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Sun Studio 12: Fortran Programming Guide

9.1.4.3 Rationalizing Tangled Code

Complicated conditional operations within a computationally intensive loop can dramatically inhibit the compiler’s attempt at optimization. In general, a good rule to follow is to eliminate all arithmetic and logical IF’s, replacing them with block IF’s:


Original Code:
    IF(A(I)-DELTA) 10,10,11
10  XA(I) = XB(I)*B(I,I)
    XY(I) = XA(I) - A(I)
    GOTO 13
11  XA(I) = Z(I)
    XY(I) = Z(I)
    IF(QZDATA.LT.0.) GOTO 12
    ICNT = ICNT + 1
    ROX(ICNT) = XA(I)-DELTA/2.
12  SUM = SUM + X(I)
13  SUM = SUM + XA(I)

Untangled Code:
    IF(A(I).LE.DELTA) THEN
      XA(I) = XB(I)*B(I,I)
      XY(I) = XA(I) - A(I)
    ELSE
      XA(I) = Z(I)
      XY(I) = Z(I)
      IF(QZDATA.GE.0.) THEN
        ICNT = ICNT + 1
        ROX(ICNT) = XA(I)-DELTA/2.
      ENDIF
      SUM = SUM + X(I)
    ENDIF
    SUM = SUM + XA(I)

Using block IF not only improves the opportunities for the compiler to generate optimal code, it also improves readability and assures portability.