[Bug c/78824] New: multiple add should in my opinion be optimized to multiplication

[pj at hugeone dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78824

            Bug ID: 78824
           Summary: multiple add should in my opinion be optimized to
                    multiplication
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pj at hugeone dot co.uk
  Target Milestone: ---

Hi

Example  (-O3)

volatile uint16_t y;
uint8_t nvx8;

int main(void) {


        y = nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8
+ nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8;
}

translated to:
        y = nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8
+ nvx8 + nvx8 + nvx8 + nvx8 + nvx8 + nvx8;
  94:   80 91 00 01     lds     r24, 0x0100
  98:   28 2f           mov     r18, r24
  9a:   30 e0           ldi     r19, 0x00       ; 0
  9c:   c9 01           movw    r24, r18
  9e:   88 0f           add     r24, r24
  a0:   99 1f           adc     r25, r25
  a2:   88 0f           add     r24, r24
  a4:   99 1f           adc     r25, r25
  a6:   82 0f           add     r24, r18
  a8:   93 1f           adc     r25, r19
  aa:   82 0f           add     r24, r18
  ac:   93 1f           adc     r25, r19
  ae:   82 0f           add     r24, r18
  b0:   93 1f           adc     r25, r19
  b2:   82 0f           add     r24, r18
  b4:   93 1f           adc     r25, r19
  b6:   82 0f           add     r24, r18
  b8:   93 1f           adc     r25, r19
  ba:   82 0f           add     r24, r18
  bc:   93 1f           adc     r25, r19
  be:   82 0f           add     r24, r18
  c0:   93 1f           adc     r25, r19
  c2:   82 0f           add     r24, r18
  c4:   93 1f           adc     r25, r19
  c6:   82 0f           add     r24, r18
  c8:   93 1f           adc     r25, r19
  ca:   82 0f           add     r24, r18
  cc:   93 1f           adc     r25, r19
  ce:   82 0f           add     r24, r18
  d0:   93 1f           adc     r25, r19
  d2:   82 0f           add     r24, r18
  d4:   93 1f           adc     r25, r19
  d6:   90 93 02 01     sts     0x0102, r25
  da:   80 93 01 01     sts     0x0101, r24

It should be optimized to multiplication I think.