Mike Schaeffer's Weblog
Mon, 20 Mar 2006
Macros and Inline Functions
I recently converted a bunch of accessor macros in vCalc over to inline functions. These macros are a legacy of vCalc's siod heritage, and generally looked something like this:
#define FLONM(x) ((*x).storage_as.flonum.data)
In this macro, x is expected to be a pointer to an LObject, which is basically a struct consisting of union, storage_as, and a type field that specifies how to interpret the union. vCalc has a set of these macros to refer to fields in each possible interpretation of the storage_as union. These little jewels remove a lot of reduntant storage_as's from the code, and generally make the code much easier to work with. However, like all C/C++ macros, these macros have finicky syntax, bypass typechecking, and have limited ability to be extended to do other things (like verifying type fields, etc.). Fortunantly, they are a perfect candidate to be replaced by C++ inline functions and references:
inline flonum_t &FLONM(LRef x) 
{ return ((*x).storage_as.flonum.data); }
Even better, with function inlining turned on, there's no apparant performance penalty in making this transformation; even with inlining off, the penalty seems pretty modest at 20-20%. In other words, inline functions works exactly as advertised. It works well enough, if fact, that I made the 'logical extension', and added some type checking logic.
inline flonum_t &FLONM(LRef x)
{      assert(FLONUMP(x));
	return ((*x).storage_as.flonum.data); }
This version of the code verifies that x is actually a flonum before trying to interpret it as such. Normally, the code using these accessor functions explicitly checks the type of an object before making a reference, but sometimes, due to coding errors, invalid references can slip through the cracks. With the old style macros, these invalid references could result in data corruption with no warning. With checks, there's at least an attempt to check for bad references before they are made.

Adding these checks proved profitable: they revealed three logic errors in about 5 minutes of testing, two related to reading complex numbers, and one related to macroexpansion of a specific class of form. Adding these type checks also killed performance, but that was pretty easy to solve by making the additional checks independently switchable.

reddit this! Digg Me!

[/tech/general] permanent link