The volatile modifier adds instructions

> it must do something with the "rest" of an uint8_t

ldrb already sets the high 24 bits of the register to zero; there is nothing left to be done as long as the result is unsigned.

The godbolt link shows the uxtb instruction used regardless of optimization level (well, -O0 does does something different but awful, as expected.)

There's an interesting comment that shows up in the disassembly:

  ldrb r3, [r2, #1] @ zero_extendqisi2
  uxtb r3, r3

And there's a clue here: stackoverflow.com/.../meaning-of-zero-extendqisi2

I guess the intermediate language has a generic "fetch and zero-extend" internal instruction, and it isn't smart enough to realize that in the case that the fetch is from memory, the extend has effectively already happened. (if the source of the fetch was a register, it would have been a mov instruction, and the uxtb would be necessary because there is no mov variant for bytes to 32bit register.)

> it must do something with the "rest" of an uint8_t

ldrb already sets the high 24 bits of the register to zero; there is nothing left to be done as long as the result is unsigned.

The godbolt link shows the uxtb instruction used regardless of optimization level (well, -O0 does does something different but awful, as expected.)

There's an interesting comment that shows up in the disassembly:

  ldrb r3, [r2, #1] @ zero_extendqisi2
  uxtb r3, r3

And there's a clue here: stackoverflow.com/.../meaning-of-zero-extendqisi2

I guess the intermediate language has a generic "fetch and zero-extend" internal instruction, and it isn't smart enough to realize that in the case that the fetch is from memory, the extend has effectively already happened. (if the source of the fetch was a register, it would have been a mov instruction, and the uxtb would be necessary because there is no mov variant for bytes to 32bit register.)

I changed the example a little, and it turned out to be even more interesting.
The structure now contains int8_t, which are read in two different ways.
Simple variable "a" is read by ldrsb instruction, automatically expanded to int32_t. Because after it the "cmp" comparison instruction is used, which does not know how to work otherwise, and is a natural barrier.
And then "b" is read with the "volatile" modifier. An instruction to read an unsigned variable is used, followed by a separate extension.

#include "stdint.h"
/// -Os -mcpu=cortex-m7 
struct _st
{
    int8_t       a;
    volatile  int8_t    b;
}st;

int32_t test(int32_t c)
{
    int32_t out;
    if(st.a > c) out = st.a;
    else out = st.b; 
    return out;
};

test:
  ldr r2, .L3
  ldrsb r3, [r2]
  cmp r3, r0
  bgt .L1
  ldrb r3, [r2, #1] @ zero_extendqisi2
  sxtb r3, r3
.L1:
  mov r0, r3
  bx lr
.L3:
  .word .LANCHOR0
st: