The volatile modifier adds extension instructions

[home]

The volatile modifier adds instructions to expand a variable to a 32-bit value (UXTB, UXTH, SXTB, SXTH).
https://godbolt.org/z/x17nj5Gxq

#include "stdint.h"
struct _st
{
     uint8_t a;
    volatile uint8_t b;
}st;
uint32_t test(uint8_t c)
{
    uint8_t out;
    if(st.a > c) out = st.a;
    else out = st.b;
    return out;
};

test:
  ldr r2, .L3
  ldrb r3, [r2] //reading st.a
  cmp r3, r0
  bhi .L2
  ldrb r3, [r2, #1] //reading st.b
  uxtb r3, r3 // <<<<<
.L2:
  mov r0, r3
  bx lr
.L3:
  .word .LANCHOR0
st: