linux-kernel - Re: [PATCH v2] arch/sparc: Avoid DCTI Couples

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Date:   Mon, 27 Mar 2017 21:52:05 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     babu.moger@...cle.com
Cc:     rob.gardner@...cle.com, khalid.aziz@...cle.com,
        allen.pais@...cle.com, viro@...iv.linux.org.uk,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arch/sparc: Avoid DCTI Couples

From: Babu Moger <babu.moger@...cle.com>
Date: Fri, 17 Mar 2017 14:52:21 -0600

> Avoid un-intended DCTI Couples. Use of DCTI couples is deprecated.
> Also address the "Programming Note" for optimal performance.
> 
> Here is the complete text from Oracle SPARC Architecture Specs.
> 
> 6.3.4.7 DCTI Couples
> "A delayed control transfer instruction (DCTI) in the delay slot of
> another DCTI is referred to as a “DCTI couple”. The use of DCTI couples
> is deprecated in the Oracle SPARC Architecture; no new software should
> place a DCTI in the delay slot of another DCTI, because on future Oracle
> SPARC Architecture implementations DCTI couples may execute either
> slowly or differently than the programmer assumes it will.
> 
> SPARC V8 and SPARC V9 Compatibility Note
> The SPARC V8 architecture left behavior undefined for a DCTI couple. The
> SPARC V9 architecture defined behavior in that case, but as of
> UltraSPARC Architecture 2005, use of DCTI couples was deprecated.
> Software should not expect high performance from DCTI couples, and
> performance of DCTI couples should be expected to decline further in
> future processors.
> 
> Programming Note
> As noted in TABLE 6-5 on page 115, an annulled branch-always
> (branch-always with a = 1) instruction is not architecturally a DCTI.
> However, since not all implementations make that distinction, for
> optimal performance, a DCTI should not be placed in the instruction word
> immediately following an annulled branch-always instruction (BA,A or
> BPA,A)."
> 
> Signed-off-by: Babu Moger <babu.moger@...cle.com>
> Reviewed-by: Rob Gardner <rob.gardner@...cle.com>

Applied, thanks.