>>> On 30.07.12 at 19:03, "H.J. Lu" <hjl.tools@gmail.com> wrote:
> Please provide a testcase to show the correct behavior.

Here you go.

Jan

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).

gas/
2012-08-07  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.

gas/testsuite/
2012-08-07  Jan Beulich <jbeulich@suse.com>

	* gas/i386/x86-64-segovr.{s,l}: New.
	* gas/i386/i386.exp: Run new test.

--- 2012-08-07/gas/config/tc-i386.c	2012-07-31 09:45:03.000000000 +0200
+++ 2012-08-07/gas/config/tc-i386.c	2012-08-07 12:13:39.000000000 +0200
@@ -5729,18 +5729,14 @@ 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)
--- 2012-08-07/gas/testsuite/gas/i386/i386.exp	2012-07-24 14:52:59.000000000 +0200
+++ 2012-08-07/gas/testsuite/gas/i386/i386.exp	2012-08-07 12:13:39.000000000 +0200
@@ -311,6 +311,7 @@ if [expr ([istarget "i*86-*-*"] || [ista
     run_dump_test "x86-64-stack-suffix"
     run_list_test "x86-64-inval" "-al"
     run_list_test "x86-64-segment" "-al"
+    run_dump_test "x86-64-segovr"
     run_list_test "x86-64-inval-seg" "-al"
     run_dump_test "x86-64-branch"
     run_dump_test "x86-64-relax-1"
--- 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.d	1970-01-01 01:00:00.000000000 +0100
+++ 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.d	2012-08-03 14:06:44.000000000 +0200
@@ -0,0 +1,41 @@
+#objdump: -dw
+#name: x86-64 segment overrides
+
+.*: +file format .*
+
+Disassembly of section .text:
+
+0+ <segovr>:
+[ 	]*[a-f0-9]+:	8b 00[ 	]+mov[ 	]+\(%rax\),%eax
+[ 	]*[a-f0-9]+:	8b 01[ 	]+mov[ 	]+\(%rcx\),%eax
+[ 	]*[a-f0-9]+:	8b 02[ 	]+mov[ 	]+\(%rdx\),%eax
+[ 	]*[a-f0-9]+:	8b 03[ 	]+mov[ 	]+\(%rbx\),%eax
+[ 	]*[a-f0-9]+:	3e 8b 04 24[ 	]+mov[ 	]+%ds:\(%rsp\),%eax
+[ 	]*[a-f0-9]+:	3e 8b 45 00[ 	]+mov[ 	]+%ds:((0x)?0)?\(%rbp\),%eax
+[ 	]*[a-f0-9]+:	8b 06[ 	]+mov[ 	]+\(%rsi\),%eax
+[ 	]*[a-f0-9]+:	8b 07[ 	]+mov[ 	]+\(%rdi\),%eax
+[ 	]*[a-f0-9]+:	41 8b 00[ 	]+mov[ 	]+\(%r8\),%eax
+[ 	]*[a-f0-9]+:	41 8b 01[ 	]+mov[ 	]+\(%r9\),%eax
+[ 	]*[a-f0-9]+:	41 8b 02[ 	]+mov[ 	]+\(%r10\),%eax
+[ 	]*[a-f0-9]+:	41 8b 03[ 	]+mov[ 	]+\(%r11\),%eax
+[ 	]*[a-f0-9]+:	41 8b 04 24[ 	]+mov[ 	]+\(%r12\),%eax
+[ 	]*[a-f0-9]+:	41 8b 45 00[ 	]+mov[ 	]+((0x)?0)?\(%r13\),%eax
+[ 	]*[a-f0-9]+:	41 8b 06[ 	]+mov[ 	]+\(%r14\),%eax
+[ 	]*[a-f0-9]+:	41 8b 07[ 	]+mov[ 	]+\(%r15\),%eax
+[ 	]*[a-f0-9]+:	36 8b 00[ 	]+mov[ 	]+%ss:\(%rax\),%eax
+[ 	]*[a-f0-9]+:	36 8b 01[ 	]+mov[ 	]+%ss:\(%rcx\),%eax
+[ 	]*[a-f0-9]+:	36 8b 02[ 	]+mov[ 	]+%ss:\(%rdx\),%eax
+[ 	]*[a-f0-9]+:	36 8b 03[ 	]+mov[ 	]+%ss:\(%rbx\),%eax
+[ 	]*[a-f0-9]+:	8b 04 24[ 	]+mov[ 	]+\(%rsp\),%eax
+[ 	]*[a-f0-9]+:	8b 45 00[ 	]+mov[ 	]+((0x)?0)?\(%rbp\),%eax
+[ 	]*[a-f0-9]+:	36 8b 06[ 	]+mov[ 	]+%ss:\(%rsi\),%eax
+[ 	]*[a-f0-9]+:	36 8b 07[ 	]+mov[ 	]+%ss:\(%rdi\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 00[ 	]+mov[ 	]+%ss:\(%r8\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 01[ 	]+mov[ 	]+%ss:\(%r9\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 02[ 	]+mov[ 	]+%ss:\(%r10\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 03[ 	]+mov[ 	]+%ss:\(%r11\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 04 24[ 	]+mov[ 	]+%ss:\(%r12\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 45 00[ 	]+mov[ 	]+%ss:((0x)?0)?\(%r13\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 06[ 	]+mov[ 	]+%ss:\(%r14\),%eax
+[ 	]*[a-f0-9]+:	36 41 8b 07[ 	]+mov[ 	]+%ss:\(%r15\),%eax
+#pass
--- 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.s	1970-01-01 01:00:00.000000000 +0100
+++ 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.s	2012-08-03 13:54:08.000000000 +0200
@@ -0,0 +1,9 @@
+# 64bit segment overrides
+
+	.text
+segovr:
+.irp seg, ds, ss
+ .irp reg, ax, cx, dx, bx, sp, bp, si, di, 8, 9, 10, 11, 12, 13, 14, 15
+	mov	%\seg:(%r\reg), %eax
+ .endr
+.endr