PowerPC64 ld toc misoptimization

[Alan Modra <amodra at gmail dot com>]

I built a gcc-4.6 based powerpc compiler with some -mcmodel=medium
changes (http://gcc.gnu.org/ml/gcc-patches/2011-09/msg00304.html)
today and found an unpleasant number of gcc testsuite failures.  These
turned out to be due to gcc-4.6 not allocating a base reg for the
largetoc_low insn, giving code like:

 addis 0,2,foo@toc@ha
 addic 0,0,foo@toc@l

which ld then misoptimized when it nopped out the first insn but left
the second unchanged.  The second insn should have transformed into

 addic 0,2,foo@toc

or even better

 addi 0,2,foo@toc

As far as I'm aware, this code sequence won't happen with current
gcc-4.7 or gcc-4.6 sources, but ld needs fixing.

This patch does that, performing the addic toc optimization, and more
importantly checks that both high and low intructions are as expected
before optimizing.  Applying mainline and branch.

bfd/
	* elf64-ppc.c (struct ppc64_elf_obj_tdata): Rename
	ha_relocs_not_using_r2 to unexpected_toc_insn.
	(ok_lo_toc_insn): New function.
	(ppc64_elf_edit_toc): Check insn on lo toc reloc.  Emit warning.
	(ppc64_elf_relocate_section): Don't check insn on lo toc reloc here.
	Handle addic on lo toc reloc.
ld/testsuite/
	* ld-powerpc/powerpc.exp: Expect ld warnings for tocopt test.
	* ld-powerpc/tocopt.out: New file.

Index: bfd/elf64-ppc.c
===================================================================
RCS file: /cvs/src/src/bfd/elf64-ppc.c,v
retrieving revision 1.368
diff -u -p -r1.368 elf64-ppc.c
--- bfd/elf64-ppc.c	20 Oct 2011 16:27:03 -0000	1.368
+++ bfd/elf64-ppc.c	8 Nov 2011 10:51:27 -0000
@@ -2614,8 +2614,9 @@ struct ppc64_elf_obj_tdata
      the reloc to be in the range -32768 to 32767.  */
   unsigned int has_small_toc_reloc : 1;
 
-  /* Set if toc/got ha relocs detected not using r2.  */
-  unsigned int ha_relocs_not_using_r2 : 1;
+  /* Set if toc/got ha relocs detected not using r2, or lo reloc
+     instruction not one we handle.  */
+  unsigned int unexpected_toc_insn : 1;
 };
 
 #define ppc64_elf_tdata(bfd) \
@@ -8016,6 +8017,32 @@ adjust_toc_syms (struct elf_link_hash_en
   return TRUE;
 }
 
+/* Return TRUE iff INSN is one we expect on a _LO variety toc/got reloc.  */
+
+static bfd_boolean
+ok_lo_toc_insn (unsigned int insn)
+{
+  return ((insn & (0x3f << 26)) == 14u << 26 /* addi */
+	  || (insn & (0x3f << 26)) == 32u << 26 /* lwz */
+	  || (insn & (0x3f << 26)) == 34u << 26 /* lbz */
+	  || (insn & (0x3f << 26)) == 36u << 26 /* stw */
+	  || (insn & (0x3f << 26)) == 38u << 26 /* stb */
+	  || (insn & (0x3f << 26)) == 40u << 26 /* lhz */
+	  || (insn & (0x3f << 26)) == 42u << 26 /* lha */
+	  || (insn & (0x3f << 26)) == 44u << 26 /* sth */
+	  || (insn & (0x3f << 26)) == 46u << 26 /* lmw */
+	  || (insn & (0x3f << 26)) == 47u << 26 /* stmw */
+	  || (insn & (0x3f << 26)) == 48u << 26 /* lfs */
+	  || (insn & (0x3f << 26)) == 50u << 26 /* lfd */
+	  || (insn & (0x3f << 26)) == 52u << 26 /* stfs */
+	  || (insn & (0x3f << 26)) == 54u << 26 /* stfd */
+	  || ((insn & (0x3f << 26)) == 58u << 26 /* lwa,ld,lmd */
+	      && (insn & 3) != 1)
+	  || ((insn & (0x3f << 26)) == 62u << 26 /* std, stmd */
+	      && ((insn & 3) == 0 || (insn & 3) == 3))
+	  || (insn & (0x3f << 26)) == 12u << 26 /* addic */);
+}
+
 /* Examine all relocs referencing .toc sections in order to remove
    unused .toc entries.  */
 
