Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

Dan Cross: I'll confess I haven't looked _that_ closely, but I rather imagine that the V10 compiler is a descendant of PCC rather than Dennis's V6/V7 PDP-11 compiler. ==== Correct. From 8/e to the end of official Research UNIX, cc was pcc2 with a few research-specific hacks. As Dan says, lcc was there too, but not used a lot. I'm not sure which version of lcc it was; probably it was already out-of-date. In my private half-backed 10/e descendant system, which runs only on MicroVAXes in my basement, cc is an lcc descendant instead. I took the lcc on which the book was based and re-ported it to the VAX to get an ISO- compliant C compiler, and made small changes to libc and /usr/include to afford ISO-C compliance there too. The hardest but most-interesting part was optimizing. lcc does a lot of optimization work by itself, and initially I'd hoped to dispense with a separate c2 pass entirely, but that turns out not to be feasible on machines like the VAX or the PDP-11: internally lcc separates something like c = *p++; into two operations c = *p; p++; and makes two distinct calls to the code generator. To sew them back together from cvtbl (p),c incl p to cvtbl (p)+,c requires external help; lcc just can't see that what it thinks of as two distinct expressions can be combined. It's more than 15 years since I last looked at any of this stuff, but I vaguely remember that lcc has its own interesting (but ISO/POSIX-compatible) memory-allocation setup. It allows several nested contexts' worth of allocation, freeing an inner context when there's no longer any need for it. For example, once the compiler has finished with a function and has no further need for its local symbols, it frees the associated memory. See the lcc book for details. Read the book anyway; it's the one case I know of in which the authors followed strict Literate Programming rules and made a big success of it. Not only is the compiler well- documented, but the result is a wonderful tour through the construction and design decisions of a large program that does real work. Norman Wilson Toronto ON