[Bug target/54673] [SH] Unnecessary sign extension of logical operation results

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54673

Oleg Endo <olegendo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2012-10-27
         AssignedTo|unassigned at gcc dot       |olegendo at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #1 from Oleg Endo <olegendo at gcc dot gnu.org> 2012-10-27 13:15:12 UTC ---
Created attachment 28542
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28542
WIP patch (little endian only)

I've been trying out a couple of things.  It seems that disallowing subregs in
operands of logical ops introduces other missed optimization opportunities,
such as:

void test02452 (unsigned char* a, int* b, int c)
{
  b[0] = a[0] & 251;
}

where this would expand into something like 'a[0] & -5' and the 'and #imm,r0'
insn would thus not be used.

The actual problem seems to be the original expansion of memory loads, where
sometimes e.g. the movqi pattern is used to load a QImode mem into a QImode reg
without zero/sign extensions.  Thus subsequent uses of the loaded mem do not
know that it's been sign-extended by the load.
The attached patch fixes this in the movqi / movhi expanders and the redundant
extensions disappear.  However, it has an impact on the tst insns and overall
register allocation.  The tst insns cases can be handled by a few additional
patterns, but the register allocation issue leads to a couple of rather huge
code size increases in some of the CSiBE files (-O2 -m4-single -ml
-mpretend-cmove), with the following peaks:

bzip2-1.0.2
  compress       12172 -> 12708       +536 / +4.403549 %

mpeg2dec-0.3.1
  libmpeg2/motion_comp            4860 -> 5020        +160 / +3.292181 %
  libvo/yuv2rgb                   3856 -> 4056        +200 / +5.186722 %