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 right 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.
I'll attach a patch which appears to fix this problem. If it looks OK, please
check it in.
Note: the new policy won't be optimal for Itanium (Merced) chips. I don't think
anyone will care.
--
Summary: gas should avoid F-unit NOPs (and B-unit probably, too)
Product: binutils
Version: 2.17 (HEAD)
Status: NEW
Severity: normal
Priority: P2
Component: gas
AssignedTo: unassigned at sources dot redhat dot com
ReportedBy: davidm at hpl dot hp dot com
CC: bug-binutils at gnu dot org,hjl at lucon dot org
GCC build triplet: ia64-linux
GCC host triplet: ia64-linux
GCC target triplet: ia64-linux
http://sources.redhat.com/bugzilla/show_bug.cgi?id=803
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