lower-bitint: Fix up __builtin_{add,sub}_overflow{,_p} bitint lowering [PR116501]

The following testcase is miscompiled.  The problem is in the last_ovf step.
The second operand has signed _BitInt(513) type but has the MSB clear,
so range_to_prec returns 512 for it (i.e. it fits into unsigned
_BitInt(512)).  Because of that the last step actually doesn't need to get
the most significant bit from the second operand, but the code was deciding
what to use purely from TYPE_UNSIGNED (type1) - if unsigned, use 0,
otherwise sign-extend the last processed bit; but that in this case was set.
We don't want to treat the positive operand as if it was negative regardless
of the bit below that precision, and precN >= 0 indicates that the operand
is in the [0, inf) range.

2024-09-03  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/116501
	* gimple-lower-bitint.cc (bitint_large_huge::lower_addsub_overflow):
	In the last_ovf case, use build_zero_cst operand not just when
	TYPE_UNSIGNED (typeN), but also when precN >= 0.

	* gcc.dg/torture/bitint-73.c: New test.

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index b10593035c36b0cd09df6f5c281e555b5a2f0f27..58deaf253e93926dfcea5a714597a7d4eb8ae3f4 100644 (file)
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -4192,7 +4192,7 @@ bitint_large_huge::lower_addsub_overflow (tree obj, gimple *stmt)
       else
        {
          m_data_cnt = data_cnt;
-         if (TYPE_UNSIGNED (type0))
+         if (TYPE_UNSIGNED (type0) || prec0 >= 0)
            rhs1 = build_zero_cst (m_limb_type);
          else
            {
@@ -4210,7 +4210,7 @@ bitint_large_huge::lower_addsub_overflow (tree obj, gimple *stmt)
                  rhs1 = add_cast (m_limb_type, gimple_assign_lhs (g));
                }
            }
-         if (TYPE_UNSIGNED (type1))
+         if (TYPE_UNSIGNED (type1) || prec1 >= 0)
            rhs2 = build_zero_cst (m_limb_type);
          else
            {

diff --git a/gcc/testsuite/gcc.dg/torture/bitint-73.c b/gcc/testsuite/gcc.dg/torture/bitint-73.c
new file mode 100644 (file)
index 0000000..1e15f39
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/bitint-73.c
@@ -0,0 +1,20 @@
+/* PR tree-optimization/116501 */
+/* { dg-do run { target bitint575 } } */
+/* { dg-options "-std=c23" } */
+/* { dg-skip-if "" { ! run_expensive_tests }  { "*" } { "-O0" "-O2" } } */
+/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */
+
+_BitInt (4) a;
+
+int
+foo (_BitInt(513) b)
+{
+  return __builtin_sub_overflow_p (a, b, (_BitInt (511)) 0);
+}
+
+int
+main ()
+{
+  if (!foo (0xffffffffffffffff0000000000000000ffffffffffffffff0000000000000000ffffffffffffffff0000000000000000ffffffffffffffff0000000000000000wb))
+    __builtin_abort ();
+}