• Babu Moger's avatar
    arch/sparc: Avoid DCTI Couples · 0ae2d26f
    Babu Moger authored
    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: default avatarBabu Moger <babu.moger@oracle.com>
    Reviewed-by: default avatarRob Gardner <rob.gardner@oracle.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    0ae2d26f
head_64.S 22.4 KB