[Bug optimization/6585] Reduntant store/load instruction pairs on ix86

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

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=6585


steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|useless memory store        |Reduntant store/load
                   |instructions on x86         |instruction pairs on ix86


------- Additional Comments From steven at gcc dot gnu dot org  2003-07-10 20:09 -------
After looking into this a bit further and after consulting with Zdenek, I
believe we should blaim this bug on something in the register allocator.  But
then again the same problem exists with -fnew-ra, so maybe it's something in the
i386 backend (register sets???).  Maybe someone can try on a different 32bits
target and see if the problem exists there as well.

Anyway, before register allocation it
looks reasonable, but after global reg alloc we get this insns:

(insn 39 18 24 0 (set (mem:SI (plus:SI (reg/f:SI 7 esp)
                (const_int 4 [0x4])) [3 S4 A8])
        (reg:SI 2 ecx)) 39 {*movsi_1_nointernunit} (nil)
    (nil))
 
(note:HI 24 39 27 0 NOTE_INSN_FUNCTION_END)
 
(insn:HI 27 24 30 0 (set (reg/i:DI 0 rax [ <result> ])
        (mem:DI (reg/f:SI 7 esp) [3 S8 A8])) 60 {*movdi_2} (insn_list 18
(nil))
    (nil))

%ecx is put in the stack and subsequently loaded into %edx when the
result insn is expanded.  (After some black magic apparently %ecx is
replaced with %ebx post reload...)