAND (expr, 0), type);
> ? ? ? ? ? ?tree arg1 = get_unwidened (TREE_OPERAND (expr, 1), type);
>
> + ? ? ? ? ? /* Do not try to narrow operands of pointer subtraction;
> + ? ? ? ? ? ? ?that will interfere with other folding. ?*/
> + ? ? ? ? ? if (ex_form == MINUS_EXPR
> + ? ? ? ? ? ? ? && CONVERT_EXPR_P (arg0)
> + ? ? ? ? ? ? ? && CONVERT_EXPR_P (arg1)
> + ? ? ? ? ? ? ? && POINTER_TYPE_P (TREE_TYPE (TREE_OPERAND (arg0, 0)))
> + ? ? ? ? ? ? ? && POINTER_TYPE_P (TREE_TYPE (TREE_OPERAND (arg1, 0))))
> + ? ? ? ? ? ? break;
> +
> ? ? ? ? ? ?if (outprec >= BITS_PER_WORD
> ? ? ? ? ? ? ? ?|| TRULY_NOOP_TRUNCATION (outprec, inprec)
> ? ? ? ? ? ? ? ?|| inprec > TYPE_PRECISION (TREE_TYPE (arg0))
>
> --
> Joseph S. Myers
> joseph@codesourcery.com
>