在thumb模式下编译时的GCC arm指令模式

Question

我想知道 GCC 如何配置使用--with-mode=thumb句柄编译/组装代码，如果-marm未指定标志，则使用 ARM 模式部分。那是：

• GCC 是用 --with-mode=thumb
• 程序编译时没有-marm（默认为拇指模式）
• 该程序的汇编部分使用 ARM 模式

我尝试在 Raspberry Pi 4 上使用 Ubuntu 18.04.4 内核 5.3.0-1018-raspi2 编译一个小型测试程序，并注意到该.arm部分正在 16 位拇指指令模式下执行，这促使我对此进行调查。这自然会导致分段错误，因为程序计数器增加了 2 个字节而不是 4 个字节。

以下是 gdb in layout asmmode 在我的程序分支到 .arm 汇编代码和执行单个stepi命令后所说的内容：

0x400900 <asm_maxfilter>        push   {r4, lr}
0x400904 <asm_maxfilter+4>      mov    r3, #0
0x400908 <filter_loop>          vld1.8 {d0-d1}, [r0]

pc 0x400902 0x400902 <asm_maxfilter+2>
^ The program counter is between instructions

我的代码如下：

#include <arm_neon.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

void asm_maxfilter(unsigned char* upbuffer, unsigned char* longterm_buffer, int grid_size);

int main(int argc, char** argv) {

    const int pixels_per = 16;
    const int grid_reso = 256;
    const int grid_size = grid_reso * grid_reso;
    const int remainder = grid_size % pixels_per;
    const int work_count = grid_size - remainder;

    unsigned char* longterm_up = (unsigned char*)malloc(grid_reso * grid_reso);
    memset(longterm_up, 0, grid_reso * grid_reso);

    unsigned char* up_buffers[60];
    int u;
    int i;

    for (u = 0; u < 60; ++u) {
        up_buffers[u] = (unsigned char*)malloc(grid_reso * grid_reso);

        if (up_buffers[u] == NULL) {
            fprintf(stderr, "Failed mallocing\n");
            return 1;
        }

        memset(up_buffers[u], 0, grid_reso * grid_reso);
    }

    for (u = 0; u < 60; ++u) {

        asm_maxfilter(up_buffers[u], longterm_up, work_count);

        // non-SIMD version handles the remainder that did not fit in NEON registers
        for (i = grid_size - remainder; i < grid_size; ++i) {
            if (longterm_up[i] < up_buffers[u][i]) {
                longterm_up[i] = up_buffers[u][i];
            }
        }
    }

    for (u = 0; u < 60; ++u) {
        free(up_buffers[u]);
    }

    free(longterm_up);

    return 0;
}

集会：

@ ARM NEON version of a max filter. Performs the following operation:
@
@ for (int i = 0; i < buf_size; ++i) {
@   if (buf_b[i] < buf_a[i]) {
@       buf_b[i] = buf_a[i];
@   }
@ }

.arm
.section .text
.align 4
.globl asm_maxfilter

@ parameters
@ r0: buf_a
@ r1: buf_b
@ r2: buf_size, multiple of 16
asm_maxfilter:

    @ Store register states in stack. They must be restored before returning
    push { r4, lr }

    @ Reset counter
    mov r3, #0

    filter_loop:

        @ Load 16 bytes into vectors
        vld1.u8 {q0}, [r0]
        vld1.u8 {q1}, [r1]

        @ Find greater values in each vector
        vcgt.u8 q2, q0, q1

        @ Bitselect the greater value into q2
        vbsl.u8 q2, q0, q1

        @ Store the larger value in output buffer
        vst1.u8 {q2}, [r1]

        @ Increment counter by 16
        add r3, r3, #16

        @ Increment pointers
        add r0, r0, #16
        add r1, r1, #16

        @ Check if loop is done
        cmp r3, r2
        blt filter_loop

    @ Restore registers to their original state
    pop { r4, lr }

    @ lr register contains return address
    bx lr

.end

代码编译使用：

gcc -Wall -Wpedantic -O0 -g -march=armv8-a -mfloat-abi=hard -mtune=cortex-a72 -mfpu=neon -c -o main.o main.c
gcc -Wall -Wpedantic -O0 -g -march=armv8-a -mfloat-abi=hard -mtune=cortex-a72 -mfpu=neon -o neon_test ./main.o ./asm_test.s

根据 ARM 文档所说的，如果处理器需要在拇指/手臂之间切换，程序应该使用BLXorBX指令执行分支：

https://developer.arm.com/docs/100076/0100/instruction-set-overview/overview-of-aarch32-state/changed-between-a32-and-t32-instruction-set-states

引用：

To direct armasm to generate A32 or T32 instruction encodings, you must set the assembler mode using an ARM or THUMB directive. Assembly code using CODE32 and CODE16 directives can still be assembled, but Arm recommends you use the ARM and THUMB directives for new code.

These directives do not change the instruction set state of the processor. To do this, you must use an appropriate instruction, for example BX or BLX to change between A32 and T32 states when performing a branch.

反汇编我的程序后，我发现这种模式切换没有完成。这是程序员必须在他们的汇编代码中自己做的事情（即使分支发生在 C 代码中），还是编译器/汇编器应该处理这个？

我还尝试__attribute__((target("arm")))在 C 文件函数声明中指定，即：

__attribute__((target("arm")))
void asm_maxfilter(unsigned char* upbuffer, unsigned char* longterm_buffer, int grid_size);

然而，这似乎并没有改变任何事情。只要我编译-marm或使用没有的 GCC，一切都会正常工作--with-mode=thumb

Answer 1

正如 old_timer 在评论中所建议的，问题是汇编源代码没有包含.type asm_maxfilter, %function在标签之前。工作汇编代码开始如下：

.arm
.section .text
.align 4
.globl asm_maxfilter

.type asm_maxfilter, %function
asm_maxfilter:

    @ Store register states in stack. They must be restored before returning
    push { r4, lr }

    @ Reset counter
    mov r3, #0
    ...

如果情况相反（ARM 模式程序使用拇指功能），则.type asm_maxfilter, %function类型应该是.thumb_func.

根据Jester的回复，我注意到C代码目标文件确实有一个R_ARM_THM_CALL重定位段，但是如果不使用.type宏，分支指令不会被bx指令替换。

如果不使用外部汇编在 C 文件中实现 ARM 函数__attribute__((target("arm")))，即：

#include <stdio.h>
#include <stdlib.h>

__attribute__((target("arm")))
void foo(int a) {
    int b = 6*a;
    fprintf(stderr, "%d\n", b*5);
}

int main(int argc, char** argv) {
    int asd = atoi(argv[1]);
    foo(asd);
    return 0;
}

然后我们可以观察到blx生成的二进制文件中正确使用了一条指令。我遇到的问题只是当人们在不通过编译器的单独文件中使用汇编代码时才需要担心。