LF95 Linux User's Guide - Lahey Computer Systems

Chapter 2 Developing with LF64--[n]apArithmetic PrecisionCompile only. Default: --napSpecify --ap to guarantee the consistency of REAL and COMPLEX calculations, regardlessof optimization level; user variables are not assigned to registers. Consider the followingexample:ExampleX = S - T2 Y = X - U...3 Y = X - UBy default (--nap), during compilation of statement 2, the compiler recognizes the value X isalready in a register and does not cause the value to be reloaded from memory. At statement3, the value X may or may not already be in a register, and so the value may or may not bereloaded accordingly. Because the precision of the datum is greater in a register than in memory,a difference in precision at statements 2 and 3 may occur.Specify --ap to choose the memory reference for non-INTEGER operands; that is, registersare reloaded. --ap must be specified when testing for the equality of randomly-generatedvalues.The default, --nap, allows the compiler to take advantage of the current values in registers,with possibly greater accuracy in low-order bits.Specifying --ap will usually generate slower executables.--block blocksizeDefault I/O block sizeCompile only. Default: 8 MbytesSpecify --block to change the default block size on OPEN statements. See “BLOCKSIZE=”in the LF64 Language Reference. blocksize must be a decimal INTEGER constant.Specifying an optimal blocksize can make a significant improvement in the speed of yourexecutable. The program tryblock.f90 demonstrates how changing blocksize can affectexecution speed. Some experimentation with blocksize in your program is usually necessaryto determine the optimal value. This optimal value varies from one machine to the next;therefore, if your program is moved to another machine and optimal performance is desired,then blocksize should be re-evaluated. See also details of the runtime switch “-g[num] 1