Re: [PATCH] avoid warning on constant strncpy until next statement is reachable (PR 87028)

[Martin Sebor <msebor@gmail.com>]

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>> Sadly, dstbase is the PARM_DECL for d.  That's where things are going
>> "wrong".  Not sure why you're getting the PARM_DECL in that case.  I'd
>> debug get_addr_base_and_unit_offset to understand what's going on.
>> Essentially you're getting different results of
>> get_addr_base_and_unit_offset in a case where they arguably should be
>> the same.
>
> Probably get_attr_nonstring_decl has the same "mistake" and returns
> the PARM_DECL instead of the SSA name pointer.  So we're comparing
> apples and oranges here.

Returning the SSA_NAME_VAR from get_attr_nonstring_decl() is
intentional but the function need not (perhaps should not)
also set *REF to it.

>
> Yeah:
>
> /* If EXPR refers to a character array or pointer declared attribute
>    nonstring return a decl for that array or pointer and set *REF to
>    the referenced enclosing object or pointer.  Otherwise returns
>    null.  */
>
> tree
> get_attr_nonstring_decl (tree expr, tree *ref)
> {
>   tree decl = expr;
>   if (TREE_CODE (decl) == SSA_NAME)
>     {
>       gimple *def = SSA_NAME_DEF_STMT (decl);
>
>       if (is_gimple_assign (def))
>         {
>           tree_code code = gimple_assign_rhs_code (def);
>           if (code == ADDR_EXPR
>               || code == COMPONENT_REF
>               || code == VAR_DECL)
>             decl = gimple_assign_rhs1 (def);
>         }
>       else if (tree var = SSA_NAME_VAR (decl))
>         decl = var;
>     }
>
>   if (TREE_CODE (decl) == ADDR_EXPR)
>     decl = TREE_OPERAND (decl, 0);
>
>   if (ref)
>     *ref = decl;
>
> I see a lot of "magic" here again in the attempt to "propagate"
> a nonstring attribute.

That's the function's purpose: to look for the attribute.  Is
there a better way to do this?

> Note
>
> foo (char *p __attribute__(("nonstring")))
> {
>   p = "bar";
>   strlen (p); // or whatever is necessary to call get_attr_nonstring_decl
> }
>
> is perfectly valid and p as passed to strlen is _not_ nonstring(?).

I don't know if you're saying that it should get a warning or
shouldn't.  Right now it doesn't because the strlen() call is
folded before we check for nonstring.

I could see an argument for diagnosing it but I suspect you
wouldn't like it because it would mean more warning from
the folder.  I could also see an argument against it because,
as you said, it's safe.

If you take the assignment to p away then a warning is issued,
and that's because p is declared with attribute nonstring.
That's also why get_attr_nonstring_decl looks at SSA_NAME_VAR.

> I think in your code comparing bases you want to look at the _original_
> argument to the string function rather than what get_attr_nonstring_decl
> returned as ref.

I've adjusted get_attr_nonstring_decl() to avoid setting *REF
to SSA_NAME_VAR.  That let me remove the GIMPLE_NOP code from
the patch.  I've also updated the comment above SSA_NAME_VAR
to clarify its purpose per Jeff's comments.

Attached is an updated revision with these changes.

Martin

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PR tree-optimization/87028 - false positive -Wstringop-truncation strncpy with global variable source string
gcc/ChangeLog:

	PR tree-optimization/87028
	* calls.c (get_attr_nonstring_decl): Avoid setting *REF to
	SSA_NAME_VAR.
	* gimple-fold.c (gimple_fold_builtin_strncpy): Avoid folding
	when statement doesn't belong to a basic block.
	* tree.h (SSA_NAME_VAR): Update comment.
	* tree-ssa-strlen.c (maybe_diag_stxncpy_trunc): Simplify.

gcc/testsuite/ChangeLog:

	PR tree-optimization/87028
	* c-c++-common/Wstringop-truncation.c: Remove xfails.
	* gcc.dg/Wstringop-truncation-5.c: New test.

