求助这跳指令ADDLT PC,PC,R1；CC5.2编译不过

■ A branch or any instruction that modifies the PC must either be outside an IT block or must be
the last instruction inside the IT block. These are:
– ADD PC, PC, Rm
– MOV PC, Rm
– B, BL, BX, BLX
– any LDM, LDR, or POP instruction that writes to the PC

这里提到的 IT block 是什么意思？把指令换成ADD   PC,PC,R1；倒是可以编译通过。