[TUHS] Comments on "C"

Doug McIlroy doug at cs.dartmouth.edu
Fri Sep 9 12:43:35 AEST 2016


I sent a similar message some time ago, but I haven't
seen it appear in the mailing list, so here goes again.
Apologies if it ends up as a duplicate.

> After about 30 years of C, there are only three things I would have liked
> to see:
> 
> 1.  Computed goto
> ...
> Computed goto's are good for interpreters.

A computed goto is an optimized switch, and that optimization
goes back to the original C compiler. Mostly driven by
considerations of size and speed of the Unix kernel, Dennis
quite early on taught the compiler to choose among three
compilation strategies for a switch: a chain of comparisons, 
a tree of comparisons, or a computed goto, depending on the
number and density of alternatives.

The compilation of the system-call dispatch table was
a perfect example of "good for interpreters."

I have always assumed that other mainline compilers behave
similarly, but I have no solid knowledge about that.

doug


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