x86/Intel: issue diagnostics for redundant segment override prefixes

While we shouldn't outright reject such (as was wrongly done by commit
4d36230d59 ("x86: Update segment register check in Intel syntax"), as
MASM accepts them even silently, issue (by default) a warning for such
questionable constructs.

gas/ChangeLog

@@ -1,3 +1,15 @@
+2017-11-29  Jan Beulich  <jbeulich@suse.com>
+
+	PR gas/21874
+	* config/tc-i386-intel.c (i386_intel_simplify): Chain together
+	multiple segment override expressions.
+	(i386_intel_operand): Issue diagnostic for redundant segment
+	overrides.
+	* testsuite/gas/i386/intelok.e: New.
+	* testsuite/gas/i386/intelok.d: Reference intelok.e.
+	* testsuite/gas/i386/inval-seg.s: Add redundant override checks.
+	* testsuite/gas/i386/inval-seg.l: Adjust expectations.
+
 2017-11-29  Jim Wilson  <jimw@sifive.com>
 	    Palmer Dabbelt  <palmer@sifive.com>
 

gas/config/tc-i386-intel.c

@@ -413,7 +413,19 @@ static int i386_intel_simplify (expressionS *e)
 			       intel_state.index))
 	return 0;
       if (!intel_state.in_offset)
-	intel_state.seg = e->X_add_symbol;
+	{
+	  if (!intel_state.seg)
+	    intel_state.seg = e->X_add_symbol;
+	  else
+	    {
+	      expressionS exp;
+
+	      exp.X_op = O_full_ptr;
+	      exp.X_add_symbol = e->X_add_symbol;
+	      exp.X_op_symbol = intel_state.seg;
+	      intel_state.seg = make_expr_symbol (&exp);
+	    }
+	}
       i386_intel_fold (e, e->X_op_symbol);
       break;
 
@@ -956,10 +968,12 @@ i386_intel_operand (char *operand_string, int got_a_float)
 
       if (intel_state.seg)
 	{
-	  for (;;)
+	  for (ret = check_none; ; ret = operand_check)
 	    {
 	      expP = symbol_get_value_expression (intel_state.seg);
-	      if (expP->X_op != O_full_ptr)
+	      if (expP->X_op != O_full_ptr 
+		  || symbol_get_value_expression (expP->X_op_symbol)->X_op
+		     != O_register)
 		break;
 	      intel_state.seg = expP->X_add_symbol;
 	    }
@@ -974,6 +988,15 @@ i386_intel_operand (char *operand_string, int got_a_float)
 	      as_bad (_("invalid use of register"));
 	      return 0;
 	    }
+	  switch (ret)
+	    {
+	    case check_error:
+	      as_bad (_("redundant segment overrides"));
+	      return 0;
+	    case check_warning:
+	      as_warn (_("redundant segment overrides"));
+	      break;
+	    }
 	  switch (i386_regtab[expP->X_add_number].reg_num)
 	    {
 	    case 0: i.seg[i.mem_operands] = &es; break;

gas/testsuite/gas/i386/intelok.d

@@ -1,6 +1,7 @@
 #as: -J --divide
 #objdump: -dwMintel
 #name: i386 intel-ok
+#stderr: intelok.e
 
 .*: +file format .*
 

gas/testsuite/gas/i386/intelok.e

@@ -0,0 +1,2 @@
+.*: Assembler messages:
+.*:153: Warning: redundant segment overrides

gas/testsuite/gas/i386/inval-seg.l

@@ -1,10 +1,22 @@
 .*: Assembler messages:
 .*:3: Error: .*
 .*:4: Error: .*
+.*:7: Error: .*
+.*:8: Error: .*
+.*:9: Error: .*
+.*:10: Error: .*
+.*:11: Error: .*
 GAS LISTING .*
 
 
-   1 [ 	]*	.text
+[ 	]*[1-9][0-9]*[ 	]*\.text
-   2 [ 	]*# All the following should be illegal
+[ 	]*[1-9][0-9]*[ 	]*# All the following should be illegal
-   3 [ 	]*	movl	%ds,\(%eax\)
+[ 	]*[1-9][0-9]*[ 	]*movl	%ds,\(%eax\)
-   4 [ 	]*	movl	\(%eax\),%ds
+[ 	]*[1-9][0-9]*[ 	]*movl	\(%eax\),%ds
+[ 	]*[1-9][0-9]*[ 	]*
+[ 	]*[1-9][0-9]*[ 	]*\.intel_syntax noprefix
+[ 	]*[1-9][0-9]*[ 	]*mov	eax, es:foo:\[eax\]
+[ 	]*[1-9][0-9]*[ 	]*mov	eax, es:fs:foo:\[eax\]
+[ 	]*[1-9][0-9]*[ 	]*mov	eax, fs:foo:bar:\[eax\]
+[ 	]*[1-9][0-9]*[ 	]*mov	eax, fs:foo:gs:\[eax\]
+[ 	]*[1-9][0-9]*[ 	]*mov	eax, bar:gs:\[eax\]

gas/testsuite/gas/i386/inval-seg.s

@@ -2,3 +2,10 @@
 # All the following should be illegal
 	movl	%ds,(%eax)
 	movl	(%eax),%ds
+
+	.intel_syntax noprefix
+	mov	eax, es:foo:[eax]
+	mov	eax, es:fs:foo:[eax]
+	mov	eax, fs:foo:bar:[eax]
+	mov	eax, fs:foo:gs:[eax]
+	mov	eax, bar:gs:[eax]