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

The ROR instruction can only handle immediates between 1 and 31. The would-be encoding for ROR #0 is actually the RRX instruction. Reviewed By: nickdesaulniers Differential Revision: https://reviews.llvm.org/D102455
emception · May 19, 2021 · 65d0264 · 65d0264
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diff --git a/llvm/lib/Target/ARM/ARMInstrThumb2.td b/llvm/lib/Target/ARM/ARMInstrThumb2.td
@@ -2683,7 +2683,7 @@ def : T2Pat<(ARMusat (sra GPRnopc:$Rn, asr_imm:$shft), imm0_31:$pos),
 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

diff --git a/llvm/test/MC/ARM/thumb2-diagnostics.s b/llvm/test/MC/ARM/thumb2-diagnostics.s
@@ -49,6 +49,12 @@
 @ 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