[rs6000] Fix reload failures in 64-bit mode with no special constant pool

[Eric Botcazou]

Hi,

this fixes reload failures in 64-bit mode with -mcmodel=medium for targets 
that define ASM_OUTPUT_SPECIAL_POOL_ENTRY_P to 0; that's the case for the 
VxWorks port we'll submit because of the compatibility with the system linker, 
which rejects .got sections in object files.

When reload ends up calling rs6000_emit_move on a register on the LHS and 
(const (symbol_ref) (const_int)) on the RHS, the function invokes:

          operands[1] = force_const_mem (mode, operands[1]);

creating (mem (symbol_ref)), which then needs to be legitimized on TARGET_TOC 
targets.  That's done by the immediately following block:

          if (TARGET_TOC
              && GET_CODE (XEXP (operands[1], 0)) == SYMBOL_REF
              && constant_pool_expr_p (XEXP (operands[1], 0))
              && ASM_OUTPUT_SPECIAL_POOL_ENTRY_P (
                        get_pool_constant (XEXP (operands[1], 0)),
                        get_pool_mode (XEXP (operands[1], 0))))
            {
              rtx tocref = create_TOC_reference (XEXP (operands[1], 0),
                                                 operands[0]);
              operands[1] = gen_const_mem (mode, tocref);
              set_mem_alias_set (operands[1], get_TOC_alias_set ());
            }

on most targets, but is disabled if ASM_OUTPUT_SPECIAL_POOL_ENTRY_P is 0.

Now you don't need to have a special pool to call create_TOC_reference, you 
can call it for regular TOC references as well, as done a few lines above:

      /* If this is a SYMBOL_REF that refers to a constant pool entry,
         and we have put it in the TOC, we just need to make a TOC-relative
         reference to it.  */
      if (TARGET_TOC
          && GET_CODE (operands[1]) == SYMBOL_REF
          && use_toc_relative_ref (operands[1], mode))
        operands[1] = create_TOC_reference (operands[1], operands[0]);

So the attached patch does it there too.

Tested on PowerPC64/Linux (LRA) and VxWorks (reload), OK for the mainline?


2016-10-08  Eric Botcazou  <ebotca...@adacore.com>

        * config/rs6000/rs6000.c (rs6000_emit_move): Also use a TOC reference
        after forcing to constant memory when the code model is medium.

-- 
Eric Botcazou
Index: config/rs6000/rs6000.c
===================================================================
--- config/rs6000/rs6000.c	(revision 240888)
+++ config/rs6000/rs6000.c	(working copy)
@@ -10590,10 +10590,7 @@ rs6000_emit_move (rtx dest, rtx source,
 
 	  if (TARGET_TOC
 	      && GET_CODE (XEXP (operands[1], 0)) == SYMBOL_REF
-	      && constant_pool_expr_p (XEXP (operands[1], 0))
-	      && ASM_OUTPUT_SPECIAL_POOL_ENTRY_P (
-			get_pool_constant (XEXP (operands[1], 0)),
-			get_pool_mode (XEXP (operands[1], 0))))
+	      && use_toc_relative_ref (XEXP (operands[1], 0), Pmode))
 	    {
 	      rtx tocref = create_TOC_reference (XEXP (operands[1], 0),
 						 operands[0]);