[Bug target/20632] New: GCC should avoid generating F- and B-unit NOPs

[davidm at hpl dot hp dot com]

Chips derived from McKinley-core (Itanium 2, etc.) have an anomaly which can
cause stalls if an F-unit instruction (including a NOP) is issued within a
six-cycle window after reading certain application registers (such as ar.bsp). 
Furthermore, power-considerations also argue against the use of F-unit
instructions unless they're really needed.

Similarly, using B-unit NOPs is probably not a great idea for McKinley-derived
cores: certain templates with B unit instructions cause split-issue and the BBB
template in particular causes a branch-prediction anomaly (the first two
branches are predicated based on the slot 0 hints.

Unfortunately, at the moment the GCC scheduler still seems to favor using F and
B-unit NOPs whenever possible.  This made sense for Merced (since it didn't have
sufficient M and I execution units) but really should be avoided for McKinley
and newer cores.

-- 
           Summary: GCC should avoid generating F- and B-unit NOPs
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: davidm at hpl dot hp dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: ia64-linux
  GCC host triplet: ia64-linux
GCC target triplet: ia64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20632