gcc/clang如何假设字符串常量的地址是32位? [英] How can gcc/clang assume a string constant's address is 32-bit?

问题描述

如果我编译该程序:

#include <stdio.h>

int main(int argc, char** argv) {
    printf("hello world!\n");
    return 0;
}

对于x86-64，asm输出使用movl $.LC0, %edi/call puts. (

for x86-64, the asm output uses movl $.LC0, %edi / call puts. (See full asm output / compile options on godbolt.)

我的问题是:GCC如何知道字符串的地址可以放入32位立即数中?为什么不需要使用movabs $.LC0, %rdi(即mov r64, imm64，而不是零或符号扩展的imm32).

My question is: How can GCC know that the the string's address can fit in a 32bit immediate operand? Why doesn't it need to use movabs $.LC0, %rdi (i.e. a mov r64, imm64, not a zero or sign-extended imm32).

AFAIK，没有什么说加载程序必须决定将数据段加载到任何特定地址的.如果字符串存储在1ULL << 32以上的某个地址，则movl将忽略较高的位.我在clang上也有类似的行为，所以我认为这并不是GCC独有的.

AFAIK, there's nothing saying the loader has to decide to load the data section at any particular address. If the string is stored at some address above 1ULL << 32 then the higher bits will be ignored by the movl. I get similar behavior with clang, so I don't think this is unique to GCC.

我关心的原因是我想创建自己的数据段，该段位于我选择的任意地址(可能大于2 ^ 32)中的内存中.

The reason I care is I want to create my own data segment that lives in memory at any arbitrary address I choose (above 2^32 potentially).

推荐答案

在GCC手册中:

https://gcc. gnu.org/onlinedocs/gcc-4.5.3/gcc/i386-and-x86_002d64-Options.html

3.17.15 Intel 386和AMD x86-64选项

-mcmodel = small

为小代码模型生成代码:程序及其符号 必须链接在地址空间的低2 GB中.指针是64 位.程序可以静态或动态链接. 这是 默认代码模型.

Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.

-mcmodel = kernel为内核代码模型生成代码.内核在负2 GB的地址空间中运行.这个模型必须是 用于Linux内核代码.

-mcmodel=kernel Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.

-mcmodel = medium

-mcmodel=medium

为中型模型生成代码:程序链接在下部 2 GB的地址空间.小符号也放置在此处. 大小大于-mlarge-data-threshold的符号被放入 大数据或bss部分，并且可以位于2GB以上.程序可以 进行静态或动态链接.

Generate code for the medium model: The program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or bss sections and can be located above 2GB. Programs can be statically or dynamically linked.

-mcmodel =大

-mcmodel=large

为大型模型生成代码:该模型不做任何假设 关于部分的地址和大小.

Generate code for the large model: This model makes no assumptions about addresses and sizes of sections.

---

https://gcc.gnu.org/onlinedocs/gcc/AArch64- Options.html

3.18.1 AArch64选项

-mcmodel = tiny

-mcmodel=tiny

为小代码模型生成代码.该程序及其静态定义的符号之间的距离必须在1GB之内.指针 是64位.程序可以静态或动态链接.这 模型尚未完全实施，通常被视为小型".

Generate code for the tiny code model. The program and its statically defined symbols must be within 1GB of each other. Pointers are 64 bits. Programs can be statically or dynamically linked. This model is not fully implemented and mostly treated as ‘small’.

-mcmodel = small

为小代码模型生成代码.该程序及其静态定义的符号之间的距离必须在4GB之内.指针 是64位.程序可以静态或动态链接. 这是 默认代码模型.

Generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.

-mcmodel =大

-mcmodel=large

为大型代码模型生成代码.这不对节的地址和大小做任何假设.指针是64位.程式 只能静态链接.

Generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Pointers are 64 bits. Programs can be statically linked only.

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