Patchwork [cxx-mem-model] test invalid bitfield twiddling

login
register
mail settings
Submitter Aldy Hernandez
Date March 21, 2011, 4:55 p.m.
Message ID <4D87831B.7080800@redhat.com>
Download mbox | patch
Permalink /patch/87802/
State New
Headers show

Comments

Aldy Hernandez - March 21, 2011, 4:55 p.m.
This test is inspired by PR 48124.

[Jakub, let me know if you find any additional variations on this theme.]

In the C++ memory model, non contiguous bitfields are distinct memory 
locations and cannot be accessed by and/or magic if we introduce data races.

In the test below, we can trigger invalid bit twiddling by declaring a 
bit field as volatile and then accessing it.  On x86-64, we end up using 
a 32-bit access which ends up touching the rest of the fields.

Unfortunately, I can't come up with a suitable test without the 
volatile-- I guess that's a good thing (either that, or my bit twiddle 
skills are lacking).

Committing to branch.

p.s. Oh yeah, I found some small buglets in the harness which are fixed 
below.
testsuite/
	* lib/gcc-memmodel-gdb-test.exp: Return if no executable.
	* gcc.dg/memmodel/memmodel.h (memmodel_done): Add noinline
	attribute.
	* g++.dg/memmodel/bitfields.C: New.

Patch

Index: testsuite/lib/gcc-memmodel-gdb-test.exp
===================================================================
--- testsuite/lib/gcc-memmodel-gdb-test.exp	(revision 170852)
+++ testsuite/lib/gcc-memmodel-gdb-test.exp	(working copy)
@@ -35,6 +35,10 @@  proc memmodel-gdb-test { } {
     set exec_file "[file rootname [file tail $prog]].exe"
     set cmd_file "$testsuite_dir/gcc.dg/memmodel/memmodel.gdb"
 
+    if ![file exists $exec_file] {
+	return
+    }
+
     send_log "Spawning: $gdb_name -nx -nw -quiet -x $cmd_file ./$exec_file\n"
     set res [remote_spawn target "$gdb_name -nx -nw  -x $cmd_file ./$exec_file"]
     if { $res < 0 || $res == "" } {
Index: testsuite/gcc.dg/memmodel/memmodel.h
===================================================================
--- testsuite/gcc.dg/memmodel/memmodel.h	(revision 170852)
+++ testsuite/gcc.dg/memmodel/memmodel.h	(working copy)
@@ -1,6 +1,6 @@ 
 int memmodel_fini = 0;
 
-void
+void __attribute__((noinline))
 memmodel_done ()
 {
   memmodel_fini = 1;
Index: testsuite/g++.dg/memmodel/bitfields.C
===================================================================
--- testsuite/g++.dg/memmodel/bitfields.C	(revision 0)
+++ testsuite/g++.dg/memmodel/bitfields.C	(revision 0)
@@ -0,0 +1,73 @@ 
+/* { dg-do link } */
+/* { dg-options "-O2 --param allow-load-data-races=0 --param allow-store-data-races=0" } */
+/* { dg-final { memmodel-gdb-test } } */
+
+/* Test that setting <var.a> does not touch either <var.b> or <var.c>.
+   In the C++ memory model, non contiguous bitfields ("a" and "c"
+   here) should be considered as distinct memory locations, so we
+   can't use bit twiddling to set either one.  */
+
+#include <stdio.h>
+#include "memmodel.h"
+
+#define CONSTA 12
+
+static int global;
+struct S
+{
+  /* On x86-64, the volatile causes us to access <a> with a 32-bit
+     access, and thus trigger this test.  */
+  volatile unsigned int a : 4;
+
+  unsigned char b;
+  unsigned int c : 6;
+} var;
+
+void set_a()
+{
+  var.a = CONSTA;
+}
+
+void memmodel_other_threads()
+{
+  ++global;
+  var.b = global;
+  var.c = global;
+}
+
+int memmodel_step_verify()
+{
+  int ret = 0;
+  if (var.b != global)
+    {
+      printf ("FAIL: Unexpected value: var.b is %d, should be %d\n",
+	      var.b, global);
+      ret = 1;
+    }
+  if (var.c != global)
+    {
+      printf ("FAIL: Unexpected value: var.c is %d, should be %d\n",
+	      var.c, global);
+      ret = 1;
+    }
+  return ret;
+}
+
+int memmodel_final_verify()
+{
+  int ret = memmodel_step_verify();
+  if (var.a != CONSTA)
+    {
+      printf ("FAIL: Unexpected value: var.a is %d, should be %d\n",
+	      var.a, CONSTA);
+      ret = 1;
+    }
+  return ret;
+}
+
+int main()
+{
+  set_a();
+  memmodel_done();
+  return 0;
+}