bitint: Zero-extend the result of a signed->unsigned widening cast

A widening cast from a signed _BitInt operand to an unsigned _BitInt
type involves filling the extra limb(s) with sign extension.
On a target that wants _BitInts extended in memory, if this unsigned
type has a partial limb, the unused part of it should be zeroed.

e.g. Assuming limb_mode == E_DImode, at the end of

  void
  test (unsigned _BitInt(519) *t, _BitInt(512) x)
  {
    *t = -x;
  }

the most significant limb of *t should be masked with 0x7f.

This patch also fixes gcc.dg/torture/bitint-16.c, which aborts at -O2
when the extension on load is optimized away.

	* gimple-lower-bitint.cc (bitint_large_huge::lower_mergeable_stmt):
	Zero-extend the partial limb of any unsigned _BitInt LHS assigned
	with a widening sign-extension.

	* gcc.dg/torture/bitint-83.c: New test.

2 files changed, 59 insertions, 0 deletions

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index 1224173..e7c7fe5 100644
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -3181,6 +3181,17 @@ bitint_large_huge::lower_mergeable_stmt (gimple *stmt, tree_code &cmp_code,
 	    {
 	      tree l = limb_access (nlhs ? NULL_TREE : lhs_type,
 				    nlhs ? nlhs : lhs, idx, true);
+
+	      if (bitint_extended
+		  && sext
+		  && TYPE_UNSIGNED (lhs_type)
+		  && tree_fits_uhwi_p (idx)
+		  && !nlhs)
+		{
+		  rhs1 = add_cast (limb_access_type (lhs_type, idx), rhs1);
+		  rhs1 = add_cast (TREE_TYPE (l), rhs1);
+		}
+
 	      g = gimple_build_assign (l, rhs1);
 	    }
 	  insert_before (g);
diff --git a/gcc/testsuite/gcc.dg/torture/bitint-83.c b/gcc/testsuite/gcc.dg/torture/bitint-83.c
new file mode 100644
index 0000000..8a9df44
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/bitint-83.c
@@ -0,0 +1,48 @@
+/* Derived from a test in gcc.dg/torture/bitint-16.c */
+/* { dg-do run { target bitint } } */
+/* { dg-options "-std=c23 -pedantic-errors" } */
+/* { dg-skip-if "" { ! run_expensive_tests }  { "*" } { "-O0" "-O2" } } */
+/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */
+
+#include "../bitintext.h"
+
+#define BASIC_TESTS \
+  TEST(8)	    \
+  TEST(16)	    \
+  TEST(32)
+
+#if __BITINT_MAXWIDTH__ >= 519
+#define ALL_TESTS \
+  BASIC_TESTS	  \
+  TEST(64)	  \
+  TEST(128)	  \
+  TEST(256)	  \
+  TEST(512)
+#else
+#define ALL_TESTS BASIC_TESTS
+#endif
+
+#define TEST(N) \
+void						    \
+test##N (unsigned _BitInt(N + 7) *t, _BitInt(N) x)  \
+{						    \
+  *t = -x;					    \
+}
+ALL_TESTS
+#undef TEST
+
+volatile int y = 0;
+
+int
+main (void)
+{
+#define TEST(N)	\
+  {				\
+    unsigned _BitInt(N + 7) t;	\
+    _BitInt(N) x = y + N;	\
+    test##N (&t, x);		\
+    BEXTC (t);			\
+  }
+  ALL_TESTS
+#undef TEST
+}