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Re: 6 GCC regressions, 2 new, with your patch on 2002-07-20T23:26:42Z.
> Date: Sun, 21 Jul 2002 21:38:43 +0200
> From: Jan Hubicka <jh@suse.cz>
> This is really interesting case. The beggining of function contains:
> (insn 4 3 6 0 (nil) (set (reg:SI 62)
> (mem/f:SI (plus:SI (reg/f:SI 16 argp)
> (const_int 4 [0x4])) [4 y+0 S4 A32])) 45 {*movsi_1}
> (nil)
> (expr_list:REG_EQUIV (mem/f:SI (plus:SI (reg/f:SI 16 argp)
> (const_int 4 [0x4])) [4 y+0 S4 A32])
> (nil)))
>
> And later also:
>
> (insn 12 6 21 0 0x401a3a80 (set (reg/v/f:SI 64)
> (mem/f:SI (plus:SI (reg/f:SI 16 argp)
> (const_int 4 [0x4])) [4 S4 A8])) 45 {*movsi_1} (nil)
> (nil))
>
> cselib notices the equivalence and cprop actually does the replacement
> of reg 64 by reg 62. But the REG_EQUIV note is not really valid, as the
> value of memory copy y is not valid during the function (overwriten by
> bar call). Just before register allocation the load of register 62
> appears to be moved just before the assignment. I am not sure who does
> the movement but it is valid given the information about equivalency
> with memory.
>
> cse.c seems to contain code preventing such a substitution as gcse does,
> so I am attaching simple patch that does the same for cprop and I will
> install it as obvious as I am not quite sure whether I will be able to
> read my email tomorrow.
>
> Sun Jul 21 21:35:20 CEST 2002 Jan Hubicka <jh@suse.cz>
> * gcse.c (do_local_cprop): Do not extend lifetimes of registers set by
> do_local_cprop.
I think that this patch just papers over the real problem, which is
that the REG_EQUIV note is not correct; it should be REG_EQUAL. The
whole purpose of REG_EQUIV notes is that you can always make the
substitution safely.
--
- Geoffrey Keating <geoffk@geoffk.org> <geoffk@redhat.com>