[Bug target/50467] New: Compiler can move stack cleanup before last memory reference involving the stack

[meissner at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50467

             Bug #: 50467
           Summary: Compiler can move stack cleanup before last memory
                    reference involving the stack
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: meissner@gcc.gnu.org
              Host: powerpc64-linux
            Target: powerpc64-linux
             Build: powerpc64-linux


In looking at the 462.libquantum spec 2006 benchmark, we noticed that the addi
used to undo the stack frame before the return is done can be moved above
memory operations involving the stack.  In the example we noticed (specrand.c),
it was using a convert 32-bit integer to floating point sequence.  On all
powerpc processors except for power6x, this conversion involves doing a store
to the stack and a reload into the FPR, since there is no move GPR to FPR
instruction.  However, the LFIWAX and LFIWZX instructions do not have a
reg+offset form, and only support reg+reg.  The Altivec and VSX vector
instructions are similar in only having reg+reg forms.  The address that is
used to reference the stack does not have the stack pointer in it, so the
scheduler thinks there is no conflict in moving it up.