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Add section about cost of accessing params in a variadic function

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git-svn-id: svn://svn.cc65.org/cc65/trunk@657 b7a2c559-68d2-44c3-8de9-860c34a00d81
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cuz 2001-03-23 20:19:22 +00:00
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doc/coding.sgml
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@ -368,5 +368,22 @@ or
instead will give shorter and faster code.
<sect>Access to parameters in variadic functions is expensive<p>
Since cc65 has the "wrong" calling order, the location of the fixed parameters
in a variadic function (a function with a variable parameter list) depends on
the number and size of variable arguments passed. Since this number and size
is unknown at compiler time, the compiler will generate code to calculate the
location on the stack when needed.
Because of this additional code, accessing the fixed parameters in a variadic
function is much more expensive than access to parameters in a "normal"
function. Unfortunately, this additional code is also invisible to the
programmer, so it is easy to forget.
As a rule of thumb, if you access such a parameter more than once, you should
think about copying it into a normal variable and using this variable instead.
</article>