[PowerPC] Add the 'attn' instruction

The attn instruction is not part of the Power ISA, but is documented in the A2
user manual, and is accepted by the GNU assembler for the A2 and the POWER4+.
Reported as part of PR21650.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222712 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCInstrFormats.td

@@ -385,6 +385,12 @@ class XForm_tlb<bits<10> xo, dag OOL, dag IOL, string asmstr,
   let RST = 0;
 }
 
+class XForm_attn<bits<6> opcode, bits<10> xo, dag OOL, dag IOL, string asmstr,
+                 InstrItinClass itin>
+  : I<opcode, OOL, IOL, asmstr, itin> {
+  let Inst{21-30} = xo;
+}
+
 // This is the same as XForm_base_r3xo, but the first two operands are swapped
 // when code is emitted.
 class XForm_base_r3xo_swapped

lib/Target/PowerPC/PPCInstrInfo.td

@@ -3232,6 +3232,8 @@ def MFDCR : XFXForm_1<31, 323, (outs gprc:$RT), (ins i32imm:$SPR),
 def MTDCR : XFXForm_1<31, 451, (outs), (ins gprc:$RT, i32imm:$SPR),
                       "mtdcr $SPR, $RT", IIC_SprMTSPR>, Requires<[IsPPC4xx]>;
 
+def ATTN : XForm_attn<0, 256, (outs), (ins), "attn", IIC_BrB>;
+
 //===----------------------------------------------------------------------===//
 // PowerPC Assembler Instruction Aliases
 //