A few simple DImode improvements

[Bernd Schmidt <bernds at codesourcery dot com>]

There are ways to improve register allocation for DImode without going
to extremes like the patch I posted on Monday.  The attached patch is
quite simple and fixes two issues I noticed while working on the DCE
patch I posted earlier.

First, when we set DImode registers in a sequence of multiple SImode
accesses, we emit a clobber at the start of the sequence to tell life
analysis that the register is dead above this point.  Scheduling can
move these clobbers, extending the lifetime as far as IRA is able to
tell.  The changes in ira-lives.c make us move these clobbers downwards
as far as possible.  This may sometimes help even for single-word regs
that are initialized with a sequence of partial sets involving zero_extract.

The changes in the lower-subreg pass are motivated by nonsensical
transformations like the following:

-(insn 205 204 26 4  (set (reg:DI 68)
-        (reg:DI 178)) 49 (REG_DEAD (reg:DI 178)
+(insn 261 204 262 4  (clobber (reg:DI 193))
+
+(insn 262 261 263 4  (set (subreg:SI (reg:DI 193) 0) (reg:SI 191))
+
+(insn 263 262 264 4  (set (subreg:SI (reg:DI 193) 4) (reg:SI 192))
+
+(insn 264 263 26 4  (set (reg:DI 68) (reg:DI 193))

In resolve_simple_move, we generate a new DImode temporary pseudo rather
than storing into the DImode pseudo which is the actual destination.
This, I think, is due to confusion about the use of can_decompose_p; it
makes sense to use it in this context only for hard regs.  For pseudos,
it tests the non_decomposable_context bitmap, which is irrelevant here.

Bootstrapped and regression tested on i686-linux.  Ok?


Bernd

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