lower-bitint: Fix up lowering of huge _BitInt 0 PHI args [PR113120]

The PHI argument expansion of INTEGER_CSTs where bitint_min_cst_precision
returns significantly smaller precision than the PHI result precision is
optimized by loading the much smaller constant (if any) from memory and
then either setting the remaining limbs to {} or calling memset with -1.
The case where no constant is loaded (i.e. c == NULL) is when the
INTEGER_CST is 0 or all_ones - in that case we can just set all the limbs
to {} or call memset with -1 on everything.
While for the all ones extension case that is what the code was already
doing, I missed one spot in the zero extension case, where constricting
the offset of the MEM_REF lhs of the = {} store it was using unconditionally
the byte size of c, which obviously doesn't work if c is NULL.  In that case
we want to use zero offset.

2024-01-08  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113120
	* gimple-lower-bitint.cc (gimple_lower_bitint): Fix handling of very
	large _BitInt zero INTEGER_CST PHI argument.

	* gcc.dg/bitint-62.c: New test.

2 files changed, 38 insertions, 2 deletions

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index 3869a16..8993a61 100644
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -6606,8 +6606,12 @@ gimple_lower_bitint (void)
 			= build_array_type_nelts (large_huge.m_limb_type,
 						  nelts);
 		      tree ptype = build_pointer_type (TREE_TYPE (v1));
-		      tree off = fold_convert (ptype,
-					       TYPE_SIZE_UNIT (TREE_TYPE (c)));
+		      tree off;
+		      if (c)
+			off = fold_convert (ptype,
+					    TYPE_SIZE_UNIT (TREE_TYPE (c)));
+		      else
+			off = build_zero_cst (ptype);
 		      tree vd = build2 (MEM_REF, vtype,
 					build_fold_addr_expr (v1), off);
 		      g = gimple_build_assign (vd, build_zero_cst (vtype));
diff --git a/gcc/testsuite/gcc.dg/bitint-62.c b/gcc/testsuite/gcc.dg/bitint-62.c
new file mode 100644
index 0000000..2c3139c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/bitint-62.c
@@ -0,0 +1,32 @@
+/* PR tree-optimization/113120 */
+/* { dg-do compile { target bitint } } */
+/* { dg-options "-std=c23 -O2" } */
+
+_BitInt(8) a;
+_BitInt(55) b;
+
+#if __BITINT_MAXWIDTH__ >= 401
+static __attribute__((noinline, noclone)) void
+foo (unsigned _BitInt(1) c, _BitInt(401) d)
+{
+  c /= d << b;
+  a = c;
+}
+
+void
+bar (void)
+{
+  foo (1, 4);
+}
+#endif
+
+#if __BITINT_MAXWIDTH__ >= 6928
+_BitInt(6928)
+baz (int x, _BitInt(6928) y)
+{
+  if (x)
+    return y;
+  else
+    return 0;
+}
+#endif