[Bug rtl-optimization/20972] [4.0/4.1 Regression] Can't describe an early-clobber by an auto-inc

[rearnsha at gcc dot gnu dot org]

------- Additional Comments From rearnsha at gcc dot gnu dot org  2005-05-12 11:36 -------
The ARM ARM says that

  STR Rd, [Rn], #4

and 

  STR Rd, [Rn, #4]!

are unpredictable if Rd == Rn.  That is, the auto-inc side effect is permitted
to take place before Rd is read.
 
There's currently no way for a back-end to describe that an auto-inc operation
might be an early-clobber.  The obvious '&<>' in the constraint doesn't work,
causing an abort in find_reloads().

This bug isn't really a regression (the limitation has always been there), but
the vagaries of register allocation means that it is only recently that it's
started doing this (either that, or the assembler has only recently started
whining about it :-).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rearnsha at gcc dot gnu dot
                   |                            |org
          Component|target                      |rtl-optimization
   Last reconfirmed|2005-04-12 17:08:16         |2005-05-12 11:36:06
               date|                            |
            Summary|[4.0/4.1 Regression]        |[4.0/4.1 Regression] Can't
                   |Compiler-generated code     |describe an early-clobber by
                   |produces an assembler       |an auto-inc
                   |warning                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972