Re: [C++ PATCH] Detect UB in shifts in constexpr functions

[Marek Polacek <polacek at redhat dot com>]

On Mon, Nov 24, 2014 at 05:05:08PM +0100, Jakub Jelinek wrote:
> On Mon, Nov 24, 2014 at 04:58:22PM +0100, Marek Polacek wrote:
> > > Consider say:
> > > 
> > > constexpr int p = 1;
> > > constexpr int foo (int a)
> > > {
> > >   return a << (int) &p;
> > > }
> > > constexpr int bar (int a)
> > > {
> > >   return ((int) &p) << a;
> > > }
> > > constexpr int q = foo (5);
> > > constexpr int r = bar (2);
> > > constexpr int s = bar (0);
> > > 
> > > Now, for foo (5) and bar (2) fold_binary_loc returns NULL and thus
> > > your cxx_eval_check_shift_p is not called, for bar (0) it returns
> > > non-NULL and while the result still is not a constant expression and
> > > right now is diagnosed, with your patch it would ICE.
> > > 
> > > So, I'd just return false if either lhs or rhs are not INTEGER_CSTs.
> >  
> > Ok, I'll add that.  Thank for pointing that out.
> 
> Note, the above only with -m32 obviously, or supposedly for bar
> you could change return type and the cast and type of r/s to
> __PTRDIFF_TYPE__ or so.  foo, as gcc always casts the shift count to int,
> would supposedly need to stay the way it is and might produce different
> diagnostics between -m32 and -m64.

I changed int to __PTRDIFF_TYPE__.
You're right: with -m64 I get:
i.C: In function âconstexpr int foo(int)â:
i.C:4:22: error: cast from âconst int*â to âintâ loses precision [-fpermissive]
   return a << (int) &p;
                      ^
but not with -m32.  I dropped the foo from the testcase.

> I meant for the above computation, there you don't check any overflow.

I see.  I'm happy to change it, whatever you/Jason prefer.

The following is a version with INTEGER_CST check and a new test.

2014-11-24  Marek Polacek  <polacek@redhat.com>

	* constexpr.c (cxx_eval_check_shift_p): New function.
	(cxx_eval_binary_expression): Call it.

	* g++.dg/cpp0x/constexpr-shift1.C: New test.
	* g++.dg/cpp1y/constexpr-shift1.C: New test.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 2678223..049ec0f 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -1451,6 +1451,45 @@ verify_constant (tree t, bool allow_non_constant, bool *non_constant_p,
   return *non_constant_p;
 }
 
+/* Return true if the shift operation on LHS and RHS is undefined.  */
+
+static bool
+cxx_eval_check_shift_p (enum tree_code code, tree lhs, tree rhs)
+{
+  if ((code != LSHIFT_EXPR && code != RSHIFT_EXPR)
+      || TREE_CODE (lhs) != INTEGER_CST
+      || TREE_CODE (rhs) != INTEGER_CST)
+    return false;
+
+  tree lhstype = TREE_TYPE (lhs);
+  unsigned HOST_WIDE_INT uprec = TYPE_PRECISION (TREE_TYPE (lhs));
+
+  /* [expr.shift] The behavior is undefined if the right operand
+     is negative, or greater than or equal to the length in bits
+     of the promoted left operand.  */
+  if (tree_int_cst_sgn (rhs) == -1 || compare_tree_int (rhs, uprec) >= 0)
+    return true;
+
+  /* The value of E1 << E2 is E1 left-shifted E2 bit positions; [...]
+     if E1 has a signed type and non-negative value, and E1x2^E2 is
+     representable in the corresponding unsigned type of the result type,
+     then that value, converted to the result type, is the resulting value;
+     otherwise, the behavior is undefined.  */
+  if (code == LSHIFT_EXPR && !TYPE_UNSIGNED (lhstype))
+    {
+      if (tree_int_cst_sgn (lhs) == -1)
+	return true;
+      tree t = build_int_cst (unsigned_type_node, uprec - 1);
+      t = fold_build2 (MINUS_EXPR, unsigned_type_node, t, rhs);
+      tree ulhs = fold_convert (unsigned_type_for (lhstype), lhs);
+      t = fold_build2 (RSHIFT_EXPR, TREE_TYPE (ulhs), ulhs, t);
+      if (tree_int_cst_lt (integer_one_node, t))
+	return true;
+    }
+
+  return false;
+}
+
 /* Subroutine of cxx_eval_constant_expression.
    Attempt to reduce the unary expression tree T to a compile time value.
    If successful, return the value.  Otherwise issue a diagnostic
@@ -1506,7 +1545,7 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx, tree t,
   enum tree_code code = TREE_CODE (t);
   tree type = TREE_TYPE (t);
   r = fold_binary_loc (loc, code, type, lhs, rhs);
-  if (r == NULL_TREE)
+  if (r == NULL_TREE || cxx_eval_check_shift_p (code, lhs, rhs))
     {
       if (lhs == orig_lhs && rhs == orig_rhs)
 	r = t;
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-shift1.C gcc/testsuite/g++.dg/cpp0x/constexpr-shift1.C
index e69de29..2551fbe 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-shift1.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-shift1.C
@@ -0,0 +1,73 @@
+// { dg-do compile { target c++11 } }
+
+constexpr int
+fn1 (int i, int j)
+{
+  return i << j; // { dg-error "is not a constant expression" }
+}
+
+constexpr int i1 = fn1 (1, -1);
+
+constexpr int
+fn2 (int i, int j)
+{
+  return i << j; // { dg-error "is not a constant expression" }
+}
+
+constexpr int i2 = fn2 (1, 200);
+
+constexpr int
+fn3 (int i, int j)
+{
+  return i << j; // { dg-error "is not a constant expression" }
+}
+
+constexpr int i3 = fn3 (-1, 2);
+
+constexpr int
+fn4 (int i, int j)
+{
+  return i << j; // { dg-error "is not a constant expression" }
+}
+
+constexpr int i4 = fn4 (__INT_MAX__, 2);
+
+constexpr int
+fn5 (int i, int j)
+{
+  return i << j;
+}
+
+constexpr int i5 = fn5 (__INT_MAX__, 1);
+
+constexpr int
+fn6 (unsigned int i, unsigned int j)
+{
+  return i << j; // { dg-error "is not a constant expression" }
+}
+
+constexpr int i6 = fn6 (1, -1);
+
+constexpr int
+fn7 (int i, int j)
+{
+  return i >> j; // { dg-error "is not a constant expression" }
+}
+
+constexpr int i7 = fn7 (1, -1);
+
+constexpr int
+fn8 (int i, int j)
+{
+  return i >> j;
+}
+
+constexpr int i8 = fn8 (-1, 1);
+
+constexpr int
+fn9 (int i, int j)
+{
+  return i >> j;  // { dg-error "is not a constant expression" }
+}
+
+constexpr int i9 = fn9 (1, 200);
diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-shift1.C gcc/testsuite/g++.dg/cpp1y/constexpr-shift1.C
index e69de29..a739fd2 100644
--- gcc/testsuite/g++.dg/cpp1y/constexpr-shift1.C
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-shift1.C
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++14 } }
+
+constexpr int p = 1;
+constexpr __PTRDIFF_TYPE__ bar (int a)
+{
+  return ((__PTRDIFF_TYPE__) &p) << a; // { dg-error "is not a constant expression" }
+}
+constexpr __PTRDIFF_TYPE__ r = bar (2);
+constexpr __PTRDIFF_TYPE__ s = bar (0); // { dg-error "conversion from pointer" }

	Marek