[PATCH] Fix ubsan tree sharing (PR sanitizer/66908)

[Marek Polacek]

In this testcase we were generating an uninitialized variable when doing
-fsanitize=shift,bounds sanitization.  The shift instrumentation is done
first; after that, the IR looks like

  res[i] = (m > 31) ? __ubsan (... tab[i] ...) ? 0, ... tab[i] ...;

where tab[i] are identical.  That means that when we instrument the first
tab[i] (we shouldn't do this I suppose), the second tab[i] is changed as
well as they're shared.  But that doesn't play well with SAVE_EXPRs, because
SAVE_EXPR <i> would only be initialized on one path.  Fixed by unsharing
the operands when constructing the ubsan check.  The .gimple diff is in
essence just

+  i.2 = i;
+  UBSAN_BOUNDS (0B, i.2, 21);
-  UBSAN_BOUNDS (0B, i.1, 21);

(Merely not instrumenting __ubsan_* wouldn't help exactly because of the
sharing.)

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

2015-07-22  Marek Polacek  <pola...@redhat.com>

        PR sanitizer/66908
        * c-ubsan.c: Include gimplify.h.
        (ubsan_instrument_division): Unshare OP0 and OP1.
        (ubsan_instrument_shift): Likewise.

        * c-c++-common/ubsan/pr66908.c: New test.

diff --git gcc/c-family/c-ubsan.c gcc/c-family/c-ubsan.c
index 0baf118..3869511 100644
--- gcc/c-family/c-ubsan.c
+++ gcc/c-family/c-ubsan.c
@@ -38,6 +38,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "internal-fn.h"
 #include "stor-layout.h"
 #include "builtins.h"
+#include "gimplify.h"
 
 /* Instrument division by zero and INT_MIN / -1.  If not instrumenting,
    return NULL_TREE.  */
@@ -54,6 +55,9 @@ ubsan_instrument_division (location_t loc, tree op0, tree op1)
   gcc_assert (TYPE_MAIN_VARIANT (TREE_TYPE (op0))
              == TYPE_MAIN_VARIANT (TREE_TYPE (op1)));
 
+  op0 = unshare_expr (op0);
+  op1 = unshare_expr (op1);
+
   if (TREE_CODE (type) == INTEGER_TYPE
       && (flag_sanitize & SANITIZE_DIVIDE))
     t = fold_build2 (EQ_EXPR, boolean_type_node,
@@ -134,6 +138,9 @@ ubsan_instrument_shift (location_t loc, enum tree_code code,
   HOST_WIDE_INT op0_prec = TYPE_PRECISION (type0);
   tree uprecm1 = build_int_cst (op1_utype, op0_prec - 1);
 
+  op0 = unshare_expr (op0);
+  op1 = unshare_expr (op1);
+
   t = fold_convert_loc (loc, op1_utype, op1);
   t = fold_build2 (GT_EXPR, boolean_type_node, t, uprecm1);
 
diff --git gcc/testsuite/c-c++-common/ubsan/pr66908.c 
gcc/testsuite/c-c++-common/ubsan/pr66908.c
index e69de29..5f731f0 100644
--- gcc/testsuite/c-c++-common/ubsan/pr66908.c
+++ gcc/testsuite/c-c++-common/ubsan/pr66908.c
@@ -0,0 +1,15 @@
+/* PR sanitizer/66908 */
+/* { dg-do compile } */
+/* { dg-options "-fsanitize=shift,bounds -O2 -Werror=maybe-uninitialized" } */
+/* { dg-additional-options "-std=gnu90" { target c } } */
+
+struct S { int a[22]; };
+static int const e[22] = { };
+
+void
+foo (struct S const *s, unsigned int m, unsigned int *res)
+{
+  unsigned int i;
+  for (i = 0; i < 22; ++i)
+    res[i] = ((s->a[i] + e[i]) << m);
+}

        Marek