c-family: Tweak ptr +- (expr +- cst) FE optimization [PR120837]
The following testcase is miscompiled with -fsanitize=undefined but we
introduce UB into the IL even without that flag.
The optimization ptr +- (expr +- cst) when expr/cst have undefined
overflow into (ptr +- cst) +- expr is sometimes simply not valid,
without careful analysis on what ptr points to we don't know if it
is valid to do (ptr +- cst) pointer arithmetics.
E.g. on the testcase, ptr points to start of an array (actually
conditionally one or another) and cst is -1, so ptr - 1 is invalid
pointer arithmetics, while ptr + (expr - 1) can be valid if expr
is at runtime always > 1 and smaller than size of the array ptr points
to + 1.
Unfortunately, removing this 1992-ish optimization altogether causes
FAIL: c-c++-common/restrict-2.c -Wc++-compat scan-tree-dump-times lim2 "Moving statement" 11
FAIL: gcc.dg/tree-ssa/copy-headers-5.c scan-tree-dump ch2 "is now do-while loop"
FAIL: gcc.dg/tree-ssa/copy-headers-5.c scan-tree-dump-times ch2 " if " 3
FAIL: gcc.dg/vect/pr57558-2.c scan-tree-dump vect "vectorized 1 loops"
FAIL: gcc.dg/vect/pr57558-2.c -flto -ffat-lto-objects scan-tree-dump vect "vectorized 1 loops"
regressions (restrict-2.c also for C++ in all std modes). I've been thinking
about some match.pd optimization for signed integer addition/subtraction of
constant followed by widening integral conversion followed by multiplication
or left shift, but that wouldn't help 32-bit arches.
So, instead at least for now, the following patch keeps doing the
optimization, just doesn't perform it in pointer arithmetics.
pointer_int_sum itself actually adds the multiplication by size_exp,
so ptr + expr is turned into ptr p+ expr * size_exp,
so this patch will try to optimize
ptr + (expr +- cst)
into
ptr p+ ((sizetype)expr * size_exp +- (sizetype)cst * size_exp)
and
ptr - (expr +- cst)
into
ptr p+ -((sizetype)expr * size_exp +- (sizetype)cst * size_exp)
2025-07-04 Jakub Jelinek <jakub@redhat.com>
PR c/120837
* c-common.cc (pointer_int_sum): Rewrite the intop PLUS_EXPR or
MINUS_EXPR optimization into extension of both intop operands,
their separate multiplication and then addition/subtraction followed
by rest of pointer_int_sum handling after the multiplication.
* gcc.dg/ubsan/pr120837.c: New test.
(cherry picked from commit e16820d4f7ab1d8a40f70beef722e6f8a4c2392c)
diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index 587d764..65478c3 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -3437,20 +3437,41 @@
an overflow error if the constant is negative but INTOP is not. */
&& (TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (intop))
|| (TYPE_PRECISION (TREE_TYPE (intop))
- == TYPE_PRECISION (TREE_TYPE (ptrop)))))
+ == TYPE_PRECISION (TREE_TYPE (ptrop))))
+ && TYPE_PRECISION (TREE_TYPE (intop)) <= TYPE_PRECISION (sizetype))
{
- enum tree_code subcode = resultcode;
- tree int_type = TREE_TYPE (intop);
- if (TREE_CODE (intop) == MINUS_EXPR)
- subcode = (subcode == PLUS_EXPR ? MINUS_EXPR : PLUS_EXPR);
- /* Convert both subexpression types to the type of intop,
- because weird cases involving pointer arithmetic
- can result in a sum or difference with different type args. */
- ptrop = build_binary_op (EXPR_LOCATION (TREE_OPERAND (intop, 1)),
- subcode, ptrop,
- convert (int_type, TREE_OPERAND (intop, 1)),
- true);
- intop = convert (int_type, TREE_OPERAND (intop, 0));
+ tree intop0 = TREE_OPERAND (intop, 0);
+ tree intop1 = TREE_OPERAND (intop, 1);
+ if (TYPE_PRECISION (TREE_TYPE (intop)) != TYPE_PRECISION (sizetype)
+ || TYPE_UNSIGNED (TREE_TYPE (intop)) != TYPE_UNSIGNED (sizetype))
+ {
+ tree optype = c_common_type_for_size (TYPE_PRECISION (sizetype),
+ TYPE_UNSIGNED (sizetype));
+ intop0 = convert (optype, intop0);
+ intop1 = convert (optype, intop1);
+ }
+ tree t = fold_build2_loc (loc, MULT_EXPR, TREE_TYPE (intop0), intop0,
+ convert (TREE_TYPE (intop0), size_exp));
+ intop0 = convert (sizetype, t);
+ if (TREE_OVERFLOW_P (intop0) && !TREE_OVERFLOW (t))
+ intop0 = wide_int_to_tree (TREE_TYPE (intop0), wi::to_wide (intop0));
+ t = fold_build2_loc (loc, MULT_EXPR, TREE_TYPE (intop1), intop1,
+ convert (TREE_TYPE (intop1), size_exp));
+ intop1 = convert (sizetype, t);
+ if (TREE_OVERFLOW_P (intop1) && !TREE_OVERFLOW (t))
+ intop1 = wide_int_to_tree (TREE_TYPE (intop1), wi::to_wide (intop1));
+ intop = build_binary_op (EXPR_LOCATION (intop), TREE_CODE (intop),
+ intop0, intop1, true);
+
+ /* Create the sum or difference. */
+ if (resultcode == MINUS_EXPR)
+ intop = fold_build1_loc (loc, NEGATE_EXPR, sizetype, intop);
+
+ ret = fold_build_pointer_plus_loc (loc, ptrop, intop);
+
+ fold_undefer_and_ignore_overflow_warnings ();
+
+ return ret;
}
/* Convert the integer argument to a type the same size as sizetype
diff --git a/gcc/testsuite/gcc.dg/ubsan/pr120837.c b/gcc/testsuite/gcc.dg/ubsan/pr120837.c
new file mode 100644
index 0000000..97c85c7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/ubsan/pr120837.c
@@ -0,0 +1,32 @@
+/* PR c/120837 */
+/* { dg-do run } */
+/* { dg-options "-O1 -fsanitize=undefined -fno-sanitize-recover=undefined" } */
+
+[[gnu::noipa]] void
+bar (void **x, void **y)
+{
+ x[0] = 0;
+ x[1] = 0;
+ x[2] = 0;
+ y[0] = 0;
+ y[1] = 0;
+ y[2] = 0;
+ y[3] = 0;
+ y[4] = 0;
+}
+
+[[gnu::noipa]] void *
+foo (int x, int y)
+{
+ void *a[3];
+ void *b[5];
+ bar (a, b);
+ return (x > y ? b : a)[y - 1];
+}
+
+int
+main ()
+{
+ if (foo (2, 1) != 0)
+ __builtin_abort ();
+}
