[llvm-bugs] [Bug 48981] New: [AVR] Bad optimization of multiplication when mul instruction is available

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48981

            Bug ID: 48981
           Summary: [AVR] Bad optimization of multiplication when mul
                    instruction is available
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AVR
          Assignee: unassignedbugs at nondot.org
          Reporter: gabravier at gmail.com
                CC: llvm-bugs at lists.llvm.org

int square(int num)
{
    return num * num;
}

GCC -O3 generates this:

square(int):
  movw r18,r24
  mul r18,r18
  movw r24,r0
  mul r18,r19
  add r25,r0
  add r25,r0
  clr r1
  ret

LLVM generates this:

square(int):
  muls r25, r24
  mov r18, r0
  clr r1
  muls r24, r25
  mov r19, r0
  clr r1
  mul r24, r24
  mov r20, r1
  clr r1
  add r20, r19
  add r20, r18
  lsl r20
  rol r21
  lsl r20
  rol r21
  lsl r20
  rol r21
  lsl r20
  rol r21
  lsl r20
  rol r21
  lsl r20
  rol r21
  lsl r20
  rol r21
  lsl r20
  rol r21
  mov r24, r0
  clr r25
  or r24, r20
  or r25, r21
  ret

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