Subject: [Boost-bugs] [Boost C++ Libraries] #4455: clean up preprocessor/seq/fold_left.hpp
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-07-20 15:06:02
#4455: clean up preprocessor/seq/fold_left.hpp
Reporter: anonymous | Owner:
Type: Bugs | Status: new
Milestone: Boost 1.44.0 | Component: None
Version: Boost 1.44.0 | Severity: Problem
BOOST_PP_SEQ_FOLD_LEFT is a macro that applies a submitted macro
(henceforth called operation) to all elements of a sequence, starting with
element 0 (leftmost). The operation macro op(n, state, elem) takes three
arguments, the first being a counter starting from 2 and incremented after
each operation, second, an arbitrary sequence of preprocessing tokens
comprising the intermediate result (or current state), and finally the
current element, stripped of the embracing parentheses. The result of the
operation is an updated state, passed on to the next call to the
BOOST_PP_SEQ_FOLD_LEFT is called with three parameters: first, the name of
the operation macro, second, the initial state, and third, the sequence to
which to apply the operation element-wise. It returnes the final state.
Since the above mentioned counter starts from 2, submitting a sequence of
length 256 to BOOST_SEQ_FOLD_LEFT will result in a final call to the
operation macro with its first parameter set to 257. I neither expect a
counter starting from anything but 0 or 1, nor do I expect, it might get
me out of the arithmetic range, so I consider this a really bad design
The 1.43 implementation of BOOST_PP_SEQ_FOLD_LEFT contains a huge amount
of what seems to be leftovers from a major rework, not having been removed
yet, and, in addition, allows for significant reduction of evaluation
fold_left.hpp is now simplified in 3 major steps.
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,
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.
Hence simplify the definition of BOOST_PP_SEQ_FOLD_LEFT:
# define BOOST_PP_SEQ_FOLD_LEFT BOOST_PP_SEQ_FOLD_LEFT_1
Now, that BOOST_PP_SEQ_FOLD_LEFT_P has been eliminated, its definition may
be removed. Then, none of the BOOST_PP_SEQ_FOLD_LEFT_CHECK_* macros are
called any more (not only in fold_left.hpp, but throughout the whole
preprocessor branch). The same holds for some other macros as well. Here
is the list:
- BOOST_PP_SEQ_FOLD_LEFT_n for 2 <= n <= 256;
Drop their definitions.
The loop iterating over the elements has been unrolled disadvantageously.
Although the length of the sequence is determined in advance, the loop
starts in a forward manner and, thus, has to permanently check when to
terminate. A countdown strategy is more appropriate here, automatically
terminating when iteration 1 is reached. This saves a couple of macro
replacements per iteration. Because of the 257 quirk of
BOOST_PP_SEQ_FOLD_LEFT mentioned in the first section, an extra parameter
q257 has to be introduced, controlling the setting of the counter to the
somewhat irregular value 257 when a sequence of length 256 is encountered.
While the reduction and dropping in step 1 and 2 should not be affected by
preprocessor quirks, the optimization carried out here might be
susceptible to them. So, for safety reasons, this optimization takes
effect only when BOOST_PP_CONFIG_FLAGS() is equal to
-- Ticket URL: <https://svn.boost.org/trac/boost/ticket/4455> Boost C++ Libraries <http://www.boost.org/> Boost provides free peer-reviewed portable C++ source libraries.
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