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.
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;
+}
