From patchwork Wed Mar 30 14:11:58 2011
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Subject: [cxx-mem-model] bitfield tests
Date: Wed, 30 Mar 2011 04:11:58 -0000
From: Aldy Hernandez <aldyh@redhat.com>
X-Patchwork-Id: 88924
Message-Id: <4D933A2E.9030105@redhat.com>
To: Richard Guenther <richard.guenther@gmail.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>, Jakub Jelinek <jakub@redhat.com>

> The memory model is not implementable on strict-alignment targets
> that do not have a byte store operation.  But we previously said that ;)

Yes.  I think we should issue an error when we have such a target and 
the user tries -fmemory-model=c++0x.  However, how many strict-alignment 
targets are not byte addressable nowadays?

> Also consider global vars
>
> char a;
> char b;
>
> accessing them on strict-align targets may access adjacent globals
> (that's a problem anyway, also with alias analysis).

Good point.  I am adding a test to that effect (see attached patch).

BTW, I assume you mean strict-align targets WITHOUT byte-addressability 
as above.  I have spot-checked your scenario on a handful of important 
targets that have strict alignment, and all of them work without 
touching adjacent global vars:

	arm-elf		OK
	sparc-linux	OK
	ia64-linux	OK
	alpha-linux	OK, but only with -mbwx (byte addressability)

rth tells me that we shouldn't worry about ancient non-byte addressable 
Alphas, so the last isn't an issue.

So... do you have any important targets in mind, because I don't see 
this being a problem for most targets?  As can be expected, I am only 
interested in x86*, powerpc*, and s390, especially since a cursory 
glance on other important targets didn't exhibit any problems.  However, 
given my target bias, I am willing to look into any important targets 
that are problematic (I'm hoping none :)).

Let me know if you see anything else, and please take a quick peek at 
the attached patch below, which I will be committing shortly.

As usual, thanks.
Aldy


Index: testsuite/gcc.dg/memmodel/strict-align-global.c
===================================================================
--- testsuite/gcc.dg/memmodel/strict-align-global.c	(revision 0)
+++ testsuite/gcc.dg/memmodel/strict-align-global.c	(revision 0)
@@ -0,0 +1,46 @@
+/* { dg-do link } */
+/* { dg-options "-O2 --param allow-packed-store-data-races=0" } */
+/* { dg-final { memmodel-gdb-test } } */
+
+#include <stdio.h>
+#include "memmodel.h"
+
+/* This test verifies writes to globals do not write to adjacent
+   globals.  This mostly happens on strict-align targets that are not
+   byte addressable (old Alphas, etc).  */
+
+char a = 0;
+char b = 77;
+
+void memmodel_other_threads() 
+{
+}
+
+int memmodel_step_verify()
+{
+  if (b != 77)
+    {
+      printf("FAIL: Unexpected value.  <b> is %d, should be 77\n", b);
+      return 1;
+    }
+  return 0;
+}
+
+/* Verify that every variable has the correct value.  */
+int memmodel_final_verify()
+{
+  int ret = memmodel_step_verify ();
+  if (a != 66)
+    {
+      printf("FAIL: Unexpected value.  <a> is %d, should be 66\n", a);
+      return 1;
+    }
+  return ret;
+}
+
+int main ()
+{
+  a = 66;
+  memmodel_done();
+  return 0;
+}