[Bug rtl-optimization/44249] [4.4/4.5/4.6 Regression] IRA generates extra register move

[vmakarov at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44249

--- Comment #2 from Vladimir Makarov <vmakarov at redhat dot com> 2010-11-24 17:40:56 UTC ---
Reload creates additional insn for insn

(insn 9 7 11 2 (parallel [
            (set (reg:DI 71)
                (lshiftrt:DI (reg/v:DI 60 [ tag ])
                    (const_int 4 [0x4])))
            (clobber (reg:CC 17 flags))
        ]) b.i:5 533 {*lshrdi3_1}
     (expr_list:REG_DEAD (reg/v:DI 60 [ tag ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

That is because r60 and r71 got different registers (0 an 1) even
although there is a copy between r71 and r60 which should result in
getting r70 hard register 0 as r60 one.  It does not happen because
r68 already got 0 and it conflicts with r71:

    r71: preferred GENERAL_REGS, alternative NO_REGS, cover GENERAL_REGS
    r68: preferred AREG, alternative GENERAL_REGS, cover GENERAL_REGS
    r60: preferred GENERAL_REGS, alternative NO_REGS, cover GENERAL_REGS

;; a0(r68,l0) conflicts: a1(r71,l0)

;; a4(r67,l0) conflicts:  cp0:a1(r71)<->a3(r60)@1000:constraint

      Popping a0(r68,l0)  -- assign reg 0
      Popping a3(r60,l0)  -- assign reg 0
      Popping a1(r71,l0)  -- assign reg 1

Analogous insn for gcc-4.3 looks like

(insn:HI 9 7 11 2 b.i:4 (parallel [
            (set (reg/v:DI 58 [ tag ])
                (lshiftrt:DI (reg/v:DI 58 [ tag ])
                    (const_int 4 [0x4])))
            (clobber (reg:CC 17 flags))
        ]) 514 {*lshrdi3_1_rex64} (expr_list:REG_UNUSED (reg:CC 17 flags)
        (nil)))

It means there is no such problem as in gcc4.4+.

Insn 9 for gcc-4.3 is a result of regmove transformation.  I have no
idea why regmove (which is present in gcc4.4+) does not do the same
for gcc4.4+ (probably because of some changes since 4.3).

The problem could be fixed in regmove or in IRA (which is probably
harder).  But I don't know is it worth to do it.  Because such
transformations result in longer live ranges of pseudos and might
result in worse code for other programs.