Re: [patch] Remove unused crossjump code

[Richard Guenther <richard dot guenther at gmail dot com>]

On Sat, Jun 13, 2009 at 1:00 AM, Steven Bosscher<stevenb.gcc@gmail.com> wrote:
> Hi,
>
> Apparently GCC ran crossjumping before reload once, and it was useful
> to substitute REG_EQUIV values for registers because it "helps take
> care of G++ cleanups."
>
> There are several problems with this code now:
>
> 1. It does not run. ?Crossjumping only runs after reload right now
> (before reload, crossjumping almost always fails because structurally
> equivalent code looks different because different input registers are
> used -- I have a patch to deal with that separately).
>
> 2. It does not help. As far as I know, GCC currently only has
> REG_EQUIV notes for incoming arguments, and the arguments registers
> don't overlap so substituting them can never result in a new
> successful match. And a failure to substitute a REG_EQUAL note earlier
> in the pipeline would mean there is a missed optimization somewhere.
> (Cases with different insns but the same REG_EQUAL value are not
> handled at all.)
>
> 3. I could not figure out what it may have been helpful for in the
> past. ?The code comes from the old crossjumping code in jump.c so it
> predates even the CFG-based crossjumping code. And in jump.c the code
> is already in the oldest available revision in SVN. So basically the
> code has always been there but its raison-d'etre is lost... ?Maybe
> some (hi Jeff!) remembers what this was for?
>
> 4. The code as-is modifies the insns stream in a _p function. Bad.
>
> Anyway, enough reasons for me to propose this patch, to just remove the code.
> Bootstrapped&tested on {ia64,x86_64}-unknown-linux-gnu and
> cross-built&tested on x86_64 X arm-elf, x86_64 X mips-elf.

Ok.

Thanks,
Richard.

> Ciao!
> Steven
>
>
> * cfgcleanup.c (old_insns_match_p): Remove code to substitute
> REG_EQUAL/REG_EQUIV notes.
>
> Index: cfgcleanup.c
> ===================================================================
> --- cfgcleanup.c ? ? ? ?(revision 148444)
> +++ cfgcleanup.c ? ? ? ?(working copy)
> @@ -1009,40 +1009,6 @@ old_insns_match_p (int mode ATTRIBUTE_UN
> ? ? ? ? rtx_renumbered_equal_p (p1, p2) : rtx_equal_p (p1, p2))
> ? ? return true;
>
> - ?/* Do not do EQUIV substitution after reload. ?First, we're undoing the
> - ? ? work of reload_cse. ?Second, we may be undoing the work of the post-
> - ? ? reload splitting pass. ?*/
> - ?/* ??? Possibly add a new phase switch variable that can be used by
> - ? ? targets to disallow the troublesome insns after splitting. ?*/
> - ?if (!reload_completed)
> - ? ?{
> - ? ? ?/* The following code helps take care of G++ cleanups. ?*/
> - ? ? ?rtx equiv1 = find_reg_equal_equiv_note (i1);
> - ? ? ?rtx equiv2 = find_reg_equal_equiv_note (i2);
> -
> - ? ? ?if (equiv1 && equiv2
> - ? ? ? ? /* If the equivalences are not to a constant, they may
> - ? ? ? ? ? ?reference pseudos that no longer exist, so we can't
> - ? ? ? ? ? ?use them. ?*/
> - ? ? ? ? && (! reload_completed
> - ? ? ? ? ? ? || (CONSTANT_P (XEXP (equiv1, 0))
> - ? ? ? ? ? ? ? ? && rtx_equal_p (XEXP (equiv1, 0), XEXP (equiv2, 0)))))
> - ? ? ? {
> - ? ? ? ? rtx s1 = single_set (i1);
> - ? ? ? ? rtx s2 = single_set (i2);
> - ? ? ? ? if (s1 != 0 && s2 != 0
> - ? ? ? ? ? ? && rtx_renumbered_equal_p (SET_DEST (s1), SET_DEST (s2)))
> - ? ? ? ? ? {
> - ? ? ? ? ? ? validate_change (i1, &SET_SRC (s1), XEXP (equiv1, 0), 1);
> - ? ? ? ? ? ? validate_change (i2, &SET_SRC (s2), XEXP (equiv2, 0), 1);
> - ? ? ? ? ? ? if (! rtx_renumbered_equal_p (p1, p2))
> - ? ? ? ? ? ? ? cancel_changes (0);
> - ? ? ? ? ? ? else if (apply_change_group ())
> - ? ? ? ? ? ? ? return true;
> - ? ? ? ? ? }
> - ? ? ? }
> - ? ?}
> -
> ? return false;
> ?}
>