[MC][ARM] Reject Thumb "ror rX, #0"

[rprichard]

Summary

The ROR instruction can only handle immediates between 1 and 31. The
would-be encoding for ROR #0 is actually the RRX instruction.

[nickdesaulniers]

F5.1.160 ROR (immediate)
ROR{<c>}{<q>} {<Rd>,} <Rm>, #<imm>
<imm> For encoding T3: is the shift amount, in the range 1 to 31, encoded in the "imm3:imm2" field.

llvm/lib/Target/ARM/ARMInstrThumb2.td

 
 //===----------------------------------------------------------------------===//
 //  Shift and rotate Instructions.
 //
 
 defm t2LSL  : T2I_sh_ir<0b00, "lsl", imm1_31, shl>;
 defm t2LSR  : T2I_sh_ir<0b01, "lsr", imm_sr,  srl>;
 defm t2ASR  : T2I_sh_ir<0b10, "asr", imm_sr,  sra>;
-defm t2ROR  : T2I_sh_ir<0b11, "ror", imm0_31, rotr>;
+defm t2ROR  : T2I_sh_ir<0b11, "ror", imm1_31, rotr>;
 
 // LSL #0 is actually MOV, and has slightly different permitted registers to
 // LSL with non-zero shift
 def : t2InstAlias<"lsl${s}${p} $Rd, $Rm, #0",
                   (t2MOVr GPRnopc:$Rd, GPRnopc:$Rm, pred:$p, cc_out:$s)>;
 def : t2InstAlias<"lsl${s}${p}.w $Rd, $Rm, #0",
                   (t2MOVr GPRnopc:$Rd, GPRnopc:$Rm, pred:$p, cc_out:$s)>;
 

llvm/test/MC/ARM/thumb2-diagnostics.s

 @ CHECK-ERRORS-V7: operand must be an immediate in the range [0,7]
 @ CHECK-ERRORS-V7: operand must be an immediate in the range [0,7]
 @ CHECK-ERRORS-V8: invalid instruction
 @ CHECK-ERRORS-V8: too many operands for instruction
 @ CHECK-ERRORS: operand must be an immediate in the range [0,15]
 @ CHECK-ERRORS-V7: operand must be an immediate in the range [0,15]
 @ CHECK-ERRORS-V8: invalid instruction
 
+        @ Out of range immediate for ROR.
+        @ (Assembling this instruction to "mov r1, r1" might also be OK.)
+        ror r1, r1, #0
+@ CHECK-ERRORS: invalid instruction
+@ CHECK-ERRORS: operand must be an immediate in the range [1,31]
+
         isb  #-1
         isb  #16
 @ CHECK-ERRORS: error: immediate value out of range
 @ CHECK-ERRORS: error: immediate value out of range
 
         itt eq
         bkpteq #1
 @ CHECK-ERRORS: error: instruction 'bkpt' is not predicable, but condition code specified