Thumb rsb #0 -> neg

[Paul Brook <paul at codesourcery dot com>]

The patch below teaches gas how to use the 16-bit negs instruction to encode 
rsbs reg, reg, #0.

Tested on arm-none-eabi.
Applied to CVS head.

Paul

2007-04-18  Paul Brook  <paul@codesourcery.com>

	gas/testsuite/
	* gas/arm/thumb2_add.s: Add rsb #0 test.
	* gas/arm/thumb2_add.d: Update expected output.

	gas/
	* config/tc-arm.c (do_t_rsb): Use 16-bit encoding when possible.

Index: gas/testsuite/gas/arm/thumb2_add.d
===================================================================
--- gas/testsuite/gas/arm/thumb2_add.d	(revision 169105)
+++ gas/testsuite/gas/arm/thumb2_add.d	(working copy)
@@ -27,4 +27,4 @@ Disassembly of section .text:
 0+04c <[^>]+> a840      	add	r0, sp, #256
 0+04e <[^>]+> f50d 6580 	add.w	r5, sp, #1024	; 0x400
 0+052 <[^>]+> f20d 1901 	addw	r9, sp, #257	; 0x101
-0+056 <[^>]+> bf00      	nop
+0+056 <[^>]+> 4271      	negs	r1, r6
Index: gas/testsuite/gas/arm/thumb2_add.s
===================================================================
--- gas/testsuite/gas/arm/thumb2_add.s	(revision 169105)
+++ gas/testsuite/gas/arm/thumb2_add.s	(working copy)
@@ -29,4 +29,4 @@ thumb2_add:
 	add r0, sp, #0x100
 	add r5, sp, #0x400
 	add r9, sp, #0x101
-	nop
+	rsbs r1, r6, #0
Index: gas/config/tc-arm.c
===================================================================
--- gas/config/tc-arm.c	(revision 169105)
+++ gas/config/tc-arm.c	(working copy)
@@ -9942,8 +9942,37 @@ do_t_rsb (void)
   inst.instruction |= Rs << 16;
   if (!inst.operands[2].isreg)
     {
-      inst.instruction = (inst.instruction & 0xe1ffffff) | 0x10000000;
-      inst.reloc.type = BFD_RELOC_ARM_T32_IMMEDIATE;
+      bfd_boolean narrow;
+
+      if ((inst.instruction & 0x00100000) != 0)
+	narrow = (current_it_mask == 0);
+      else
+	narrow = (current_it_mask != 0);
+
+      if (Rd > 7 || Rs > 7)
+	narrow = FALSE;
+
+      if (inst.size_req == 4 || !unified_syntax)
+	narrow = FALSE;
+
+      if (inst.reloc.exp.X_op != O_constant
+	  || inst.reloc.exp.X_add_number != 0)
+	narrow = FALSE;
+
+      /* Turn rsb #0 into 16-bit neg.  We should probably do this via
+         relaxation, but it doesn't seem worth the hassle.  */
+      if (narrow)
+	{
+	  inst.reloc.type = BFD_RELOC_UNUSED;
+	  inst.instruction = THUMB_OP16 (T_MNEM_negs);
+	  inst.instruction |= Rs << 3;
+	  inst.instruction |= Rd;
+	}
+      else
+	{
+	  inst.instruction = (inst.instruction & 0xe1ffffff) | 0x10000000;
+	  inst.reloc.type = BFD_RELOC_ARM_T32_IMMEDIATE;
+	}
     }
   else
     encode_thumb32_shifted_operand (2);