[Bug target/30829] extra register zero extends

["dje at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from dje at google dot com  2007-11-09 00:57 -------
I looked into what's going on here.

This is a problem in the i386.md lshr+zext combiner patterns (or a problem in
the combine pass, depending on one's point of view).  There are patterns in
i386.md that are supposed to handle this case but they're not being used.

lshrsi3_1_one_bit_zext doesn't handle the lshr(1) case. I don't understand why
the lshr is outside of the zero_extend in its definition.  Looking at the dump
of the combine pass I see it trying to match a zero_extract, and if I code
lshrsi3_1_one_bit_zext to use a zero_extract then I get the expected code (i.e.
the superfluous move is gone).

lshrsi3_1_zext doesn't handle the lshr(n) case.  It intuitively matches but
combine tries to "simplify" this case to (and:DI (subreg:DI (lshr ...) 0)
0xffffffff) and there's no pattern that matches this.  If I add a pattern to
match this it still doesn't work because the rtx_cost of the two separate insns
lshr,zext (= 4 + 1) is less than the rtx cost of the combined pattern (= 4 + 3
+ 4) so combine rejects the change because it thinks it's more expensive. 
ix86_rtx_costs has code to treat zero_extend SI->DI as a cheap case but it
doesn't get used here unfortunately because the zero_extend gets converted to
an AND (which isn't to say that's bad for the general case).

There are two other related patterns that I also don't understand:
lshrsi3_cmp_one_bit_zext and lshrsi3_cmp_zext.  It's not clear to me that they
will match anything.

It's unfortunate that the intuitive pattern can't work here.  i.e. for the one
bit shift one needs to use a zero_extract and for the N bit shift one needs to
use AND and hack ix86_rtx_costs.  Maybe if the "simplified" pattern doesn't
work combine could try a "less simplified" pattern, or maybe combine could be
told not to simplify this particular case (either via target dependent means or
by detecting the specific case of SI->DI).  Maybe I'm missing something.


-- 

dje at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dje at google dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30829