Avoid expecting nonzero size for access none void* arguments [PR101751].

Resolves:
PR middle-end/101751 - attribute access none with void pointer expects nonzero size

gcc/ChangeLog:

	PR middle-end/101751
	* doc/extend.texi (attribute access): Adjust.
	* gimple-ssa-warn-access.cc (pass_waccess::maybe_check_access_sizes):
	Treat access mode none on a void* argument as expecting as few as
	zero bytes.

gcc/testsuite/ChangeLog:

	PR middle-end/101751
	* gcc.dg/Wstringop-overflow-86.c: New test.

gcc/doc/extend.texi

@@ -2524,7 +2524,6 @@ The following attributes are supported on most targets.
 @table @code
 @c Keep this table alphabetized by attribute name.  Treat _ as space.
 
-@item access
 @itemx access (@var{access-mode}, @var{ref-index})
 @itemx access (@var{access-mode}, @var{ref-index}, @var{size-index})
 
@@ -2598,7 +2597,9 @@ __attribute__ ((access (write_only, 1, 2), access (read_write, 3))) int fgets (c
 The access mode @code{none} specifies that the pointer to which it applies
 is not used to access the referenced object at all.  Unless the pointer is
 null the pointed-to object must exist and have at least the size as denoted
-by the @var{size-index} argument.  The object need not be initialized.
+by the @var{size-index} argument.  When the optional @var{size-index}
+argument is omitted for an argument of @code{void*} type the actual pointer
+agument is ignored.  The referenced object need not be initialized.
 The mode is intended to be used as a means to help validate the expected
 object size, for example in functions that call @code{__builtin_object_size}.
 @xref{Object Size Checking}.

gcc/gimple-ssa-warn-access.cc

@@ -2998,6 +2998,10 @@ pass_waccess::maybe_check_access_sizes (rdwr_map *rwm, tree fndecl, tree fntype,
 	  if (access.second.minsize
 	      && access.second.minsize != HOST_WIDE_INT_M1U)
 	    access_nelts = build_int_cstu (sizetype, access.second.minsize);
+	  else if (VOID_TYPE_P (argtype) && access.second.mode == access_none)
+	    /* Treat access mode none on a void* argument as expecting
+	       as little as zero bytes.  */
+	    access_nelts = size_zero_node;
 	  else
 	    access_nelts = size_one_node;
 	}

gcc/testsuite/gcc.dg/Wstringop-overflow-86.c

@@ -0,0 +1,63 @@
+/* PR middle-end/101751 - attribute access none with void pointer expects
+   nonzero size
+   { dg-do compile }
+   { dg-options "-Wall" } */
+
+__attribute__ ((access (none, 1))) void
+fvp_m1 (const void*);
+
+void nowarn_m1 (void)
+{
+  /* Verify these don't trigger a warning for calls to a function
+     declared with attribute access none.  */
+  fvp_m1 ((void*)-1);         // { dg-bogus "-Wstringop-" }
+  fvp_m1 ((void*)1);          // { dg-bogus "-Wstringop-" }
+}
+
+
+__attribute__ ((access (none, 1))) void
+fvp_none (void*);
+
+void nowarn_c_cp1 (void)
+{
+  char c;
+  fvp_none (&c);
+  fvp_none (&c + 1);          // { dg-bogus "-Wstringop-" }
+}
+
+void nowarn_f_fp1 (void)
+{
+  fvp_none ((char*)&nowarn_f_fp1);
+  fvp_none ((char*)&nowarn_f_fp1 + 1);
+}
+
+void nowarn_sp1_sp_4 (void)
+{
+  fvp_none ("" + 1);          // { dg-bogus "-Wstringop-" }
+  fvp_none ("123" + 4);       // { dg-bogus "-Wstringop-" }
+}
+
+
+__attribute__ ((access (none, 1))) void
+wfvp_none (void*);            // { dg-message "in a call to function 'wfvp_none' declared with attribute 'access \\\(none, 1\\\)'" }
+
+void warn_cm1_p1 (void)
+{
+  char c;
+  /* With optimization both of the following are diagnosed by -Warray-bounds.
+     The second also without optimization by -Wstringop-overread.  They
+     should both be diagnosed by the same warning even without optimization. */
+  wfvp_none (&c - 1);         // { dg-warning "" "pr??????" { xfail *-*-* } }
+  wfvp_none (&c + 2);         // { dg-warning "" }
+}
+
+void warn_fp2 (void)
+{
+  void *p = (char*)&warn_fp2 + sizeof warn_fp2;
+  fvp_none (p);               // { dg-warning "" "pr??????" { xfail *-*-* } }
+}
+
+void warn_sp2 (void)
+{
+  wfvp_none ("" + 2);         // { dg-warning "" }
+}