Mark H Weaver <m...@netris.org> writes: > Hi David, > > David Kastrup <d...@gnu.org> writes: >> Mark H Weaver <m...@netris.org> writes: >>> Simpler data structures can usually be implemented with less memory, >>> shorter code sequences with fewer conditional branches and less space in >>> the instruction cache, which in turn means they can be implemented more >>> efficiently. Therefore, to allow efficient compilation, primitive data >>> structures should be very simple, with more complex structures built on >>> simpler ones instead of the other way around. >>> >>> For example, consider resizable vectors vs fixed-size vectors. A >>> fixed-size vector can be represented as a single memory block that >>> contains both the length and the elements together. A resizable vector >>> requires an additional level of pointer indirection, which inevitably >>> means slower accesses and greater code size. Furthermore, fixed-size >>> vectors allow the possibility of compiling in an unsafe mode where >>> out-of-bounds checks can be skipped. >> >> I have a really hard time swallowing an efficiency argument for Scheme >> that is weak enough in comparison with the associated drawbacks not to >> find consideration in the C++ standard template library. > > C++, like Scheme, already supports fixed-size vectors in the core > language, so it would be redundant to include them in a library.
A vector with run-time determined size? Which variant of C++ offers that? >> What kind of performance gains are we talking about in a typical >> vector-heavy application? > > If C++ supported _only_ resizable vectors, such that there was no way > to avoid the additional level of pointer indirection, and all derived > data structures had to be built upon these doubly-indirected vectors, > then I'd expect that the efficiency impact would be quite significant > in both time and space. Reality check: C++ does not offer structs/classes containing vectors of run-time determinable size. You need to allocate a pointer for them. You are apparently confusing fixed size with compile-time size. -- David Kastrup
