[Bug tree-optimization/18137] New: arguments being gimple registers cause redundant memory loads

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

Since the arguments are gimple registers, gimple optimizers are happy to create many references to it.
While lowering them to RTL these however compile to memory loads causing number of redundant loads.
The attached quicksort loop on when compiled with -O2 -fno-loop-optimize (the other being needed only for clarity of the testcase) produces such a funny sequence:
        movl    8(%ebp), %eax   # 143   *movsi_1/1      [length = 3]
        movl    8(%ebp), %edx   # 171   *movsi_1/1      [length = 3]
        movl    8(%ebp), %ebx   # 145   *movsi_1/1      [length = 3]
These are comming from:
  median = data[start];
  pos.22 = start + 1;
  if (end - start <= 1) goto <L6>; else goto <L25>;
where each of these compiles into RTL expression that looks different to CSE:
(insn 16 15 18 1 (set (reg/v:SI 66 [ median ])
        (mem/s:SI (plus:SI (mult:SI (reg/v:SI 68 [ start ])
                    (const_int 4 [0x4]))
                (reg/f:SI 70)) [3 data S4 A32])) -1 (nil)
    (nil))

(insn 18 16 20 1 (parallel [
            (set (reg/v:SI 60 [ pos.22 ])
                (plus:SI (reg/v:SI 68 [ start ])
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil)
    (nil))

(insn 20 18 21 1 (parallel [
            (set (reg:SI 71)
                (minus:SI (reg/v:SI 69 [ end ])
                    (reg/v:SI 68 [ start ])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil)
    (nil))

(insn 21 20 22 1 (set (reg:CCGC 17 flags)
        (compare:CCGC (reg:SI 71)
            (const_int 1 [0x1]))) -1 (nil)
    (nil))
Similarly we get redudnant loads inside the loop itself.

Not sure about sollution - making arguments nongimple registers does not lead optimizers to deal with them very nicely, forcing
expander to load memory operands to register in prologue would lead to unnecesarly long lifetimes...
forcing memory operands to registers in RTL generation is something we want to avoid ;)
Ideas?

-- 
           Summary: arguments being gimple registers cause redundant memory
                    loads
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hubicka at gcc dot gnu dot org
                CC: dnovillo at redhat dot com,gcc-bugs at gcc dot gnu dot
                    org,rth at redhat dot com,stevenb at suse dot de
GCC target triplet: i686-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18137