问题描述
我的理解是ARMv8 A64程序集中的立即参数可以是12位长.如果是这种情况，为什么这行汇编代码:
My understanding is that immediate parameters in ARMv8 A64 assembly can be 12 bits long. If that is the case, why does this line of assembly code:
AND X12, X10, 0xFEF
产生此错误(使用gcc编译时)
Produce this error (when compiled with gcc)
Error: immediate out of range at operand 3 -- `AND X12, X10, 0xFEF'
有趣的是，这一行汇编代码可以很好地编译:
Interestingly enough, this line of assembly code compiles fine:
ADD X12, X10, 0xFEF
我正在使用aarch64-linux-gnu-gcc(Linaro GCC 2014.11)4.9.3(预发行版)
I'm using aarch64-linux-gnu-gcc (Linaro GCC 2014.11) 4.9.3 (prerelease)
推荐答案
与A32的灵活的第二操作数"不同，A64中没有通用的立即数格式.对于立即操作数数据处理指令(忽略无聊而直接的指令，如移位)，
Unlike A32's "flexible second operand", there is no common immediate format in A64. For immediate-operand data-processing instructions (ignoring the boring and straightforward ones like shifts),
- 算术指令(
add {s}，sub {s}，cmp，cmn)12位无符号立即数，并可以选择左移12位. - 移动指令(
movz，movn，movk)采用16位立即数，可以选择移动到其中的任何16位对齐位置寄存器. - 地址计算(
adr，adrp)采用21位带符号立即数，尽管没有实际的语法可以直接指定它-为此，您必须借助汇编程序表达式技巧来生成适当的标签". - 逻辑指令(
和{s}，orr，eor，tst)采用立即数位掩码"，我不确定我什至不能解释，因此我只引用令人难以置信的复杂定义:
- Arithmetic instructions (
add{s},sub{s},cmp,cmn) take a 12-bit unsigned immediate with an optional 12-bit left shift. - Move instructions (
movz,movn,movk) take a 16-bit immediate optionally shifted to any 16-bit-aligned position within the register. - Address calculations (
adr,adrp) take a 21-bit signed immediate, although there's no actual syntax to specify it directly - to do so you'd have to resort to assembler expression trickery to generate an appropriate "label". - Logical instructions (
and{s},orr,eor,tst) take a "bitmask immediate", which I'm not sure I can even explain, so I'll just quote the mind-bogglingly complicated definition:
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