From patchwork Thu Sep  2 15:35:10 2010
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Subject: Default to -fstrict-volatile-bitfields for ARM EABI
Date: Thu, 02 Sep 2010 05:35:10 -0000
From: Jie Zhang <jie@codesourcery.com>
X-Patchwork-Id: 63497
Message-Id: <4C7FC42E.10208@codesourcery.com>
To: gcc-patches@gcc.gnu.org

-fstrict-volatile-bitfields should also be default for ARM EABI. This 
patch does it. I also copied volatile-bitfields-1.c and 
volatile-bitfields-2.c from i386 target subdirectory and tweaked for 
ARM. For gcc.target/arm/volatile-bitfields-3.c, we need the change of 
stor-layout.c in this patch.

Tested for arm-none-eabi. There are some regressions for 
gcc.c-torture/execute/20010518-2.c. When compile the test, gcc reports 
the warning and note:

20010518-2.c: In function 'main':
20010518-2.c:34:21: warning: mis-aligned access used for structure 
member [-fstrict-volatile-bitfields]
20010518-2.c:34:21: note: When a volatile object spans multiple 
type-sized locations, the compiler must choose between using a single 
mis-aligned access to preserve the volatility, or using multiple aligned 
accesses to avoid runtime faults.  This code may fail at runtime if the 
hardware does not allow this access.

So I think this test should be disabled for all targets which default to 
-fstrict-volatile-bitfields. If this is what we should do, I will update 
my patch to disable it for all such targets.

Any comments?


Thanks,



	* stor-layout.c (layout_decl): Use the field's type to
	determine the mode and keep DECL_BIT_FIELD for a volatile
	bit-field.
	* config/arm/arm.c (arm_override_options): Default to
	-fstrict-volatile-bitfields.

	testsuite/
	* gcc.target/arm/volatile-bitfields-1.c: New test.
	* gcc.target/arm/volatile-bitfields-2.c: New test.
	* gcc.target/arm/volatile-bitfields-3.c: New test.

Index: testsuite/gcc.target/arm/volatile-bitfields-1.c
===================================================================
--- testsuite/gcc.target/arm/volatile-bitfields-1.c	(revision 0)
+++ testsuite/gcc.target/arm/volatile-bitfields-1.c	(revision 0)
@@ -0,0 +1,17 @@
+/* { dg-do compile { target arm*-*-eabi* } } */
+/* { dg-options "-O2" } */
+
+typedef struct {
+  char a:1;
+  char b:7;
+  int c;
+} BitStruct;
+
+volatile BitStruct bits;
+
+int foo ()
+{
+  return bits.b;
+}
+
+/* { dg-final { scan-assembler "ldrb\[\\t \]+.*,\[\\t \]*\\\[.*\\\]" } } */
Index: testsuite/gcc.target/arm/volatile-bitfields-2.c
===================================================================
--- testsuite/gcc.target/arm/volatile-bitfields-2.c	(revision 0)
+++ testsuite/gcc.target/arm/volatile-bitfields-2.c	(revision 0)
@@ -0,0 +1,17 @@
+/* { dg-do compile { target arm*-*-eabi* } } */
+/* { dg-options "-O2" } */
+
+typedef struct {
+  volatile unsigned long a:8;
+  volatile unsigned long b:8;
+  volatile unsigned long c:16;
+} BitStruct;
+
+BitStruct bits;
+
+unsigned long foo ()
+{
+  return bits.b;
+}
+
+/* { dg-final { scan-assembler "ldr\[\\t \]+.*,\[\\t \]*\\\[.*\\\]" } } */
Index: testsuite/gcc.target/arm/volatile-bitfields-3.c
===================================================================
--- testsuite/gcc.target/arm/volatile-bitfields-3.c	(revision 0)
+++ testsuite/gcc.target/arm/volatile-bitfields-3.c	(revision 0)
@@ -0,0 +1,17 @@
+/* { dg-do compile { target arm*-*-eabi* } } */
+/* { dg-options "-O2" } */
+
+typedef struct {
+  volatile unsigned long a:8;
+  volatile unsigned long b:8;
+  volatile unsigned long c:16;
+} BitStruct;
+
+BitStruct bits;
+
+unsigned long foo ()
+{
+  return bits.c;
+}
+
+/* { dg-final { scan-assembler "ldr\[\\t \]+.*,\[\\t \]*\\\[.*\\\]" } } */
Index: stor-layout.c
===================================================================
--- stor-layout.c	(revision 163756)
+++ stor-layout.c	(working copy)
@@ -591,11 +591,14 @@ layout_decl (tree decl, unsigned int kno
 	    }
 
 	  /* See if we can use an ordinary integer mode for a bit-field.
-	     Conditions are: a fixed size that is correct for another mode
-	     and occupying a complete byte or bytes on proper boundary.  */
+	     Conditions are: a fixed size that is correct for another mode,
+	     occupying a complete byte or bytes on proper boundary,
+	     and not volatile or not -fstrict-volatile-bitfields.  */
 	  if (TYPE_SIZE (type) != 0
 	      && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST
-	      && GET_MODE_CLASS (TYPE_MODE (type)) == MODE_INT)
+	      && GET_MODE_CLASS (TYPE_MODE (type)) == MODE_INT
+	      && !(TREE_THIS_VOLATILE (decl)
+		   && flag_strict_volatile_bitfields > 0))
 	    {
 	      enum machine_mode xmode
 		= mode_for_size_tree (DECL_SIZE (decl), MODE_INT, 1);
Index: config/arm/arm.c
===================================================================
--- config/arm/arm.c	(revision 163756)
+++ config/arm/arm.c	(working copy)
@@ -1916,6 +1916,10 @@ arm_override_options (void)
        calculation, which is 2 instructions.  */
     set_param_value ("gcse-unrestricted-cost", 2);
 
+  /* ARM EABI defaults to strict volatile bitfields.  */
+  if (TARGET_AAPCS_BASED && flag_strict_volatile_bitfields < 0)
+    flag_strict_volatile_bitfields = 1;
+
   /* Register global variables with the garbage collector.  */
   arm_add_gc_roots ();
 }