[PATCH 2/4, middle-end] Factor out low_bitmask_len from try_widen_shift_mode

[Adam Nemet <anemet at caviumnetworks dot com>]

This patch should have no effect on the generated code.  It simply factors out
a helper, low_bitmask_len, to be used from the MIPS backend in the final patch
of the series.

Bootstrapped and regtested on {mips64octeon,x86_64}-linux.  Also regtested on
mipsisa64r2-elf.

OK to install?

Adam

	* combine.c (try_widen_shift_mode): Factor out code to check if an
        interger constant is a low-order bitmask from here ...
	* rtlanal.c (low_bitmask_len): ... to here.
	* rtl.h (low_bitmask_len): Declare.

Index: gcc/rtl.h
===================================================================
--- gcc.orig/rtl.h	2009-07-28 12:19:56.000000000 -0700
+++ gcc/rtl.h	2009-07-28 13:41:46.000000000 -0700
@@ -1095,6 +1095,7 @@ extern unsigned HOST_WIDE_INT nonzero_bi
 extern unsigned int num_sign_bit_copies (const_rtx, enum machine_mode);
 extern bool constant_pool_constant_p (rtx);
 extern bool truncated_to_mode (enum machine_mode, const_rtx);
+extern int low_bitmask_len (enum machine_mode, unsigned HOST_WIDE_INT);
 
 
 /* 1 if RTX is a subreg containing a reg that is already known to be
Index: gcc/combine.c
===================================================================
--- gcc.orig/combine.c	2009-07-28 12:19:56.000000000 -0700
+++ gcc/combine.c	2009-07-28 12:20:45.000000000 -0700
@@ -9024,13 +9024,9 @@ try_widen_shift_mode (enum rtx_code code
 
       /* We can also widen if the bits brought in will be masked off.  This
 	 operation is performed in ORIG_MODE.  */
-      if (outer_code == AND
-	  && GET_MODE_BITSIZE (orig_mode) <= HOST_BITS_PER_WIDE_INT)
+      if (outer_code == AND)
 	{
-	  int care_bits;
-
-	  outer_const &= GET_MODE_MASK (orig_mode);
-	  care_bits = exact_log2 (outer_const + 1);
+	  int care_bits = low_bitmask_len (orig_mode, outer_const);
 
 	  if (care_bits >= 0
 	      && GET_MODE_BITSIZE (orig_mode) - care_bits >= count)
Index: gcc/rtlanal.c
===================================================================
--- gcc.orig/rtlanal.c	2009-07-28 12:19:56.000000000 -0700
+++ gcc/rtlanal.c	2009-07-28 13:50:34.000000000 -0700
@@ -5032,3 +5032,18 @@ constant_pool_constant_p (rtx x)
   x = avoid_constant_pool_reference (x);
   return GET_CODE (x) == CONST_DOUBLE;
 }
+
+/* If M is a bitmask that selects a field of low-order bits within an item but
+   not the entire word, return the length of the field.  Return -1 otherwise.
+   M is used in machine mode MODE.  MODE can be VOIDmode if it's unknown.  */
+
+int
+low_bitmask_len (enum machine_mode mode, unsigned HOST_WIDE_INT m)
+{
+  if (GET_MODE_BITSIZE (mode) > HOST_BITS_PER_WIDE_INT)
+    return -1;
+  if (mode != VOIDmode)
+    m &= GET_MODE_MASK (mode);
+
+  return exact_log2 (m + 1);
+}