[Xtensa] fix subword loads from constant pool

[Xtensa] fix subword loads from constant pool

[Bob Wilson]

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I'm committing this patch to fix an ICE for the Xtensa port.  A QImode or HImode 
reload from the constant pool requires a scratch register because Xtensa can 
only load 32-bit values from the constant pool.  I ran the testsuite for an 
xtensa-elf target to check for regressions and I verified that this fixes the 
original problem for an xtensa-linux-uclibc target.  I'll apply this on the 4.2 
branch as well.

2008-04-03  Bob Wilson  <bob.wilson@acm.org>

         * config/xtensa/xtensa.c (xtensa_secondary_reload_class): Use a
         secondary input reload for subword loads from the constant pool.

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Index: config/xtensa/xtensa.c
===================================================================
--- config/xtensa/xtensa.c	(revision 133873)
+++ config/xtensa/xtensa.c	(working copy)
@@ -2839,7 +2839,8 @@
 
   if (!isoutput)
     {
-      if (class == FP_REGS && constantpool_mem_p (x))
+      if ((class == FP_REGS || GET_MODE_SIZE (mode) < UNITS_PER_WORD)
+	  && constantpool_mem_p (x))
 	return RL_REGS;
     }