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From: "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/25186] (short)(((int)short_var) <<1) should be folded so that the shift is done in the short type Date: Thu, 01 Dec 2005 12:28:00 -0000 [thread overview] Message-ID: <20051201122812.1200.qmail@sourceware.org> (raw) In-Reply-To: <bug-25186-6528@http.gcc.gnu.org/bugzilla/> ------- Comment #4 from rguenth at gcc dot gnu dot org 2005-12-01 12:28 ------- convert_to_integer contains case LSHIFT_EXPR: /* We can pass truncation down through left shifting when the shift count is a nonnegative constant and the target type is unsigned. */ if (TREE_CODE (TREE_OPERAND (expr, 1)) == INTEGER_CST && tree_int_cst_sgn (TREE_OPERAND (expr, 1)) >= 0 && TYPE_UNSIGNED (type) && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST) which for our case then (should) falls through to ... /* Don't do unsigned arithmetic where signed was wanted, or vice versa. Exception: if both of the original operands were unsigned then we can safely do the work as unsigned. Exception: shift operations take their type solely from the first argument. Exception: the LSHIFT_EXPR case above requires that we perform this operation unsigned lest we produce signed-overflow undefinedness. And we may need to do it as unsigned if we truncate to the original size. */ if (TYPE_UNSIGNED (TREE_TYPE (expr)) || (TYPE_UNSIGNED (TREE_TYPE (arg0)) && (TYPE_UNSIGNED (TREE_TYPE (arg1)) || ex_form == LSHIFT_EXPR || ex_form == RSHIFT_EXPR || ex_form == LROTATE_EXPR || ex_form == RROTATE_EXPR)) || ex_form == LSHIFT_EXPR) typex = lang_hooks.types.unsigned_type (typex); else typex = lang_hooks.types.signed_type (typex); now, this path seems to handle LSHIFT_EXPR of signed types, so the exeption above does not need to apply. Further, I don't understand the reasoning why we need to do the shift unsigned - we invoke undefined behavior for signed overflow, but the original code, (short) int << n invoked undefined behavior in truncating the int to short in case the value doesn't fit. Which of course still happens for the unsigned case. So, what would break with Index: convert.c =================================================================== *** convert.c (revision 107813) --- convert.c (working copy) *************** convert_to_integer (tree type, tree expr *** 512,518 **** the target type is unsigned. */ if (TREE_CODE (TREE_OPERAND (expr, 1)) == INTEGER_CST && tree_int_cst_sgn (TREE_OPERAND (expr, 1)) >= 0 - && TYPE_UNSIGNED (type) && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST) { /* If shift count is less than the width of the truncated type, --- 490,495 ---- *************** convert_to_integer (tree type, tree expr *** 607,614 **** || ex_form == LSHIFT_EXPR || ex_form == RSHIFT_EXPR || ex_form == LROTATE_EXPR ! || ex_form == RROTATE_EXPR)) ! || ex_form == LSHIFT_EXPR) typex = lang_hooks.types.unsigned_type (typex); else typex = lang_hooks.types.signed_type (typex); --- 584,590 ---- || ex_form == LSHIFT_EXPR || ex_form == RSHIFT_EXPR || ex_form == LROTATE_EXPR ! || ex_form == RROTATE_EXPR))) typex = lang_hooks.types.unsigned_type (typex); else typex = lang_hooks.types.signed_type (typex); ? -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25186
next prev parent reply other threads:[~2005-12-01 12:28 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2005-11-30 18:21 [Bug middle-end/25186] New: (short)(((int)short_var) <<1) should be done in short pinskia at gcc dot gnu dot org 2005-11-30 18:32 ` [Bug middle-end/25186] " pinskia at gcc dot gnu dot org 2005-12-01 10:53 ` [Bug middle-end/25186] (short)(((int)short_var) <<1) should be folded so that the shift is done in the short type rguenth at gcc dot gnu dot org 2005-12-01 11:01 ` rguenth at gcc dot gnu dot org 2005-12-01 12:28 ` rguenth at gcc dot gnu dot org [this message] 2005-12-01 12:46 ` rguenth at gcc dot gnu dot org 2005-12-01 14:17 ` rguenth at gcc dot gnu dot org 2006-04-05 13:49 ` rguenth at gcc dot gnu dot org [not found] <bug-25186-4@http.gcc.gnu.org/bugzilla/> 2021-09-02 2:03 ` gabravier at gmail dot com 2021-09-02 7:15 ` rguenth at gcc dot gnu.org 2023-04-22 8:06 ` roger at nextmovesoftware dot com
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