Index: gcc/tree.h
===================================================================
--- gcc/tree.h	(revision 263925)
+++ gcc/tree.h	(working copy)
@@ -1697,7 +1697,10 @@ extern tree maybe_wrap_with_location (tree, locati
    : NULL_TREE)
 
 /* Returns the variable being referenced.  This can be NULL_TREE for
-   temporaries not associated with any user variable.
+   temporaries not associated with any user variable.  The result
+   is mainly useful for debugging, diagnostics, or as the target
+   declaration referenced by an SSA_NAME.  Otherwise, because
+   it has no dataflow information, it should not be used.
    Once released, this is the only field that can be relied upon.  */
 #define SSA_NAME_VAR(NODE)					\
   (SSA_NAME_CHECK (NODE)->ssa_name.var == NULL_TREE		\
Index: gcc/calls.c
===================================================================
--- gcc/calls.c	(revision 263928)
+++ gcc/calls.c	(working copy)
@@ -1503,6 +1503,7 @@ tree
 get_attr_nonstring_decl (tree expr, tree *ref)
 {
   tree decl = expr;
+  tree var = NULL_TREE;
   if (TREE_CODE (decl) == SSA_NAME)
     {
       gimple *def = SSA_NAME_DEF_STMT (decl);
@@ -1515,17 +1516,25 @@ get_attr_nonstring_decl (tree expr, tree *ref)
 	      || code == VAR_DECL)
 	    decl = gimple_assign_rhs1 (def);
 	}
-      else if (tree var = SSA_NAME_VAR (decl))
-	decl = var;
+      else
+	var = SSA_NAME_VAR (decl);
     }
 
   if (TREE_CODE (decl) == ADDR_EXPR)
     decl = TREE_OPERAND (decl, 0);
 
+  /* To simplify calling code, store the referenced DECL regardless of
+     the attribute determined below, but avoid storing the SSA_NAME_VAR
+     obtained above (it's not useful for dataflow purposes).  */
   if (ref)
     *ref = decl;
 
-  if (TREE_CODE (decl) == ARRAY_REF)
+  /* Use the SSA_NAME_VAR that was determined above to see if it's
+     declared nonstring.  Otherwise drill down into the referenced
+     DECL.  */
+  if (var)
+    decl = var;
+  else if (TREE_CODE (decl) == ARRAY_REF)
     decl = TREE_OPERAND (decl, 0);
   else if (TREE_CODE (decl) == COMPONENT_REF)
     decl = TREE_OPERAND (decl, 1);
Index: gcc/gimple-fold.c
===================================================================
--- gcc/gimple-fold.c	(revision 263925)
+++ gcc/gimple-fold.c	(working copy)
@@ -1702,6 +1702,11 @@ gimple_fold_builtin_strncpy (gimple_stmt_iterator
   if (tree_int_cst_lt (ssize, len))
     return false;
 
+  /* Defer warning (and folding) until the next statement in the basic
+     block is reachable.  */
+  if (!gimple_bb (stmt))
+    return false;
+
   /* Diagnose truncation that leaves the copy unterminated.  */
   maybe_diag_stxncpy_trunc (*gsi, src, len);
 
Index: gcc/tree-ssa-strlen.c
===================================================================
--- gcc/tree-ssa-strlen.c	(revision 263925)
+++ gcc/tree-ssa-strlen.c	(working copy)
@@ -1904,8 +1904,6 @@ maybe_diag_stxncpy_trunc (gimple_stmt_iterator gsi
   if (TREE_CODE (dstdecl) == ADDR_EXPR)
     dstdecl = TREE_OPERAND (dstdecl, 0);
 
-  tree ref = NULL_TREE;
-
   if (!sidx)
     {
       /* If the source is a non-string return early to avoid warning
@@ -1914,12 +1912,14 @@ maybe_diag_stxncpy_trunc (gimple_stmt_iterator gsi
       tree srcdecl = gimple_call_arg (stmt, 1);
       if (TREE_CODE (srcdecl) == ADDR_EXPR)
 	srcdecl = TREE_OPERAND (srcdecl, 0);
-      if (get_attr_nonstring_decl (srcdecl, &ref))
+      if (get_attr_nonstring_decl (srcdecl, NULL))
 	return false;
     }
 
-  /* Likewise, if the destination refers to a an array/pointer declared
-     nonstring return early.  */
+  /* Likewise, if the destination refers to an array/pointer declared
+     nonstring return early.  REF will be set to the referenced enclosing
+     object or pointer either way.  */
+  tree ref;
   if (get_attr_nonstring_decl (dstdecl, &ref))
     return false;
 
