CODEWITHSUNDEEP
gccassemblycompilationthumb
Anand Srivastava
Anand Srivastava

Reputation: 71

How to use thumb-2 instructions with GCC

I have written the following simplistic memcpy32 function, as a means of understanding how to write assembly code for the cortex-m4.

    .section .text
    .align 2
    .global as_memcpy32
    .type as_memcpy32, %function
as_memcpy32:
     push    {r4, lr}
     movs    r3, #0
start_loop: 
     cmp     r3, r2
     bge   end_loop
     ldr   r4, [r1]
     str   r4, [r0]
     add   r0, #4
     add   r1, #4
     add    r3, #1
     b     start_loop
end_loop:   
    pop     {r4, pc}

The above code compiles and runs. These are only 16bit instructions. I want to use 32 bit thumb2 instructions also as they are supported by the Cortex-M4. And the main point of writing assembly is to run my code faster.

I should be able to use the following form of the ldr and str instruction according to the STM32F4 manual

op{type}{cond} Rt, [Rn], #offset; post-indexed

I am supplying the following options to the GCC.

arm-none-eabi-gcc" -c -g -x assembler-with-cpp -MMD -mcpu=cortex-m4 -DF_CPU=168000000L -DARDUINO=10610 -DARDUINO_STM32DiscoveryF407 -DARDUINO_ARCH_STM32F4  -DMCU_STM32F406VG -mthumb -DSTM32_HIGH_DENSITY -DSTM32F2 -DSTM32F4 -DBOARD_discovery_f4   -mthumb -D__STM32F4__ memcpy.S" -o memcpy.S.o

When I try to use the following instructions for ldr and str

ldr r4, [r1], #4
ldr r4, [r0], #4

I get the following errors.

memcpy.S: Assembler messages:

memcpy.S:11: Error: Thumb does not support this addressing mode -- `ldr r4,[r1],#4'

memcpy.S:12: Error: Thumb does not support this addressing mode -- `str r4,[r0],#4'

exit status 1
Error compiling for board STM32 Discovery F407.

I am not able to understand what the problem is. Actually the compiler itself generated a lot more complex addressing opcode.

ldr.w   r4, [r1, r3, lsl #2]
str.w   r4, [r0, r3, lsl #2]

thanks

Upvotes: 2

Views: 6112

Answers (1)

Anand Srivastava
Anand Srivastava

Reputation: 71

I just found that I should say

.syntax unified

below 

.section

The following topic deals with other things, but I saw it there and tried. It worked.

How to generate the machine code of Thumb instructions?

Upvotes: 3

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