@@ -8270,11 +8297,13 @@ ppc64_elf_edit_toc (struct bfd_link_info
 		struct elf_link_hash_entry *h;
 		Elf_Internal_Sym *sym;
 		bfd_vma val;
+		enum {no_check, check_lo, check_ha} insn_check;
 
 		r_type = ELF64_R_TYPE (rel->r_info);
 		switch (r_type)
 		  {
 		  default:
+		    insn_check = no_check;
 		    break;
 
 		  case R_PPC64_GOT_TLSLD16_HA:
@@ -8283,24 +8312,49 @@ ppc64_elf_edit_toc (struct bfd_link_info
 		  case R_PPC64_GOT_DTPREL16_HA:
 		  case R_PPC64_GOT16_HA:
 		  case R_PPC64_TOC16_HA:
-		    {
-		      bfd_vma off = rel->r_offset & ~3;
-		      unsigned char buf[4];
-		      unsigned int insn;
+		    insn_check = check_ha;
+		    break;
 
-		      if (!bfd_get_section_contents (ibfd, sec, buf, off, 4))
-			{
-			  free (used);
-			  goto error_ret;
-			}
-		      insn = bfd_get_32 (ibfd, buf);
-		      if ((insn & ((0x3f << 26) | 0x1f << 16))
-			  != ((15u << 26) | (2 << 16)) /* addis rt,2,imm */)
-			ppc64_elf_tdata (ibfd)->ha_relocs_not_using_r2 = 1;
-		    }
+		  case R_PPC64_GOT_TLSLD16_LO:
+		  case R_PPC64_GOT_TLSGD16_LO:
+		  case R_PPC64_GOT_TPREL16_LO_DS:
+		  case R_PPC64_GOT_DTPREL16_LO_DS:
+		  case R_PPC64_GOT16_LO:
+		  case R_PPC64_GOT16_LO_DS:
+		  case R_PPC64_TOC16_LO:
+		  case R_PPC64_TOC16_LO_DS:
+		    insn_check = check_lo;
 		    break;
 		  }
 
