[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" <gcc-bugzilla at gcc dot gnu dot org>]

------- 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