[ubsan] Instrument expr only when doing shift or division

[Marek Polacek]

Running
make -C gcc check RUNTESTFLAGS='--target_board=unix/-fsanitize=undefined
dg.exp=*const-expr*' unveiled a pretty stupid bug: we should
instrument expression only when it's actually needed, not otherwise.
On the C++ FE this is handled right, the thinko was only in C FE.

Tested x86_64-unknown-linux-gnu, applying to ubsan branch.

diff --git a/gcc/c/ChangeLog.ubsan b/gcc/c/ChangeLog.ubsan
index 11d167f..f41ae90 100644
--- a/gcc/c/ChangeLog.ubsan
+++ b/gcc/c/ChangeLog.ubsan
@@ -1,3 +1,8 @@
+2013-07-31  Marek Polacek  <pola...@redhat.com>
+
+       * c-typeck.c (build_binary_op): Sanitize only when
+       doing shift or division.
+
 2013-07-30  Marek Polacek  <pola...@redhat.com>
 
        * c-typeck.c (build_binary_op): Sanitize only when
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 7257166..2595382 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -10489,7 +10489,8 @@ build_binary_op (location_t location, enum tree_code 
code,
     }
 
   if (flag_sanitize & SANITIZE_UNDEFINED
-      && current_function_decl != 0)
+      && current_function_decl != 0
+      && (doing_div_or_mod || doing_shift))
     {
       /* OP0 and/or OP1 might have side-effects.  */
       op0 = c_save_expr (op0);

        Marek