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Re: [RFA 2/n] Don't lift loads above register using jumps in postreload-gcse.c
- From: Richard Earnshaw <rearnsha at arm dot com>
- To: Steven Bosscher <stevenb dot gcc at gmail dot com>
- Cc: Matthew Gretton-Dann <matthew dot gretton-dann at linaro dot org>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, "jle at rice dot edu" <jle at rice dot edu>, Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>, "law at redhat dot com" <law at redhat dot com>, "ebotcazou at libertysurf dot fr" <ebotcazou at libertysurf dot fr>, "rdsandiford at googlemail dot com" <rdsandiford at googlemail dot com>
- Date: Wed, 05 Sep 2012 13:45:38 +0100
- Subject: Re: [RFA 2/n] Don't lift loads above register using jumps in postreload-gcse.c
- References: <1721935.92zzXaUnD9@e103209-lin> <CABu31nPdrYC6Rm=wEFGq7wOacGUkFp_3HhY6V+PuDWkB+1XTug@mail.gmail.com>
On 05/09/12 13:02, Steven Bosscher wrote:
> On Wed, Sep 5, 2012 at 1:42 PM, Matthew Gretton-Dann wrote:
>> Whilst this fix works for this particular case I am not sure it is the
>> best fix for the general issue, and so if others have a better idea how
>> to fix this I would be very happy.
>
> postreload-gcse.c is broken in "interesting" ways. Look at this gem for example:
>
> static bool
> reg_changed_after_insn_p (rtx x, int cuid)
> {
> unsigned int regno, end_regno;
>
> regno = REGNO (x);
> end_regno = END_HARD_REGNO (x);
> do
> if (reg_avail_info[regno] > cuid)
> return true;
> while (++regno < end_regno);
> return false;
> }
>
> So the more conservative the fix, the better :-)
>
> The patch looks correct to me. But perhaps the pass should just punt
> on blocks not ending in a simple jump in
> bb_has_well_behaved_predecessors?
>
> Ciao!
> Steven
>
That sort of makes sense. Why would we ever want to hoist an insn out
of a cold block into a hot one? I could see it making sense to do the
reverse on occasion, but clearly care is needed here.
R.