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From: Jeff Law <>
Subject: Re: [RFA] Avoid unnecessary load-immediate in coremark
Date: Sat, 1 Oct 2022 12:58:11 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 9/29/22 01:44, Richard Biener wrote:
> On Tue, Sep 27, 2022 at 9:54 PM Jeff Law <> wrote:
>> This is another minor improvement to coremark.   I suspect this only
>> improves code size as the load-immediate was likely issuing with the ret
>> statement on multi-issue machines.
>> Basically we're failing to utilize conditional equivalences during the
>> post-reload CSE pass.  So if a particular block is only reached when a
>> certain condition holds (say for example a4 == 0) and the block has an
>> assignment like a4 = 0, we would fail to eliminate the unnecessary
>> assignment.
> conditional equivalences on RTL - ick ;)

That was my first reaction as well.

> I'm not familiar with RTL pattern matching so somebody else has to
> comment on that, but
> +                     /* If this is not the first time through, then
> +                        verify the source and destination match.  */
> +                     else if (dest == XEXP (cond, 0) && src == XEXP (cond, 1))
> +                       ;
> shouldn't you restrict dest/src somehow?  It might be a MEM?
> The way you create the fake insn suggests only REG_P dest are OK
> (not SUBREGs for example?)?

You're absolutely right, as is Richard S WRT unexpected sharing. I'll 
adjust the patch appropriately.

> Should you use rtx_equal_p (not using that possibly exempts MEM,
> but being more explicit would be nice).  Should you restrict this to
> MODE_INT compares?

rtx_equal_p would be better, yes.  I'll adjust that too.

This should work regardless of hte mode type though.  The key is the 
post-reload cse bits have to check that the pattern matches and that the 
constraints are satisfied when a replacement is made.   My only concern 
would be MODE_CC stuff.  I'll think a bit more about that case.


  reply	other threads:[~2022-10-01 18:58 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-27 19:53 Jeff Law
2022-09-29  7:44 ` Richard Biener
2022-10-01 18:58   ` Jeff Law [this message]
2022-09-30 10:47 ` Richard Sandiford
2022-10-01 19:03   ` Jeff Law

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