Suggest you habe a fortran module with a collection of mathematical constant like PI.<\/p>\n
\r\nmodule constant_m\r\n real, parameter :: PI = 3.1314_8\r\nend module\r\n<\/code><\/pre>\nNow you use<\/strong> this module in a function which calculate the circumcenter of a circle with given radius. <\/p>\n\r\nfunction U(r) \r\n use constant_m\r\n implicit none\r\n real, intent(in) :: r\r\n real :: U\r\n U = 2*PI*r\r\nend function\r\n<\/code><\/pre>\nWill the expression 2*PI<\/strong> be evaluated a compile or runtime?
\nLets check the generated assember code!
\n...
\nAnd you'll see only crap.<\/p>\nLet's start with an simpler example. Replace every real to integer. Truncate PI to 3, thats fair enough. And remove the \"2*\" expression. Somewhere in the assembler code are there must be the expression y=3*x.
\nHere it is:
\ngfortran --save-temps -O1 -c constants.F90 circle.F90 <\/p>\n
\r\n movl\t4(%esp), %eax % Store the adress of r from stack pointer into eax\r\n movl\t(%eax), %eax % Dereference adress and store value from r into eax\r\n leal\t(%eax,%eax,2), %eax % eax = eax + 2*eax \r\n<\/code><\/pre>\nThe last line is the one. The compiler optimize the multiplicaton with 3 to an assembler instrctuin which performs a multiplication with 2 and an addition.
\nWhat we not see is a constant \"3\" in assember code. Or a assembler register filled with \"3\". <\/p>\n
Lets add the \"2*\" and see whats happens.<\/p>\n
\r\n movl\t4(%esp), %eax\r\n movl\t(%eax), %eax\r\n leal\t(%eax,%eax,2), %eax\r\n addl\t%eax, %eax\r\n<\/code><\/pre>\nOnly the last line is new. The compiler decide that x+x is more performance than 2*x. Cute.<\/p>\n
Okay lets have some fun. We force the compiler to evaluate (2*PI) first. (2*PI)*r
\nAnd the resulting assembler code is the same HAHA.<\/p>\n
Okay back to topic. We see that integer, parameter variables in modules are evaluated at compile time. Lets switch back to real.<\/p>\n
Now the assembler code looks a litte bit more complicated:<\/p>\n
\r\n flds\t.LC0@GOTOFF(%eax) % Load constant into floading point register\r\n movl\t4(%esp), %eax % Store value from r into eax\r\n fmuls\t(%eax) % Multiply the constant with r and put the result back in eax\r\n.LC0:\r\n .long\t1086918230 % The constand 2*PI\r\n<\/code><\/pre>\nNow you can see a constant in assember as a 4byte iteger. Its only one constant, so I guess its 2*PI here.
\nThis constant is loaded into a floting point register. The value from r is loaded into a second register. Then both get multiplyed. Thats all. Only a single multiplaction.<\/p>\n
PARAMETER VARIABLES IN MODULE ARE EVALUATED AT COMPILE TIME! :D<\/p>\n","protected":false},"excerpt":{"rendered":"
Suggest you habe a fortran module with a collection of mathematical constant like PI. module constant_m real, parameter :: PI = 3.1314_8 end module Now you use this module in a function which calculate the circumcenter of a circle with given radius. function U(r) use constant_m implicit none real, intent(in) :: r real :: U […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[30],"class_list":["post-3090","post","type-post","status-publish","format-standard","hentry","category-allgemein","tag-fortran"],"_links":{"self":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/3090","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3090"}],"version-history":[{"count":5,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/3090\/revisions"}],"predecessor-version":[{"id":3095,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/3090\/revisions\/3095"}],"wp:attachment":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3090"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3090"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3090"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}