+		if (insn_check != no_check)
+		  {
+		    bfd_vma off = rel->r_offset & ~3;
+		    unsigned char buf[4];
+		    unsigned int insn;
+
+		    if (!bfd_get_section_contents (ibfd, sec, buf, off, 4))
+		      {
+			free (used);
+			goto error_ret;
+		      }
+		    insn = bfd_get_32 (ibfd, buf);
+		    if (insn_check == check_lo
+			? !ok_lo_toc_insn (insn)
+			: ((insn & ((0x3f << 26) | 0x1f << 16))
+			   != ((15u << 26) | (2 << 16)) /* addis rt,2,imm */))
+		      {
+			char str[12];
+
+			ppc64_elf_tdata (ibfd)->unexpected_toc_insn = 1;
+			sprintf (str, "%#08x", insn);
+			info->callbacks->einfo
+			  (_("%P: %H: toc optimization is not supported for"
+			     " %s instruction.\n"),
+			   ibfd, sec, rel->r_offset & ~3, str);
+		      }
+		  }
+
 		switch (r_type)
 		  {
 		  case R_PPC64_TOC16:
@@ -13322,7 +13376,7 @@ ppc64_elf_relocate_section (bfd *output_
 	case R_PPC64_GOT16_HA:
 	case R_PPC64_TOC16_HA:
 	  if (htab->do_toc_opt && relocation + addend + 0x8000 < 0x10000
-	      && !ppc64_elf_tdata (input_bfd)->ha_relocs_not_using_r2)
+	      && !ppc64_elf_tdata (input_bfd)->unexpected_toc_insn)
 	    {
 	      bfd_byte *p = contents + (rel->r_offset & ~3);
 	      bfd_put_32 (input_bfd, NOP, p);
@@ -13338,33 +13392,22 @@ ppc64_elf_relocate_section (bfd *output_
 	case R_PPC64_TOC16_LO:
 	case R_PPC64_TOC16_LO_DS:
 	  if (htab->do_toc_opt && relocation + addend + 0x8000 < 0x10000
-	      && !ppc64_elf_tdata (input_bfd)->ha_relocs_not_using_r2)
+	      && !ppc64_elf_tdata (input_bfd)->unexpected_toc_insn)
 	    {
 	      bfd_byte *p = contents + (rel->r_offset & ~3);
 	      insn = bfd_get_32 (input_bfd, p);
-	      if ((insn & (0x3f << 26)) == 14u << 26 /* addi */
-		  || (insn & (0x3f << 26)) == 32u << 26 /* lwz */
-		  || (insn & (0x3f << 26)) == 34u << 26 /* lbz */
-		  || (insn & (0x3f << 26)) == 36u << 26 /* stw */
-		  || (insn & (0x3f << 26)) == 38u << 26 /* stb */
-		  || (insn & (0x3f << 26)) == 40u << 26 /* lhz */
-		  || (insn & (0x3f << 26)) == 42u << 26 /* lha */
-		  || (insn & (0x3f << 26)) == 44u << 26 /* sth */
-		  || (insn & (0x3f << 26)) == 46u << 26 /* lmw */
-		  || (insn & (0x3f << 26)) == 47u << 26 /* stmw */
-		  || (insn & (0x3f << 26)) == 48u << 26 /* lfs */
-		  || (insn & (0x3f << 26)) == 50u << 26 /* lfd */
-		  || (insn & (0x3f << 26)) == 52u << 26 /* stfs */
-		  || (insn & (0x3f << 26)) == 54u << 26 /* stfd */
-		  || ((insn & (0x3f << 26)) == 58u << 26 /* lwa,ld,lmd */
-		      && (insn & 3) != 1)
-		  || ((insn & (0x3f << 26)) == 62u << 26 /* std, stmd */
-		      && ((insn & 3) == 0 || (insn & 3) == 3)))
+	      if ((insn & (0x3f << 26)) == 12u << 26 /* addic */)
+		{
+		  /* Transform addic to addi when we change reg.  */
+		  insn &= ~((0x3f << 26) | (0x1f << 16));
+		  insn |= (14u << 26) | (2 << 16);
+		}
+	      else
 		{
 		  insn &= ~(0x1f << 16);
 		  insn |= 2 << 16;
-		  bfd_put_32 (input_bfd, insn, p);
 		}
+	      bfd_put_32 (input_bfd, insn, p);
 	    }
 	  break;
 	}
Index: ld/testsuite/ld-powerpc/powerpc.exp
===================================================================
RCS file: /cvs/src/src/ld/testsuite/ld-powerpc/powerpc.exp,v
retrieving revision 1.37
diff -u -p -r1.37 powerpc.exp
--- ld/testsuite/ld-powerpc/powerpc.exp	30 Sep 2011 05:11:04 -0000	1.37
+++ ld/testsuite/ld-powerpc/powerpc.exp	8 Nov 2011 11:54:18 -0000
@@ -204,7 +204,7 @@ set ppc64elftests {
     {"sym@tocbase" "-shared -melf64ppc" "-a64" {symtocbase-1.s symtocbase-2.s}
 	{{objdump -dj.data symtocbase.d}} "symtocbase.so"}
     {"TOC opt" "-melf64ppc" "-a64"  {tocopt.s}
-	{{objdump -s tocopt.d}} "tocopt"}
+	{{ld tocopt.out} {objdump -s tocopt.d}} "tocopt"}
     {"TOC opt2" "-melf64ppc --defsym x=2" "-a64"  {tocopt2.s}
 	{{ld tocopt2.out} {objdump -s tocopt2.d}} "tocopt2"}
     {"TOC opt3" "-melf64ppc -no-keep-memory --defsym x=2" "-a64"  {tocopt3.s}
Index: ld/testsuite/ld-powerpc/tocopt.out
===================================================================
RCS file: ld/testsuite/ld-powerpc/tocopt.out
diff -N ld/testsuite/ld-powerpc/tocopt.out
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ ld/testsuite/ld-powerpc/tocopt.out	8 Nov 2011 11:54:18 -0000
@@ -0,0 +1,4 @@
+.*
+\(\.text\+0x14\): .* 0x3fa00000 .*
+.*
+\(\.text\+0x34\): .* 0x3fa00010 .*

-- 
Alan Modra
Australia Development Lab, IBM