[google] Backport r171347 and r181549 from trunk (strict volatile bitfield)

[jingyu, 2011-12-05 20:42:06]

Hi Ahmad,

This is a backport for two upstream patches into our 4.6-mobile branch.
These two patches have been backported to google-4.6 by Doug Kwan last week.

2011-12-05  Jing Yu  <jingyu@google.com>
       Backport r171347 and r181549 from trunk.

       gcc/
       2011-03-23  Julian Brown  <julian@codesourcery.com>

		* expr.c (expand_expr_real_1): Only use BLKmode for volatile
		accesses which are not naturally aligned.

	2011-11-20  Joey Ye  <joey.ye@arm.com>
		* expr.c (expand_expr_real_1): Correctly handle strict volatile
		bitfield loads smaller than mode size.

	gcc/testsuite/
	2011-11-20  Joey Ye  <joey.ye@arm.com>
		* gcc.dg/volatile-bitfields-1.c: New.

Index: gcc/expr.c
===================================================================
--- gcc/expr.c	(revision 182019)
+++ gcc/expr.c	(working copy)
@@ -9192,8 +9192,16 @@
 		&& modifier != EXPAND_CONST_ADDRESS
 		&& modifier != EXPAND_INITIALIZER)
 	    /* If the field is volatile, we always want an aligned
-	       access.  */
-	    || (volatilep && flag_strict_volatile_bitfields > 0)
+	       access.  Do this in following two situations:
+	       1. the access is not already naturally
+	       aligned, otherwise "normal" (non-bitfield) volatile fields
+	       become non-addressable.
+	       2. the bitsize is narrower than the access size. Need
+	       to extract bitfields from the access.  */
+	    || (volatilep && flag_strict_volatile_bitfields > 0
+		&& (bitpos % GET_MODE_ALIGNMENT (mode) != 0 
+		    || (mode1 != BLKmode
+		        && bitsize < GET_MODE_SIZE (mode1) * BITS_PER_UNIT)))
 	    /* If the field isn't aligned enough to fetch as a memref,
 	       fetch it as a bit field.  */
 	    || (mode1 != BLKmode
Index: gcc/testsuite/gcc.dg/volatile-bitfields-1.c
===================================================================
--- gcc/testsuite/gcc.dg/volatile-bitfields-1.c	(revision 0)
+++ gcc/testsuite/gcc.dg/volatile-bitfields-1.c	(revision 0)
@@ -0,0 +1,23 @@
+/* { dg-options "-fstrict-volatile-bitfields" } */
+/* { dg-do run } */
+
+extern int puts(const char *);
+extern void abort(void) __attribute__((noreturn));
+
+typedef struct {
+  volatile unsigned short a:8, b:8;
+} BitStruct;
+
+BitStruct bits = {1, 2};
+
+void check(int i, int j)
+{
+  if (i != 1 || j != 2) puts("FAIL"), abort();
+}
+
+int main ()
+{
+  check(bits.a, bits.b);
+
+  return 0;
+}

--
This patch is available for review at http://codereview.appspot.com/5453050

[asharif1, 2011-12-05 22:54:55]

LGTM.