LNO: Gather-Scatter

• Special form of loop fission:

  • If the loop to be optimized contains conditional execution, it is often faster to evaluate all the conditions first.
  • The computationally intensive loop runs only over the indices for which the condition was true and can be better optimized (SWP)
  • LNO will not evaluate the nested IF conditions, unless -LNO:gather_scatter=2 is used

Subroutine fred(a,b,c,n)

real*8 a(n), b(n), c(n)

do I=1,n

if(c(I) .gt. 0) then

a(I) = c(I)/b(I)

c(I) = c(I)*b(I)

b(I) = 2*b(I)

endif

enddo

end

do I=1,n

deref_gs(inc_0+1) = I

if(c(I) .gt. 0) then

inc_0 = inc_0 + 1

endif

enddo

do ind_0=0,inc_0-1

I=deref_gs(ind_0+1)

a(I) = c(I)/b(I)

c(I) = c(I)*b(I)

b(I) = 2*b(I)

enddo

end

Conditional

execution

removed

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