[avr-gcc-list] Why is gcc promoting on an unsigned char?

[J.C. Wren]

        In this sample of code below, both i and data are defined as unsigned 
char.  
For some reason, GCC is effectively promoting i and data to an int for the 
operations.  The register tracking then fails to realize that it already has 
r18 loaded at 0x98, and moves r24 back to r18, inspite of having just moved 
r18 to r24.

        By casting the (i << 1) to unsigned char, I can eliminate one 
instruction, 
but no amount of casting can further reduce out the promotion to int side 
effects.

        --jc


      data |= (bit_is_set (DATAPIN, DATA) ? (i << 1) : 0);
  80: 33 27          eor   r19, r19
  82: c9 9b          sbis  0x19, 1  ; 25
  84: 07 c0          rjmp  .+14        ; 0x94
  86: 84 2f          mov   r24, r20
  88: 99 27          eor   r25, r25
  8a: 88 0f          add   r24, r24
  8c: 99 1f          adc   r25, r25
  8e: 82 2b          or r24, r18
  90: 93 2b          or r25, r19
  92: 02 c0          rjmp  .+4         ; 0x98
  94: 93 2f          mov   r25, r19
  96: 82 2f          mov   r24, r18
  98: 28 2f          mov   r18, r24

By casting the (i << 1) to unsigned char, I can eliminate one instruction

      data |= (bit_is_set (DATAPIN, DATA) ? (unsigned char) (i << 1) : 0);
  80: 33 27          eor   r19, r19
  82: c9 9b          sbis  0x19, 1  ; 25
  84: 06 c0          rjmp  .+12        ; 0x92
  86: 84 2f          mov   r24, r20
  88: 88 0f          add   r24, r24
  8a: 99 27          eor   r25, r25
  8c: 82 2b          or r24, r18
  8e: 93 2b          or r25, r19
  90: 02 c0          rjmp  .+4         ; 0x96
  92: 93 2f          mov   r25, r19
  94: 82 2f          mov   r24, r18
  96: 28 2f          mov   r18, r24