Boost C++ Libraries: Ticket #4455: clean up preprocessor/seq/fold_left.hpp

attachment set

anonymous — Tue, 20 Jul 2010 15:06:28 GMT

attachment → seq_fold_left.patch

severity changed

ookami1@… — Tue, 20 Jul 2010 15:08:23 GMT

severity Problem → Optimization

ookami1@… — Tue, 20 Jul 2010 15:11:03 GMT

Forgot to add my name and a greeting:

Cheers Wolf Lammen

type, component changed; owner set

ookami1@… — Tue, 20 Jul 2010 15:18:44 GMT

owner set to No-Maintainer
type Bugs → Library Submissions
component None → preprocessor

attachment set

ookami1@… — Tue, 20 Jul 2010 15:40:40 GMT

attachment → seq_fold_left.2.patch

takes BOOST_PP_SEQ_FOLD_RIGHT into account

ookami1@… — Tue, 20 Jul 2010 15:42:05 GMT

Forget step 3 for the time being, as this would break BOOST_PP_SEQ_FOLD_RIGHT

Cheers

Wolf Lammen

Steven Watanabe — Tue, 20 Jul 2010 16:05:41 GMT

Replying to anonymous:

step 1:

The definition of BOOST_PP_SEQ_FOLD_LEFT entails a call to BOOST_PP_AUTO_REC. In short, this call turns out to be completely pointless, as it will always return 1 as result, and can be simply replaced by this constant.

This can be seen as follows:

BOOST_PP_AUTO_REC operates on macros P(n), that

take integers from 1 to 256 as paramater;
are binary, i.e. yields only 0 or 1 as a result;
are monotonely rising, i.e. there is an integer n such that P(i) == 0 for i < n and == 1 otherwise.
BOOST_PP_AUTO_SEC is designed to find the 'leap position' where P assumes 1 for the first time.

BOOST_PP_SEQ_FOLD_LEFT_P is such a P(n). Although the replacement of BOOST_PP_SEQ_FOLD_LEFT_P looks complex, a closer look at
BOOST_PP_SEQ_FOLD_LEFT_I_ ## n(BOOST_PP_SEQ_FOLD_LEFT_O, BOOST_PP_NIL, (nil), 1)
reveals, that this expression, independently of n, will always expand to BOOST_PP_NIL, and after some more replacing, BOOST_PP_SEQ_FOLD_LEFT_P turns out to be constantly 1. Consequently, BOOST_PP_AUTO_REC, applied to this (constant) macro, will yield 1 as a result.

Nope. BOOST_PP_SEQ_FOLD_LEFT_P expands to 1 iff. we are not currently in an expansion of BOOST_PP_SEQ_FOLD_LEFT_I_ ## n. If BOOST_PP_SEQ_FOLD_LEFT_I_ ## n is being expanded, then it will not be expanded again, since recursive calls to a macro are not expanded. BOOST_PP_AUTO_REC is used to find the first available index that is not on the macro expansion stack. This allows nested calls to BOOST_PP_SEQ_FOLD_LEFT. Note that the s parameter to the macro is not the index in the sequence. It is the next available index for reentering BOOST_PP_FOLD_LEFT. See http://www.boost.org/libs/preprocessor/doc/ref/seq_fold_left.html.

anonymous — Wed, 21 Jul 2010 18:02:30 GMT

Yes, judging from how BOOST_PP_SEQ_FOLD_LEFT_* macros were used, I already had the idea, they might be a pool of macros shared among several loops. I just did not see how this pool was managed, so I thought 'get rid of it!'.

Your explanation fills in here, and I mark this ticket as invalid (that is if I have sufficient right to do so).

Cheers

Wolf Lammen

status changed; resolution set

anonymous — Wed, 21 Jul 2010 18:07:53 GMT

status new → closed
resolution → invalid