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assemblyarmcortex-mraspberry-pi-picothumb

Why MOV instruction is replaced by ADD instruction

I have the following instruction: mov r1, r7 in my assembly code but after looking into disassembly, I've found that actual generated code was adds r1, r7, #0

I checked with ARMv6-M Architecture Reference Manual and I found out that there's MOVS <Rd>,<Rm> instruction (A6.7.40) which is different from ADDS.

While that's not a big issue, I'm still puzzled why assembler replaces code that I wrote by different op-codes. According to the book that I'm reading, all non-jump instructions take 1 cycle (and I'd prefer for assembler to be dumb rather than trying to optimize something for me).

I'm using Raspberry Pi Pico SDK which uses GNU Assembler, AFAIK.

All my code is written in helloworld.S, full source code is:

.thumb_func
.global main

main:
mov r7, #0
bl stdio_init_all
loop:
ldr r0, =helloworld
add r7, #1
mov r1, r7
bl printf
mov r0, #250
bl sleep_ms
b loop
.data
.align 4
helloworld: .asciz "Hello World %d\n"
Solution

  • You are writing Thumb mode assembly code and the RPi Pico supports Thumb-2, which means the actual output will be in UAL format.

    If you refer to the ARMv6-M reference manual, table D4-1 specifies the conversion of pre-UAL Thumb syntax into UAL syntax, notably:

    Pre-UAL Thumb syntax Equivalent UAL Syntax Notes
    MOV <Rd>, <Rm> ADDS <Rd>, <Rm>, #0 If <Rd> and <Rm> are both R0-R7.
    MOV <Rd>, <Rm> Otherwise.

    As is suggested by Tom, you can add .syntax unified if you want to write UAL code (which will then be actually assembled as-is).