This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH] Fix PR34099, wrong-code with CCP
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: Richard Guenther <rguenther at suse dot de>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Fri, 16 Nov 2007 14:01:40 +0000 (UTC)
- Subject: Re: [PATCH] Fix PR34099, wrong-code with CCP
- References: <Pine.LNX.4.64.0711161105530.4148@zhemvz.fhfr.qr>
On Fri, 16 Nov 2007, Richard Guenther wrote:
> + case MULT_EXPR:
> + case TRUNC_DIV_EXPR:
> + case CEIL_DIV_EXPR:
> + case FLOOR_DIV_EXPR:
> + case ROUND_DIV_EXPR:
> + case TRUNC_MOD_EXPR:
> + case CEIL_MOD_EXPR:
> + case FLOOR_MOD_EXPR:
> + case ROUND_MOD_EXPR:
> + case EQ_EXPR:
> + case NE_EXPR:
> + case LT_EXPR:
> + case GT_EXPR:
> + /* Not MIN_EXPR, MAX_EXPR. One VARYING operand may be selected.
> + Not bitwise operators, one VARYING operand may specify the
> + result completely. Not logical operators for the same reason.
> + Not LE/GE comparisons or unordered comparisons. Not
> + COMPLEX_EXPR as one VARYING operand makes the result partly
> + not UNDEFINED. */
x * 0 and x % 1 are always 0 (for integer x). Comparisons may be always 0
or always 1 for one operand outside the range of the unsigned char from
which the original operand was promoted; likewise the results of
divisions, or shifts.
(Exactly what is or is not undefined behavior in the presence of
uninitialized values is the subject of DR#338
<http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_338.htm>. DR#260 was
an earlier DR concerning related issues.)
--
Joseph S. Myers
joseph@codesourcery.com