Re: How to type braces for computed gotos

glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:


> William Clodius <wclodius@earthlink.net> wrote:
>
> (snip on labeled variable GOTO, Fortran ASSIGNed GOTO, and such)
> ...
> If you consider something like Duff's device, then it isn't so obvious
> that computed GOTO is so different from C's switch/case. (Well, when
> the labels are sequential starting at one.)
>
> As I noted previously, the Fortran standard now considers SELECT CASE
> as a replacement for computed GOTO, though the latter hasn't yet been
> removed from the standard.


In the case of Anton I think for the applications he is best know for,
high performance interpreters, the lack of syntactic block is a minor
advantage in that the


100 SELECT CASE(i)
              CASE(0)
              ...
              GO TO 100
              CASE(1)
              ...
              GO TO 100
              CASE(2)
              ...


can be replaced by


              GO TO(10, 11, 12, ...) I
10 ...
              GO TO(10, 11, 12, ...) I
11 ...
              GO TO(10, 11, 12, ...) I
12 ...


Saving an extra step in indirection. The bigger problem is the fall
through semantics for out of range with its implicit additonal test and
branch before the goto's implicit branch table. That can be avoided by a
modulo operation


GO TO(10, 11, 12, ...) MODULO(I, N) +1


where N is the number of labels appearing in the computed GOTO. If I is
an 8 bit integer and N is 256 a sufficiently smart compiler should
recognize the semantics of an unsigned 8 bit integer, fit the jump table
in a single level 1 cache line, optimize out the MODULO, the addition of
1, and the test for out of range and give him the efficiency he used to
get with gcc's "computed goto". Whether that sufficintly smart compiler
is gfortran with gcc's backend would be a different matter.

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