The volatile modifier adds instructions

The volatile modifier adds instructions to expand the variable to a 32-bit value (UXTB, UXTH, SXTB, SXTH).
It doesn't make sense. checking code online

#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;
};
код
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:
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