Thursday, April 7, 2022

[SOLVED] inline ARMv7-M assembly code into c using gcc and macros

Issue

I have this code that works fine:

void function( void )
{
    __asm volatile
    (
        "   ldr r3, .ADDRESS            \n"
        "   mov r2, %0                  \n"
        "   str r2, [r3, %1]            \n"
        ".ADDRESS:  .word 0x401C4000    \n"
        :: "i" (1<<17), "i" (16)
    );
}

But to declare .ADDRESS I used the magic number 0x401C4000. I actually have a macro for this.

For Example:

#define ADDR_BASE 0x401C4000
#define ADDR ((void *)ADDR_BASE)

void function( void )
{
    __asm volatile
    (
        "   ldr r3, .ADDRESS            \n"
        "   mov r2, %0                  \n"
        "   str r2, [r3, %1]            \n"
        ".ADDRESS:    .word %2          \n"
        :: "i" (1<<17), "i" (16), "i" (ADDR)
    );
}

That doesn't build.

How can I use a macro in this case?


Solution

See the edit below. I'm only leaving this first solution as an example of what you shouldn't do.

I found this post where a solution is given to my same problem, but it involves x86. I wanted to try it anyway, and it works.

Then use %c2 instead of %2, as shown here:

#define ADDR_BASE 0x401C4000
#define ADDR ((void *)0x401C4000)

void function( void )
{
    __asm volatile
    (
        "   ldr r3, .ADDRESS            \n"
        "   mov r2, %0                  \n"
        "   str r2, [r3, %1]            \n"
        ".ADDRESS:   .word %c2          \n"
        :: "i" (1<<17), "i" (16), "i" (ADDR): "r2", "r3", "memory"
    );
}

EDIT

The solution I suggested above may not work by seeing the generated code:

  20                function:
  21                    @ Function supports interworking.
  22                    @ args = 0, pretend = 0, frame = 0
  23                    @ frame_needed = 1, uses_anonymous_args = 0
  24                    @ link register save eliminated.
  25 0000 04B02DE5      str fp, [sp, #-4]!
  26 0004 00B08DE2      add fp, sp, #0
  27                    .syntax divided
  28                @ 6 "test.c" 1
  29 0008 04309FE5         ldr r3, .ADDRESS            
  30 000c 0228A0E3     mov r2, #131072                  
  31 0010 102083E5     str r2, [r3, #16]            
  32 0014 00401C40  .ADDRESS:   .word 1075593216          
  33                
  34                @ 0 "" 2
  35                    .arm
  36                    .syntax unified
  37 0018 0000A0E1      nop
  38 001c 00D08BE2      add sp, fp, #0
  39                    @ sp needed
  40 0020 04B09DE4      ldr fp, [sp], #4
  41 0024 1EFF2FE1      bx  lr

You can see at line 32 the symbol .ADDRESS has been issued, but it is surrounded by code, and there is no instruction to skip it. I believe that it can be attempted to be executed as if it were an instruction.

Maybe a better solution, suggested by Peter, is

#define ADDR_BASE 0x401C4000
#define ADDR ((void *)0x401C4000)

void function( void )
{
    __asm volatile
    (
    "str %0, [%2, %1]"
    :: "r" (1<<17), "i" (16), "r" (ADDR) : "memory"
    );
}


Answered By - mastupristi
Answer Checked By - Marilyn (WPSolving Volunteer)
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