lower-bitint: Fix arithmetics followed by extension by many bits [PR112809]

A zero or sign extension from result of some upwards_2limb operation
is implemented in lower_mergeable_stmt as an extra loop which fills in
the extra bits with 0s or 1s.
If the delta of extended vs. unextended bit count is small, the code
doesn't use a loop and emits up to a couple of stores to constant indexes,
but if the delta is large, it uses
          cnt = (bo_bit != 0) + 1 + (rem != 0);
statements.  bo_bit is non-zero for bit-field loads and is done in that
case as straight line, the unconditional 1 in there is for a loop which
handles most of the limbs in the delta and finally (rem != 0) is for the
case when the extended precision is not a multiple of limb_prec and is
again done in straight line code (after the loop).
The testcase ICEs because the decision what idx to use was incorrect
for kind == bitint_prec_huge (i.e. when the precision delta is very large)
and rem == 0 (i.e. the extended precision is multiple of limb_prec).
In that case cnt is either 1 (if bo_bit == 0) or 2, and idx should
be either first size_int (start) and then result of create_loop (for bo_bit
!= 0) or just result of create_loop, but by mistake the last case
was size_int (end), which means when precision is multiple of limb_prec
storing above the precision (which ICEs; but also not emitting the loop
which is needed).

2023-12-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/112809
	* gimple-lower-bitint.cc (bitint_large_huge::lower_mergeable_stmt): For
	separate_ext in kind == bitint_prec_huge mode if rem == 0, create for
	i == cnt - 1 the loop rather than using size_int (end).

	* gcc.dg/bitint-48.c: New test.

gcc/gimple-lower-bitint.cc

@@ -2624,7 +2624,7 @@ bitint_large_huge::lower_mergeable_stmt (gimple *stmt, tree_code &cmp_code,
 	{
 	  if (kind == bitint_prec_large || (i == 0 && bo_bit != 0))
 	    idx = size_int (start + i);
-	  else if (i == cnt - 1)
+	  else if (i == cnt - 1 && (rem != 0))
 	    idx = size_int (end);
 	  else if (i == (bo_bit != 0))
 	    idx = create_loop (size_int (start + i), &idx_next);

gcc/testsuite/gcc.dg/bitint-48.c

@@ -0,0 +1,23 @@
+/* PR tree-optimization/112809 */
+/* { dg-do compile { target bitint } } */
+/* { dg-options "-O2" } */
+
+#if __BITINT_MAXWIDTH__ >= 512
+_BitInt (512) a;
+_BitInt (256) b;
+_BitInt (256) c;
+
+int
+foo (void)
+{
+  return a == (b | c);
+}
+
+void
+bar (void)
+{
+  a /= b - 2;
+}
+#else
+int i;
+#endif