[rs6000/rfc] -fPIC vs TLS

[Richard Henderson <rth at redhat dot com>]

The code we emit for TLS with -fPIC triggers the linker to 
turn on old-abi executable got.  I'm trying to figure out
the best way to rearrange things for TARGET_SECURE_PLT.

First of all, it seems like the existing code for the old
abi is being unnecessarily obtuse.

	bcl	20,31,2f
1:	.long	_GLOBAL_OFFSET_TABLE_-$
2:	
	mflr	1
	lwz	2,0(1)
	add	3,2,1

Isn't this an extremely long-winded way of loading _GOT_?

And, it turns out that the existance of that pc-relative
relocation vs _GOT_ is what causes the linker to go into
its old-style PLT mode.

For the case of secure-plt, I can't think of anything more
efficient than just treating _GOT_ as any other local symbol.
Which means (at least for the moment), just dropping its
address into the .got2 section and reading it from there.

A possible improvement for old-abi: as I understand it, -fPIC
code still does have a regular .got section, even if most bits
go into the local .got2 subsections.  This would mean that the
best way to get this address is with the plain old
load_toc_v4_pic_si pattern; that is

	bl	_GLOBAL_OFFSET_TABLE_@local-4
	mflr	3

Comments?


r~



Index: rs6000.c
===================================================================
--- rs6000.c	(revision 111910)
+++ rs6000.c	(working copy)
@@ -3108,36 +3108,12 @@ rs6000_legitimize_tls_address (rtx addr,
       else
 	{
 	  if (flag_pic == 1)
-	    got = gen_rtx_REG (Pmode, RS6000_PIC_OFFSET_TABLE_REGNUM);
+	    got = pic_offset_table_rtx;
 	  else
 	    {
 	      rtx gsym = rs6000_got_sym ();
 	      got = gen_reg_rtx (Pmode);
-	      if (flag_pic == 0)
-		rs6000_emit_move (got, gsym, Pmode);
-	      else
-		{
-		  rtx tempLR, tmp3, mem;
-		  rtx first, last;
-
-		  tempLR = gen_reg_rtx (Pmode);
-		  tmp1 = gen_reg_rtx (Pmode);
-		  tmp2 = gen_reg_rtx (Pmode);
-		  tmp3 = gen_reg_rtx (Pmode);
-		  mem = gen_const_mem (Pmode, tmp1);
-
-		  first = emit_insn (gen_load_toc_v4_PIC_1b (tempLR, gsym));
-		  emit_move_insn (tmp1, tempLR);
-		  emit_move_insn (tmp2, mem);
-		  emit_insn (gen_addsi3 (tmp3, tmp1, tmp2));
-		  last = emit_move_insn (got, tmp3);
-		  REG_NOTES (last) = gen_rtx_EXPR_LIST (REG_EQUAL, gsym,
-							REG_NOTES (last));
-		  REG_NOTES (first) = gen_rtx_INSN_LIST (REG_LIBCALL, last,
-							 REG_NOTES (first));
-		  REG_NOTES (last) = gen_rtx_INSN_LIST (REG_RETVAL, first,
-							REG_NOTES (last));
-		}
+	      rs6000_emit_move (got, gsym, Pmode);
 	    }
 	}