[Bug rtl-optimization/57921] New: Alias change causes perl from SPEC 2006 to abort on MIPS

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57921

            Bug ID: 57921
           Summary: Alias change causes perl from SPEC 2006 to abort on
                    MIPS
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sje at gcc dot gnu.org

Created attachment 30517
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30517&action=edit
What the CSE phase generated prior to checkin r200133

With the latest GCC (as of version r200133) the perl benchmark in SPEC 2006 is
failing for me on the mips-linux-gnu target.  The patch that caused it to start
failing is to the alias analysis phase of GCC and it affects the rtl CSE phase
of GCC.  Before this change CSE was generating this RTL:

r234 = address of PL_curpad
r211 = mem[r234]
.
.
r215 = $2 (return pointer from a function)
r237 = mem[r211]
mem[r237+4] = r215
r220 = mem[r237]

The important bit is the last statement where r220 is loaded using the
address in r237.

After the change we have:

r234 = address of PL_curpad
r211 = mem[r234]
.
.
r215 = $2 (return pointer from a function)
r237 = mem[r211]
mem[r237+4] = r215
r238 = mem[r211]
r220 = mem[238]

Instead of reusing r237, we load r238 and use that to load r220.  I believe
that PL_curpad (address in r234) is set up to have the value of its own
address minus 4.  That would mean that when we store to 'mem[r237+4]' we
changed what r238 has vs. what r237 has.  Now this actually seems like it
is more correct then the first version, but the first version worked (I could
compile and run perl) and the second version does not (perl allocate aborts
and goes into an infinite loop).

I can see this change in x86 RTL but the change in code on x86 does not seem
to cause an outward change in the behaviour of perl.  I don't know if GCC
is doing something wrong or if the code in perl is wrong.  I have attached
a cutdown test case from Perl_pp_entersub (pp_hot.c) with this bug report.
The code cannot be run but if compiled it shows the RTL differences I describe
above.