fold-const: Punt on MULT_EXPR in extract_muldiv MIN/MAX_EXPR case [PR111151]

As I've tried to explain in the comments, the extract_muldiv_1
MIN/MAX_EXPR optimization is wrong for code == MULT_EXPR.
If the multiplication is done in unsigned type or in signed
type with -fwrapv, it is fairly obvious that max (a, b) * c
in many cases isn't equivalent to max (a * c, b * c) (or min if c is
negative) due to overflows, but even for signed with undefined overflow,
the optimization could turn something without UB in it (where
say a * c invokes UB, but max (or min) picks the other operand where
b * c doesn't).
As for division/modulo, I think it is in most cases safe, except if
the problematic INT_MIN / -1 case could be triggered, but we can
just punt for MAX_EXPR because for MIN_EXPR if one operand is INT_MIN,
we'd pick that operand already.  It is just for completeness, match.pd
already has an optimization which turns x / -1 into -x, so the division
by zero is mostly theoretical.  That is also why in the testcase the
i case isn't actually miscompiled without the patch, while the c and f
cases are.

2024-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/111151
	* fold-const.cc (extract_muldiv_1) <case MAX_EXPR>: Punt for
	MULT_EXPR altogether, or for MAX_EXPR if c is -1.

	* gcc.c-torture/execute/pr111151.c: New test.

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 299c22bf391156957389a7376e7af3e9ef91fd0c..8960e5289c0b1e912075cf9ad22981f5114dd223 100644 (file)
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -7104,6 +7104,27 @@ extract_muldiv_1 (tree t, tree c, enum tree_code code, tree wide_type,
       if (TYPE_UNSIGNED (ctype) != TYPE_UNSIGNED (type))
        break;
 
+      /* Punt for multiplication altogether.
+        MAX (1U + INT_MAX, 1U) * 2U is not equivalent to
+        MAX ((1U + INT_MAX) * 2U, 1U * 2U), the former is
+        0U, the latter is 2U.
+        MAX (INT_MIN / 2, 0) * -2 is not equivalent to
+        MIN (INT_MIN / 2 * -2, 0 * -2), the former is
+        well defined 0, the latter invokes UB.
+        MAX (INT_MIN / 2, 5) * 5 is not equivalent to
+        MAX (INT_MIN / 2 * 5, 5 * 5), the former is
+        well defined 25, the latter invokes UB.  */
+      if (code == MULT_EXPR)
+       break;
+      /* For division/modulo, punt on c being -1 for MAX, as
+        MAX (INT_MIN, 0) / -1 is not equivalent to
+        MIN (INT_MIN / -1, 0 / -1), the former is well defined
+        0, the latter invokes UB (or for -fwrapv is INT_MIN).
+        MIN (INT_MIN, 0) / -1 already invokes UB, so the
+        transformation won't make it worse.  */
+      else if (tcode == MAX_EXPR && integer_minus_onep (c))
+       break;
+
       /* MIN (a, b) / 5 -> MIN (a / 5, b / 5)  */
       sub_strict_overflow_p = false;
       if ((t1 = extract_muldiv (op0, c, code, wide_type,

diff --git a/gcc/testsuite/gcc.c-torture/execute/pr111151.c b/gcc/testsuite/gcc.c-torture/execute/pr111151.c
new file mode 100644 (file)
index 0000000..89255d2
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr111151.c
@@ -0,0 +1,21 @@
+/* PR middle-end/111151 */
+
+int
+main ()
+{
+  unsigned a = (1U + __INT_MAX__) / 2U;
+  unsigned b = 1U;
+  unsigned c = (a * 2U > b * 2U ? a * 2U : b * 2U) * 2U;
+  if (c != 0U)
+    __builtin_abort ();
+  int d = (-__INT_MAX__ - 1) / 2;
+  int e = 10;
+  int f = (d * 2 > e * 5 ? d * 2 : e * 5) * 6;
+  if (f != 120)
+    __builtin_abort ();
+  int g = (-__INT_MAX__ - 1) / 2;
+  int h = 0;
+  int i = (g * 2 > h * 5 ? g * 2 : h * 5) / -1;
+  if (i != 0)
+    __builtin_abort ();
+}