Why does Knuth use this clunky decrement?

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Category:Languages

I'm looking at some of Prof. Don Knuth's code, written in CWEB that is converted to C. A specific example is dlx1.w, available from Knuth's website

At one stage, the .len value of a struct nd[cc] is decremented, and it is done in a clunky way:

  o,t=nd[cc].len-1;   o,nd[cc].len=t; 

(This is a Knuth-specific question, so maybe you already know that "o," is a preprocessor macro for incrementing "mems", which is a running total of effort expended, as measured by accesses to 64-bit words.) The value remaining in "t" is definitely not used for anything else. (The example here is on line 665 of dlx1.w, or line 193 of dlx1.c after ctangle.)

My question is: why does Knuth write it this way, rather than

nd[cc].len--; 

which he does actually use elsewhere (line 551 of dlx1.w):

oo,nd[k].len--,nd[k].aux=i-1; 

(And "oo" is a similar macro for incrementing "mems" twice -- but there is some subtlety here, because .len and .aux are stored in the same 64-bit word. To assign values to S.len and S.aux, only one increment to mems would normally be counted.)

My only theory is that a decrement consists of two memory accesses: first to look up, then to assign. (Is that correct?) And this way of writing it is a reminder of the two steps. This would be unusually verbose of Knuth, but maybe it is an instinctive aide-memoire rather than didacticism.

For what it's worth, I've searched in CWEB documentation without finding an answer. My question probably relates more to Knuth's standard practices, which I am picking up bit by bit. I'd be interested in any resources where these practices are laid out (and maybe critiqued) as a block -- but for now, let's focus on why Knuth writes it this way.

 


This whole practice seems to be based on a mistaken idea/model of how C works, that there's some correspondence between the work performed by the abstract machine and by the actual program as executed (i.e. the "C is portable assembler" fallacy). I don't think we can answer a lot more about why that exact code fragment appears, except that it happens to be an unusual idiom for counting loads and stores on the abstract machine as separate.

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