At Wed, 27 Feb 2013 16:34:46 -0500, Ray Racine wrote: > The below is just an observation. > > Historically in TR I've always struggled with Index vs Fixnum vs Natural > for the vector indexing type when iterating over a vector (or large > sub-ranges or even larger vectors). Either the Optimizer complains that an > index? check was not elided in a vector-ref OR you end up doing something > along the lines of (assert (add1 idx) index?) while looping your index > variable passing over the Vector. > > But I've just noticed that one can construct TR recognized (Sequenceof > Index). > > So in theory, TR + Compiler should be able to elide a number of checks, > such as (assert idx index?), use `unsafe-vector-ref', even elide in loop > bounds checks if the vector is supplied to the 'for'. This assumes the > (Sequenceof Index) is an efficient generator. i.e., (in-range 3 10000000) > is not allocating 10000000 somethings. ( I don't know what it "really" is > happening overall, just what I think is possible. Sufficiently smart > compiler yada yada.)
The TR optimizer is not smart enough to completely elide bounds checking[1]. If a vector index is guaranteed to be an `Index', the optimizer avoids checking whether the index is >= 0, uses unsafe fixnum operations for that check, and then calls `unsafe-vector-ref'. Not as fast as calling `unsafe-vector-ref' directly, but still an improvement over plain `vector-ref'. At Wed, 27 Feb 2013 16:51:42 -0500, Ray Racine wrote: > Just to be clear. The below does work in TR and idx is of type Index. > However when one embeds the (in-range 3 5) directly in the for/vector idx > is not of type Index. No reason why it shouldn't however. The `for' macros recognize the `in-*' forms and, instead of actually calling the corresponding functions, generate optimized code in-line. This means that TR, instead of seeing the call to `in-range', sees the optimized expansion and typechecks that. Since that code uses `+' in unrestricted ways, TR can't assign precise types. Vincent [1] Except in very limited cases, where a vector with a bounded-length vector type is accessed with an index of a singleton type that's within the bound. Basically, when vectors are used as poor man's structs. ____________________ Racket Users list: http://lists.racket-lang.org/users
