arm: correctly handle inequality comparisons against max constants [PR100563]

arm: correctly handle inequality comparisons against max constants [PR100563]

Normally we expect the gimple optimizers to fold away comparisons that
are always true, but at some lower optimization levels this is not
always the case, so the back-end has to be able to generate correct
code in these cases.

In this example, we have a comparison of the form

  (unsigned long long) op <= ~0ULL

which, of course is always true.

Normally, in the arm back-end we handle these expansions where the
immediate cannot be handled directly by adding 1 to the constant and
then adjusting the comparison operator:

  (unsigned long long) op < CONST + 1

but we cannot do that when the constant is already the largest value.

Fortunately, we observe that the comparisons we need to handle this
way are either always true or always false, so instead of forming a
comparison against the maximum value, we can replace it with a
comparison against the minimum value (which just happens to also be a
constant we can handle.  So

  op1 <= ~0ULL -> op1 >= 0U
  op1 > ~0ULL -> op1 < 0U

  op1 <= LONG_LONG_INT_MAX -> op1 >= (-LONG_LONG_INT_MAX - 1)
  op1 > LONG_LONG_INT_MAX -> op1 < (-LONG_LONG_INT_MAX - 1)

gcc:
	PR target/100563
	* config/arm/arm.c (arm_canonicalize_comparison): Correctly
	canonicalize DImode inequality comparisons against the
	maximum integral value.

gcc/testsuite:
	* gcc.dg/pr100563.c: New test.