Index: gcc/testsuite/c-c++-common/Wstringop-truncation.c
===================================================================
--- gcc/testsuite/c-c++-common/Wstringop-truncation.c	(revision 263925)
+++ gcc/testsuite/c-c++-common/Wstringop-truncation.c	(working copy)
@@ -329,9 +329,8 @@ void test_strncpy_array (Dest *pd, int i, const ch
      of the array to NUL is not diagnosed.  */
   {
     /* This might be better written using memcpy() but it's safe so
-       it probably shouldn't be diagnosed.  It currently triggers
-       a warning because of bug 81704.  */
-    strncpy (dst7, "0123456", sizeof dst7);   /* { dg-bogus "\\\[-Wstringop-truncation]" "bug 81704" { xfail *-*-* } } */
+       it isn't diagnosed.  See pr81704 and pr87028.  */
+    strncpy (dst7, "0123456", sizeof dst7);   /* { dg-bogus "\\\[-Wstringop-truncation]" } */
     dst7[sizeof dst7 - 1] = '\0';
     sink (dst7);
   }
@@ -350,7 +349,7 @@ void test_strncpy_array (Dest *pd, int i, const ch
   }
 
   {
-    strncpy (pd->a5, "01234", sizeof pd->a5);   /* { dg-bogus "\\\[-Wstringop-truncation]" "bug 81704" { xfail *-*-* } } */
+    strncpy (pd->a5, "01234", sizeof pd->a5);   /* { dg-bogus "\\\[-Wstringop-truncation]" } */
     pd->a5[sizeof pd->a5 - 1] = '\0';
     sink (pd);
   }
Index: gcc/testsuite/gcc.dg/Wstringop-truncation-5.c
===================================================================
--- gcc/testsuite/gcc.dg/Wstringop-truncation-5.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/Wstringop-truncation-5.c	(working copy)
@@ -0,0 +1,64 @@
+/* PR tree-optimization/87028 - false positive -Wstringop-truncation
+   strncpy with global variable source string
+   { dg-do compile }
+   { dg-options "-O2 -Wstringop-truncation" } */
+
+char *strncpy (char *, const char *, __SIZE_TYPE__);
+
+#define STR   "1234567890"
+
+struct S
+{
+  char a[5], b[5];
+};
+
+const char arr[] = STR;
+const char* const ptr = STR;
+
+const char arr2[][10] = { "123", STR };
+
+void test_literal (struct S *s)
+{
+  strncpy (s->a, STR, sizeof s->a - 1);     /* { dg-bogus "\\\[-Wstringop-truncation]" } */
+  s->a[sizeof s->a - 1] = '\0';
+}
+
+void test_global_arr (struct S *s)
+{
+  strncpy (s->a, arr, sizeof s->a - 1);     /* { dg-bogus "\\\[-Wstringop-truncation]" } */
+  s->a [sizeof s->a - 1] = '\0';
+}
+
+void test_global_arr2 (struct S *s)
+{
+  strncpy (s->a, arr2[1], sizeof s->a - 1); /* { dg-bogus "\\\[-Wstringop-truncation]" } */
+  s->a [sizeof s->a - 1] = '\0';
+
+  strncpy (s->b, arr2[0], sizeof s->a - 1);
+}
+
+void test_global_ptr (struct S *s)
+{
+  strncpy (s->a, ptr, sizeof s->a - 1);     /* { dg-bogus "\\\[-Wstringop-truncation]" } */
+  s->a [sizeof s->a - 1] = '\0';
+}
+
+void test_local_arr (struct S *s)
+{
+  const char arr[] = STR;
+  strncpy (s->a, arr, sizeof s->a - 1);
+  s->a [sizeof s->a - 1] = '\0';
+}
+
+void test_local_ptr (struct S *s)
+{
+  const char* const ptr = STR;
+  strncpy (s->a, ptr, sizeof s->a - 1);     /* { dg-bogus "\\\[-Wstringop-truncation]" } */
+  s->a [sizeof s->a - 1] = '\0';
+}
+
+void test_compound_literal (struct S *s)
+{
+  strncpy (s->a, (char[]){ STR }, sizeof s->a - 1);
+  s->a [sizeof s->a - 1] = '\0';
+}