Re: bin/47301: Miscompilation by bundled GCC

[Torbjorn Granlund <tg%gmplib.org@localhost>]

I looked a bit into this GCC problem, using gcc 4.7.2.

It seems to be a reload problem, and I think the vax port is innocent.

We have (slightly edited for legibility),

  (set (mem:SI (mem/f:SI (plus:SI (reg/v/f:SI 261 [ x ])
                                  (const_int 8))))
       (mem/u:SI (plus:SI (mult:SI (reg/v:SI 262 [ n ])
                                   (const_int 4))
                          (symbol_ref:SI ("__gmp_oddfac_table")))))

which then gets reloaded into,

  (set (reg:SI 1 %r1)
       (mem/f/c:SI (plus:SI (reg/f:SI 12 %ap)
                            (const_int 4))))

  (set (reg:SI 1 %r1)
       (mem/c:SI (plus:SI (reg/f:SI 12 %ap)
                          (const_int 8))))

  (set (mem:SI (mem/f:SI (plus:SI (reg:SI 1 %r1)
                                  (const_int 8))))
       (mem/u:SI (plus:SI (mult:SI (reg:SI 1 %r1)
                                   (const_int 4))
                          (symbol_ref:SI ("__gmp_oddfac_table")))))

i.e., reload puts both pseudo 261 and pseudo 262 into hard reg 1 in
the same live range.  That is bad.

Very few current backends support mem-to-mem copying in a single
instruction.  It might be something that no longer works in reload.

Assuming my assumption is correct, a possible fix would be to disallow
mem-to-mem insns, at least until reload.

-- 
Torbjörn