MIPS: smarter code for constant addresses

See http://gcc.gnu.org/ml/gcc/2008-11/msg00231.html

Backport of the original patch (by Richard Sandiford) to 4.3.2/4.3.3

--- a/gcc/config/mips/mips.c.orig	2009-01-21 08:54:56.526374343 +0100
+++ b/gcc/config/mips/mips.c	2009-01-21 08:55:03.229865079 +0100
@@ -2481,7 +2481,26 @@ mips_legitimize_tls_address (rtx loc)
     }
   return dest;
 }
-
+
+/* Return true if, for every base register BASE_REG, (plus BASE_REG X)
+   can address a value of mode MODE.  */
+
+static bool
+mips_valid_offset_p (rtx x, enum machine_mode mode)
+{
+  /* Check that X is a signed 16-bit number.  */
+  if (!const_arith_operand (x, Pmode))
+    return false;
+
+  /* We may need to split multiword moves, so make sure that every word
+     is accessible.  */
+  if (GET_MODE_SIZE (mode) > UNITS_PER_WORD
+      && !SMALL_OPERAND (INTVAL (x) + GET_MODE_SIZE (mode) - UNITS_PER_WORD))
+    return false;
+
+  return true;
+}
+
 /* This function is used to implement LEGITIMIZE_ADDRESS.  If *XLOC can
    be legitimized in a way that the generic machinery might not expect,
    put the new address in *XLOC and return true.  MODE is the mode of
@@ -2491,7 +2510,7 @@ bool
 mips_legitimize_address (rtx *xloc, enum machine_mode mode)
 {
   rtx base;
-  HOST_WIDE_INT offset;
+  HOST_WIDE_INT intval, high, offset;
 
   if (mips_tls_symbol_p (*xloc))
     {
@@ -2512,6 +2531,33 @@ mips_legitimize_address (rtx *xloc, enum
       *xloc = mips_add_offset (NULL, base, offset);
       return true;
     }
+
+  /* Handle references to constant addresses by loading the high part
+     into a register and using an offset for the low part.  */
+  if (GET_CODE (base) == CONST_INT)
+    {
+      intval = INTVAL (base);
+      high = trunc_int_for_mode (CONST_HIGH_PART (intval), Pmode);
+      offset = CONST_LOW_PART (intval);
+      /* Ignore cases in which a positive address would be accessed by a
+	 negative offset from a negative address.  The required wraparound
+	 does not occur for 32-bit addresses on 64-bit targets, and it is
+	 very unlikely that such an access would occur in real code anyway.
+
+	 If the low offset is not legitimate for MODE, prefer to load
+	 the constant normally, instead of using mips_force_address on
+	 the legitimized address.  The latter option would cause us to
+	 use (D)ADDIU unconditionally, but LUI/ORI is more efficient
+	 than LUI/ADDIU on some targets.  */
+      if ((intval < 0 || high > 0)
+	  && mips_valid_offset_p (GEN_INT (offset), mode))
+	{
+	  base = mips_force_temporary (NULL, GEN_INT (high));
+	  *xloc = plus_constant (base, offset);
+	  return true;
+	}
+    }
+
   return false;
 }