内联汇编内存偏移 [英] Memory offsets in inline assembly

问题描述

在A64汇编程序，有不同的方式来指定地址。

In A64 assembler, there are different ways to specify addresses.

/*
[base{,#0}]                 Simple register (exclusive) - Immediate Offset
[base{,#imm}]               Offset - Immediate Offset
[base,Xm{,LSL #imm}]        Offset - Register Offset
[base,Wm,(S|U)XTW {#imm}]   Offset - Extended Register Offset
[base,#imm]!                Pre-indexed - Immediate Offset
[base],#imm                 Post-indexed - Immediate Offset
label                       PC-relative (literal) load - Immediate Offset
*/

---

我想用偏移 - 即时偏移中的内联汇编

---

I would like to use "Offset - Immediate Offset" in inline assembler.

__asm__("ldp x8, x9, %0, 16 \n\t"
        :
        : "m" (*somePointer)
        : "x8", "x9");

本被翻译成

ldp x8, x9, [x0], 16

我的目标是要达到

My goal is to reach

ldp x8, x9, [x0, 16]

我怎么能写与内联汇编这样的指令？

推荐答案

我没有一个ARM 64位的工具链来进行测试，但你应该能够做这样的事：

I don't have an ARM 64-bit toolchain to test this with, but you should be able to do something like this:

asm("ldp x8, x9, %0\n\t"
    :
    : "Ump" (*((__int128 *) somePointer + 1))
    : "x8", "x9");

的 UMP 约束限制了存储器的操作数那些由整数自民党指令允许的，否则它的工作方式类似于 M 约束。如果 somePointer 已经是一个指针，你可以使用 somePointer [1] 作为操作数128位的类型。

The Ump constraint limits the memory operand to those permitted by the integer LDP instruction, otherwise it works like the m constraint. If somePointer is already a pointer to a 128-bit type you can just use somePointer[1] as the operand.

如果以上不工作，那么大卫Wohlferd的建议应该：

If the above doesn't work then David Wohlferd's suggestion should:

asm("ldp x8, x9, [%0, %1]" 
    : 
    : "r" (somePointer), "i"(16), "m" (*somePointer) 
    : "x8", "x9"); 

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