s390: Fix s390_const_int_pool_entry_p and movdi peephole2 [PR114605]

s390: Fix s390_const_int_pool_entry_p and movdi peephole2 [PR114605]

The following testcase is miscompiled, because we have initially
a movti which loads the 0x3f8000003f800000ULL TImode constant
from constant pool.  Later on we split it into a pair of DImode
loads.  Now, for the first load (why just that?, though not stage4
material) we trigger the peephole2 which uses s390_const_int_pool_entry_p.
That function doesn't check at all the constant pool mode though, sees
the constant pool at that address has a CONST_INT value and just assumes
that is the value to return, which is especially wrong for big-endian,
if it is a DImode load from offset 0, it should be loading 0 rather than
0x3f8000003f800000ULL.
The following patch adds checks if we are extracing a MODE_INT mode,
if the constant pool has MODE_INT mode as well, punts if constant pool
has smaller mode size than the extraction one (then it would be UB),
if it has the same mode as before keeps using what it did before,
if constant pool has a larger mode than the one being extracted, uses
simplify_subreg.  I'd have used avoid_constant_pool_reference
instead which can handle also offsets into the constant pool constants,
but it can't handle UNSPEC_LTREF.

Another thing is that once that is fixed, we ICE when we extract constant
like 0, ior insn predicate require non-0 constant.  So, the patch also
fixes the peephole2 so that if either 32-bit half is zero, it uses a mere
load of the constant into register rather than a pair of such load and ior.

2024-04-08  Jakub Jelinek  <jakub@redhat.com>

	PR target/114605
	* config/s390/s390.cc (s390_const_int_pool_entry_p): Punt
	if mem doesn't have MODE_INT mode, or pool constant doesn't
	have MODE_INT mode, or if pool constant mode is smaller than
	mem mode.  If mem mode is different from pool constant mode,
	try to simplify subreg.  If that doesn't work, punt, if it
	does, use the simplified constant instead of the constant pool
	constant.
	* config/s390/s390.md (movdi from const pool peephole): If
	either low or high 32-bit part is zero, just emit move insn
	instead of move + ior.

	* gcc.dg/pr114605.c: New test.