[PATCH] x86-64: correct segment override prefix generation

["Jan Beulich" <JBeulich at suse dot com>]

Despite them being ignored by the CPU, gas issues segment override
prefixes for other than FS/GS in 64-bit mode. If doing so at all, it
should clearly do this correctly. Determining the default segment,
however, requires to take into consideration RegRex (so far, RSP, RBP,
R12, and R13 were all treated equally here).

2012-07-24  Jan Beulich <jbeulich@suse.com>

	* config/tc-i386-intel.c (build_modrm_byte): Split determining
	default segment from figuring out encoding. Honor RegRex for
	the former.

--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -5733,18 +5733,14 @@ build_modrm_byte (void)
 	      i.sib.base = i.base_reg->reg_num;
 	      /* x86-64 ignores REX prefix bit here to avoid decoder
 		 complications.  */
-	      if ((i.base_reg->reg_num & 7) == EBP_REG_NUM)
-		{
+	      if (!(i.base_reg->reg_flags & RegRex)
+		  && (i.base_reg->reg_num == EBP_REG_NUM
+		   || i.base_reg->reg_num == ESP_REG_NUM))
 		  default_seg = &ss;
-		  if (i.disp_operands == 0)
-		    {
-		      fake_zero_displacement = 1;
-		      i.types[op].bitfield.disp8 = 1;
-		    }
-		}
-	      else if (i.base_reg->reg_num == ESP_REG_NUM)
+	      if (i.base_reg->reg_num == 5 && i.disp_operands == 0)
 		{
-		  default_seg = &ss;
+		  fake_zero_displacement = 1;
+		  i.types[op].bitfield.disp8 = 1;
 		}
 	      i.sib.scale = i.log2_scale_factor;
 	      if (i.index_reg == 0)

Attachment: binutils-mainline-x86_64-default-seg.patch
Description: Text document