[PATCH] Fix infinite recursion with div-by-zero (PR middle-end/70992)

[Marek Polacek]

We ended up in infinite recursion between extract_muldiv_1 and
fold_plusminus_mult_expr, because one turns this expression into the other
and the other does the reverse:

((2147483648 / 0) * 2) + 2 <-> 2 * (2147483648 / 0 + 1)

I tried (unsuccessfully) to fix it in either extract_muldiv_1 or
fold_plusminus_mult_expr, but in the end I went with just turning (x / 0) + A
to x / 0 (and similarly for %), because with that undefined division we can do
anything and this fixes the issue.  Any better ideas?

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

2017-07-18  Marek Polacek  <pola...@redhat.com>

        PR middle-end/70992
        * fold-const.c (fold_binary_loc): Fold (x / 0) + A to x / 0,
        and (x % 0) + A to x % 0.

        * gcc.dg/torture/pr70992.c: New test.
        * gcc.dg/torture/pr70992-2.c: New test.

diff --git gcc/fold-const.c gcc/fold-const.c
index 1bcbbb58154..9abdc9a8c20 100644
--- gcc/fold-const.c
+++ gcc/fold-const.c
@@ -9387,6 +9387,12 @@ fold_binary_loc (location_t loc,
                                                      TREE_TYPE (arg0), arg0,
                                                      cst0));
            }
+         /* Adding anything to a division-by-zero makes no sense and
+            can confuse extract_muldiv and fold_plusminus_mult_expr.  */
+         else if ((TREE_CODE (arg0) == TRUNC_DIV_EXPR
+                   || TREE_CODE (arg0) == TRUNC_MOD_EXPR)
+                  && integer_zerop (TREE_OPERAND (arg0, 1)))
+           return fold_convert_loc (loc, type, arg0);
        }
 
       /* Handle (A1 * C1) + (A2 * C2) with A1, A2 or C1, C2 being the same or
diff --git gcc/testsuite/gcc.dg/torture/pr70992-2.c 
gcc/testsuite/gcc.dg/torture/pr70992-2.c
index e69de29bb2d..c5d2c5f2683 100644
--- gcc/testsuite/gcc.dg/torture/pr70992-2.c
+++ gcc/testsuite/gcc.dg/torture/pr70992-2.c
@@ -0,0 +1,9 @@
+/* PR middle-end/70992 */
+/* { dg-do compile } */
+
+unsigned int *od;
+int
+fn (void)
+{
+  return (0 % 0 + 1) * *od * 2; /* { dg-warning "division by zero" } */
+}
diff --git gcc/testsuite/gcc.dg/torture/pr70992.c 
gcc/testsuite/gcc.dg/torture/pr70992.c
index e69de29bb2d..56728e09d1b 100644
--- gcc/testsuite/gcc.dg/torture/pr70992.c
+++ gcc/testsuite/gcc.dg/torture/pr70992.c
@@ -0,0 +1,41 @@
+/* PR middle-end/70992 */
+/* { dg-do compile } */
+
+typedef unsigned int uint32_t;
+typedef int int32_t;
+
+uint32_t
+fn (uint32_t so)
+{
+  return (so + so) * (0x80000000 / 0 + 1); /* { dg-warning "division by zero" 
} */
+}
+
+uint32_t
+fn5 (uint32_t so)
+{
+  return (0x80000000 / 0 + 1) * (so + so); /* { dg-warning "division by zero" 
} */
+}
+
+uint32_t
+fn6 (uint32_t so)
+{
+  return (0x80000000 / 0 - 1) * (so + so); /* { dg-warning "division by zero" 
} */
+}
+
+uint32_t
+fn2 (uint32_t so)
+{
+  return (so + so) * (0x80000000 / 0 - 1); /* { dg-warning "division by zero" 
} */
+}
+
+int32_t
+fn3 (int32_t so)
+{
+  return (so + so) * (0x80000000 / 0 + 1); /* { dg-warning "division by zero" 
} */
+}
+
+int32_t
+fn4 (int32_t so)
+{
+  return (so + so) * (0x80000000 / 0 - 1); /* { dg-warning "division by zero" 
} */
+}

        Marek