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3:24
Volume 3: Instruction Reference
br
The loop-type branches (
br.cloop
,
br.ctop
,
br.cexit
,
br.wtop
, and
br.wexit
) are
only allowed in instruction slot 2 within a bundle. Executing such an instruction in either
slot 0 or 1 will cause an Illegal Operation fault, whether the branch would have been
taken or not.
Read after Write (RAW) and Write after Read (WAR) dependency requirements are
slightly different for branch instructions. Changes to BRs, PRs, and PFS by non-branch
instructions are visible to a subsequent branch instruction in the same instruction group
(i.e., a limited RAW is allowed for these resources). This allows for a low-latency
compare-branch sequence, for example. The normal RAW requirements apply to the LC
and EC application registers, and the RRBs.
Within an instruction group, a WAR dependency on PR 63 is not allowed if both the
reading and writing instructions are branches. For example, a
br.wtop
or
br.wexit
may not use PR[63] as its qualifying predicate and PR[63] cannot be the qualifying
predicate for any branch preceding a
br.wtop
or
br.wexit
in the same instruction
group.
For dependency purposes, the loop-type branches effectively always write their
associated resources, whether they are taken or not. The cloop type effectively always
writes LC. When LC is 0, a cloop branch leaves it unchanged, but hardware may
implement this as a re-write of LC with the same value. Similarly,
br.ctop
and
br.cexit
effectively always write LC, EC, the RRBs, and PR[63].
br.wtop
and
br.wexit
effectively always write EC, the RRBs, and PR[63].
Values for various branch hint completers are shown in the following tables. Whether
Prediction Strategy hints are shown in
. Sequential Prefetch hints are shown in
. Branch Cache Deallocation hints are shown in
. See
“Branch Prediction Hints” on page 1:78
Figure 2-4.
Operation of br.wtop and br.wexit
PR[qp]?
wtop, wexit
wtop: Branch
wexit: Fall-thru
wtop: Fall-thru
wexit: Branch
EC?
EC--
PR[63] = 0
RRB--
EC--
PR[63] = 0
RRB--
> 1
== 1
== 0
EC = EC
PR[63] = 0
RRB--
EC = EC
PR[63] = 0
RRB = RRB
(Prolog /
Epilog)
(Epilog)
==0 (Prolog / Epilog)
(Special
Unrolled
Loops)
== 1
Kernel)
(Prolog /
Содержание Itanium 9150M
Страница 1: ......
Страница 209: ...3 200 Volume 3 Instruction Reference padd Interruptions Illegal Operation fault...
Страница 301: ...3 292 Volume 3 Pseudo Code Functions Intel Itanium Architecture Software Developer s Manual Rev 2 3...
Страница 405: ...3 396 Volume 3 Resource and Dependency Semantics...
Страница 406: ...3 397 Intel Itanium Architecture Software Developer s Manual Rev 2 3 Index...
Страница 407: ...3 398 Intel Itanium Architecture Software Developer s Manual Rev 2 3...
Страница 419: ...INDEX Index 12 Index for Volumes 1 2 3 and 4...
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