[Bug tree-optimization/108129] nop_atomic_bit_test_and_p is too bloated

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108129

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #0)
> match.pd has multi-pattern matcher 'nop_atomic_bit_test_and_p'.
> 
> It expands to ~38 KLOC in gimple-match.cc and ~350 KB in the compiled binary.
> 
> There has to be a better way than repeatedly emitting the match pattern for
> each member of {ATOMIC,SYNC}_FETCH_{AND,OR_XOR}_N :)

It's the way the matcher works - if you can think of a better way of
code-generating it within the constraints:
 - earlier patterns in match.pd should match first in case there are multiple
matches
 - matching time should ideally be O(size of the pattern) rather than O(size of
match.pd)

The 2nd constraint isn't currently fulfilled because of the first constraint
(or how that is implemented).  The 2nd constraint was the original design
goal to make frequent "re-folding" cheap.  The first constraint was implemented
to allow pattern ordering to disambiguate "overlapping" matches.

Writing a different code generation in genmatch should be possible, the
(lowered) AST is available.