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[PATCH] Fix VRP with -fno-delete-null-pointer-checks (PR c/88367)


Hi!

If we consider -fno-delete-null-pointer-checks as a way to support e.g. AVR
and other targets which can validly place objects at NULL rather than a way
to workaround UBs in code, I believe the following testcase must pass if
there is e.g.
char a[32];		// And this object ends up at address 0
void bar (void);
int main () { foo (&a[3]); baz (&a[6]); }
but fails right now.  As mentioned in the PR, in GCC 8 we used to do:
      else if (code == POINTER_PLUS_EXPR)
        {
          /* For pointer types, we are really only interested in asserting
             whether the expression evaluates to non-NULL.  */
          if (range_is_nonnull (&vr0) || range_is_nonnull (&vr1))
            set_value_range_to_nonnull (vr, expr_type);
and that triggered pretty much never, as range_is_nonnull requires that the
offset is ~[0, 0] exactly, e.g. if it is a constant, it is never that way,
but now we do:
	  if (!range_includes_zero_p (&vr0) || !range_includes_zero_p (&vr1))
which is e.g. always if the offset is constant non-zero.

I hope we can still say that pointer wrapping even with
-fno-delete-null-pointer-checks is UB, so this patch differentiates between
positive offsets (in ssizetype), negative offsets (in ssizetype) and zero
offsets and handles both the same for both ptr p+ offset and &MEM_REF[ptr, offset]
If offset is 0 and ptr is ~[0, 0], then the result is ~[0, 0] as before.
If offset is positive in ssizetype, then even for VARYING ptr the result is
~[0, 0] pointer.  If the offset is (or maybe could be) negative in
ssizetype, then for -fno-delete-null-pointer-checks the result is VARYING,
as we could go from a non-NULL pointer back to NULL on those targets; for
-fdelete-null-pointer-checks we do what we've done before, i.e. ~[0, 0].

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2018-12-06  Jakub Jelinek  <jakub@redhat.com>

	PR c/88367
	* tree-vrp.c (extract_range_from_binary_expr): For POINTER_PLUS_EXPR
	with -fno-delete-null-pointer-checks, set_nonnull only if the pointer
	is non-NULL and offset is known to have most significant bit clear.
	* vr-values.c (vr_values::vrp_stmt_computes_nonzero): For ADDR_EXPR
	of MEM_EXPR, return true if the MEM_EXPR has non-zero offset with
	most significant bit clear.  If offset does have most significant bit
	set and -fno-delete-null-pointer-checks, don't return true even if
	the base pointer is non-NULL.

	* gcc.dg/tree-ssa/pr88367.c: New test.

--- gcc/tree-vrp.c.jj	2018-12-04 13:00:02.408635579 +0100
+++ gcc/tree-vrp.c	2018-12-05 19:07:36.187567781 +0100
@@ -1673,9 +1673,25 @@ extract_range_from_binary_expr (value_ra
       else if (code == POINTER_PLUS_EXPR)
 	{
 	  /* For pointer types, we are really only interested in asserting
-	     whether the expression evaluates to non-NULL.  */
-	  if (!range_includes_zero_p (&vr0)
-	      || !range_includes_zero_p (&vr1))
+	     whether the expression evaluates to non-NULL.
+	     With -fno-delete-null-pointer-checks we need to be more
+	     conservative.  As some object might reside at address 0,
+	     then some offset could be added to it and the same offset
+	     subtracted again and the result would be NULL.
+	     E.g.
+	     static int a[12]; where &a[0] is NULL and
+	     ptr = &a[6];
+	     ptr -= 6;
+	     ptr will be NULL here, even when there is POINTER_PLUS_EXPR
+	     where the first range doesn't include zero and the second one
+	     doesn't either.  As the second operand is sizetype (unsigned),
+	     consider all ranges where the MSB could be set as possible
+	     subtractions where the result might be NULL.  */
+	  if ((!range_includes_zero_p (&vr0)
+	       || !range_includes_zero_p (&vr1))
+	      && (flag_delete_null_pointer_checks
+		  || (range_int_cst_p (&vr1)
+		      && !tree_int_cst_sign_bit (vr1.max ()))))
 	    vr->set_nonnull (expr_type);
 	  else if (range_is_null (&vr0) && range_is_null (&vr1))
 	    vr->set_null (expr_type);
--- gcc/vr-values.c.jj	2018-11-29 08:41:33.152749436 +0100
+++ gcc/vr-values.c	2018-12-05 19:37:56.222582823 +0100
@@ -303,8 +303,17 @@ vr_values::vrp_stmt_computes_nonzero (gi
 	  && TREE_CODE (base) == MEM_REF
 	  && TREE_CODE (TREE_OPERAND (base, 0)) == SSA_NAME)
 	{
-	  value_range *vr = get_value_range (TREE_OPERAND (base, 0));
-	  if (!range_includes_zero_p (vr))
+	  if (integer_zerop (TREE_OPERAND (base, 1))
+	      || flag_delete_null_pointer_checks)
+	    {
+	      value_range *vr = get_value_range (TREE_OPERAND (base, 0));
+	      if (!range_includes_zero_p (vr))
+		return true;
+	    }
+	  /* If MEM_REF has a "positive" offset, consider it non-NULL
+	     always.  */
+	  if (integer_nonzerop (TREE_OPERAND (base, 1))
+	      && !tree_int_cst_sign_bit (TREE_OPERAND (base, 1)))
 	    return true;
 	}
     }
--- gcc/testsuite/gcc.dg/tree-ssa/pr88367.c.jj	2018-12-05 19:24:36.386727781 +0100
+++ gcc/testsuite/gcc.dg/tree-ssa/pr88367.c	2018-12-05 19:40:43.228839763 +0100
@@ -0,0 +1,31 @@
+/* PR c/88367 */
+/* { dg-do compile } */
+/* { dg-options "-fno-delete-null-pointer-checks -O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump-not "link_error \\(\\);" "optimized" } } */
+/* { dg-final { scan-tree-dump-times "bar \\(\\);" 2 "optimized" } } */
+
+void bar (void);
+void link_error (void);
+
+void
+foo (char *p)
+{
+  if (!p)
+    return;
+  p += 3;
+  if (!p)
+    link_error ();
+  p -= 6;
+  if (!p)
+    bar ();
+}
+
+void
+baz (char *p)
+{
+  if (!p)
+    return;
+  p -= 6;
+  if (!p)
+    bar ();
+}

	Jakub


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