[PATCH] lower-bitint: Fix lowering of middle sized _BitInt operations which can throw [PR112770]

[Jakub Jelinek]

Hi!

The middle kind _BitInt lowering is mostly done by casting the BITINT_TYPE
operands (if any) to a signed/unsigned integer type which has larger/equal
precision, using such integer type also for the lhs (if BITINT_TYPE) and
and adding a cast after the statement from that new lhs to the old
(BITINT_TYPE) lhs.  Note, for middle kind this isn't done for GIMPLE_CALLs.
Most of the time that works nicely, the exception as the following testcase
shows is -fnon-call-exceptions and some operations which can trap.  Because
inserting the cast to a new lhs after the statement results in a trapping
statement in the middle of a basic block.
The following patch fixes that by emitting the cast on the fallthru edge
instead.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2023-12-01  Jakub Jelinek  <ja...@redhat.com>

        PR middle-end/112770
        * gimple-lower-bitint.cc (gimple_lower_bitint): When adjusting
        lhs of middle _BitInt setter which ends bb, insert cast on
        the fallthru edge rather than after stmt.

        * gcc.dg/bitint-45.c: New test.

--- gcc/gimple-lower-bitint.cc.jj       2023-11-30 17:42:07.172487347 +0100
+++ gcc/gimple-lower-bitint.cc  2023-11-30 19:04:08.004372919 +0100
@@ -6336,7 +6336,13 @@ gimple_lower_bitint (void)
                    type = build_nonstandard_integer_type (prec, uns);
                    tree lhs2 = make_ssa_name (type);
                    gimple *g = gimple_build_assign (lhs, NOP_EXPR, lhs2);
-                   gsi_insert_after (&gsi, g, GSI_SAME_STMT);
+                   if (stmt_ends_bb_p (stmt))
+                     {
+                       edge e = find_fallthru_edge (gsi_bb (gsi)->succs);
+                       gsi_insert_on_edge_immediate (e, g);
+                     }
+                   else
+                     gsi_insert_after (&gsi, g, GSI_SAME_STMT);
                    gimple_set_lhs (stmt, lhs2);
                  }
              unsigned int nops = gimple_num_ops (stmt);
--- gcc/testsuite/gcc.dg/bitint-45.c.jj 2023-11-30 19:17:43.923888015 +0100
+++ gcc/testsuite/gcc.dg/bitint-45.c    2023-11-30 19:17:35.502006495 +0100
@@ -0,0 +1,11 @@
+/* PR middle-end/112770 */
+/* { dg-do compile { target bitint128 } } */
+/* { dg-options "-std=c23 -fnon-call-exceptions" } */
+
+void
+foo (void)
+{
+  _BitInt(128) a = 0;
+  a /= 0;              /* { dg-warning "division by zero" } */
+  &a;
+}

        Jakub