Consider the following routine (written in 1992, from TSPACK ): C This function forces its argument X to be stored in a C memory location, thus providing a means of determining C floating point number characteristics (such as the machine C precision) when it is necessary to avoid computation in C …

Renka put this also in TRIPACK. In my understanding, saving the variable to a common is a trick to ensure that the arithmetic expression does not get simplified out by the compiler: it could decide to just check for RTOL>0, that is not what the author is trying to do here. In modern Fortran, you sh…

[image] FedericoPerini: In my understanding, saving the variable to a common is a trick to ensure that the arithmetic expression does not get simplified out by the compiler: […] This was one way to force the intermediate to be represented as a 64-bit real rather than an 80-bit real. There wer…

Here’s another odd use of this function: IF (STORE(SIG*EXP(-SIG)+.5D0) .EQ. .5D0) GO TO 5 I guess this is maybe equivalent to: if (2.0d0*sig*exp(-sig) < epsilon(0.0d0)) ...

As you noticed previously in SLSQP, old foxes were quite self-aware of floating-point spacing Hence without the necessary tooling back then, they adopted some -isms that you see a lot in old code. Another common one is res = 0.5D0 + (0.5D0 - x) from the 70s. The case they are trying to avoid is th…

Note: Fortran also has SPACING . Don’t think I’ve ever actually used it.

I have. The following is an almostEqual function patterned after a post on Intel Fortran Forum that uses SPACING. You could also use EPSILON (which is what I originally had). I use this in most places where I need to test for equlity (to at least a tolerance) of two floating point numbers. This vers…

[image] jacobwilliams: Note: Fortran also has SPACING . I think fortran has had the SPACING() intrinsic since f90. If so, that is 15 years or so before numpy existed. I don’t know where the idea originated before that.

Here is a little program that demonstrates the difference between SPACING and EPSILON. As detailed in the link referenced by @ilayn , the SPACING (or gap between representable numbers) varies with order of magnitude. EPSILON represents the gap in numbers between 1 and 2 and is therefore constant irre…

[image] rwmsu: As detailed in the link referenced by @ilayn , the SPACING (or gap between representable numbers) varies with order of magnitude. All of the floating point numbers with the same binary exponent will have the same SPACING() value, so you can think of the spacing as epsilon*2**exp…

[image] jacobwilliams: high precision registers. If 1992, this may be a reference to 80-bit registers. I recall Lahey Fortran provided compiler options to limit real*8 calculations to 64-bit precision, rather than the default of re-use of the values stored in the 80-bit registers.

Common block mystery

Help

jacobwilliams August 31, 2024, 11:39pm 3

Yep, sounds reasonable! See this earlier post for another old school way to trick the compilers to do something like that: Looking at some old code

Topic		Replies	Views
Looking at some old code	27	1227	October 24, 2022
Suggestion: FINDLOC tolerance Language enhancement	15	910	February 7, 2023
Question about the use of common blocks Help	62	5954	November 8, 2021
What do we mean by common?	25	828	October 19, 2021
Use case of single-precision real number Help	20	1277	March 17, 2024

Common block mystery

Related topics