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Asked: 2026-05-13T15:09:17+00:00

When using the following to compute PI in fortran77, will the compiler evaluate this

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When using the following to compute PI in fortran77, will the compiler evaluate this value or will it be evaluated at run time?

PI=4.D0*DATAN(1.D0)

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  1. Editorial Team

    EDIT: depends on the compiler: see my EDIT below. EDIT END

    i second Mick Sharpe’s suggestion that it will be evaluated at runtime. just out of curiosity, i compiled PI=4.D0*DATAN(1.D0) with Silverfrost’s ftn77 compiler and looked at the generated binary. the relevant part looks like so:

    fld1                 ; push 1.D0 onto the FPU register stack
call    ATAN_X
fmul    dbl_404000   ; multiply by 4.D0

    so indeed, no compiler cleverness here.

    this of course might be different with another compiler (eg. g77). EDIT: apparently, with g77 (the fortran77 front-end for gcc) it is possible (and enabled by default) to use gcc’s built-in atan function to auto-fold PI=4.D0*DATAN(1.D0) into a constant. EDIT END

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