Feature #1493: [patch] lex_state as bit field / IS_lex_state() macro
http://redmine.ruby-lang.org/issues/show/1493

Author: Dave B
Status: Open, Priority: Normal

#  ? Changelog:
#  Represent lex_state by bit field instead of serial integer enum
#   so that single or multiple values can be checked together using
#   unifying macro IS_lex_state().  States remain mutually exclusive.

Example:

Each use of current macro IS_BEG() ..
#define IS_BEG() (lex_state == EXPR_BEG   || lex_state == EXPR_MID || \
                  lex_state == EXPR_VALUE || lex_state == EXPR_CLASS)

.. results in up to 4 tests which can be reduced to 1.

An extreme use case ..

        if (IS_BEG() ||
            lex_state == EXPR_DOT ||
            IS_ARG()) {

.. requires up to 7 tests on lex_state.

To my knowledge, compilers can't optimize this.
As a bit field, there's no need to:

        if (IS_lex_state( EXPR_BEG_ANY | EXPR_ARG_ANY | EXPR_DOT ))

.. reduces to:    if (lex_state & (test_bits))  // TRUE/FALSE

All changes in this patch were scripted to eliminate the possibility
of typos.  Where some replaced sections looked similar or the same,
they were verified with MD5sum before applying!
Multiple state tests were merged using simple, strict logic and any
"surprise" code would have been left unchanged.

Therefore, it should not be necessary to check every line of the patch;
I hope that a check of a random sample will give confidence in the rest.

There were several repeated switches which could have been replaced with:

  lex_state = (IS_lex_state( EXPR_FNAME | EXPR_DOT )) ? EXPR_ARG : EXPR_BEG

 but I assumed you would prefer if-else for flexibility and legibility:

 	if (IS_lex_state( EXPR_FNAME | EXPR_DOT )) {
	    lex_state = EXPR_ARG;
	}
	else {
	    lex_state = EXPR_BEG;
	}

Say what you don't like about anything - it might be easy to change. :)

Wouldn't it be nice to use some spare bits to combine other state?

        if ((lex_state == EXPR_BEG && !cmd_state) || ...
        if (IS_ARG() && space_seen && ...

 - I haven't explored those, yet.


My motivation was not to reduce processor heat. ;)
In the future, if we need to break the dependence on Yacc/Bison, these
state transitions might need to become token information and the change
makes them far easier to store.
( e.g. Any of 'THESE states' => 'THIS state' )


daz


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