https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68528
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
at -O0 we have in .original
int t = 1;
because when we fold INT_MIN - (int) ((unsigned int) x0 - (unsigned int) x1)
we go through
/* A - B -> A + (-B) if B is easily negatable. */
if (negate_expr_p (arg1)
&& !TYPE_OVERFLOW_SANITIZED (type)
&& ((FLOAT_TYPE_P (type)
/* Avoid this transformation if B is a positive REAL_CST. */
&& (TREE_CODE (arg1) != REAL_CST
|| REAL_VALUE_NEGATIVE (TREE_REAL_CST (arg1))))
|| INTEGRAL_TYPE_P (type)))
return fold_build2_loc (loc, PLUS_EXPR, type,
fold_convert_loc (loc, type, arg0),
fold_convert_loc (loc, type,
negate_expr (arg1)));
resulting in
(int) ((unsigned int) x1 - (unsigned int) x0) + -2147483648
after canonicalization which can never be zero (considering undefined overflow)
and we determine that from
/* Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 -+ C1. */
if ((TREE_CODE (arg0) == PLUS_EXPR || TREE_CODE (arg0) == MINUS_EXPR)
&& (equality_code
|| (ANY_INTEGRAL_TYPE_P (TREE_TYPE (arg0))
&& TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0))))
&& TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
&& !TREE_OVERFLOW (TREE_OPERAND (arg0, 1))
&& TREE_CODE (arg1) == INTEGER_CST
&& !TREE_OVERFLOW (arg1))
{
const enum tree_code
reverse_op = TREE_CODE (arg0) == PLUS_EXPR ? MINUS_EXPR : PLUS_EXPR;
tree const1 = TREE_OPERAND (arg0, 1);
tree const2 = fold_convert_loc (loc, TREE_TYPE (const1), arg1);
tree variable = TREE_OPERAND (arg0, 0);
tree new_const = int_const_binop (reverse_op, const2, const1);
/* If the constant operation overflowed this can be
simplified as a comparison against INT_MAX/INT_MIN. */
if (TREE_OVERFLOW (new_const)
&& !TYPE_OVERFLOW_WRAPS (TREE_TYPE (arg0)))
{
...
/* We now can look at the canonicalized case
VARIABLE + 1 CODE2 INT_MIN
and decide on the result. */
