------- 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
