Intel Itanium 9150M, Manual

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3:32 Volume 3: Instruction Reference
brp
brp — Branch Predict
Format: brp.
ipwh
.
ih target
25
,
tag
13
ip_relative_form B6
brp.
indwh
.
ih b
2
,
tag
13
indirect_form B7
brp.ret.
indwh
.
ih b
2
,
tag
13
return_form, indirect_form B7
Description:
This instruction can be used to provide to hardware early information about a future
branch. It has no effect on architectural machine state, and operates as a
nop
instruction except for its performance effects.
The
tag
13
operand, in assembly, specifies the address of the branch instruction to which
this prediction information applies. This is encoded in the branch predict instruction as a
signed immediate displacement (
timm
9
) between the bundle containing the presaged
branch and the bundle containing this instruction (
timm
9
=
tag
13
- IP >> 4).
The
target
25
operand, in assembly, specifies the label that the presaged branch will have
as its target. This is encoded in the branch predict instruction exactly as in branch
instructions, with a signed immediate displacement (
imm
21
) between the target bundle
and the bundle containing this instruction (
imm
21
=
target
25
- IP >> 4). The indirect_form
can be used to presage an indirect branch. In the indirect_form, the target of the
presaged branch is given by BR
b
2
.
The return_form is used to indicate that the presaged branch will be a return.
Other hints can be given about the presaged branch. Values for various hint completers
are shown in the following tables. For more details, refer to
Section 4.5.2, “Branch
Prediction Hints” on page 1:78 .
The
ipwh
and
indwh
completers provide information about how best the branch condition
should be predicted, when the branch is reached.
The
ih
completer can be used to mark a small number of very important branches (e.g.,
an inner loop branch). This can signal to hardware to use faster, smaller prediction
structures for this information.
Table 2-11. IP-relative Branch Predict Whether Hint
ipwh Completer IP-relative Branch Predict Whether Hint
sptk Presaged branch should be predicted Static Taken
loop Presaged branch will be
br.cloop
,
br.ctop
, or
br.wtop
exit Presaged branch will be
br.cexit
or
br.wexit
dptk Presaged branch should be predicted Dynamically
Table 2-12. Indirect Branch Predict Whether Hint
indwh Completer Indirect Branch Predict Whether Hint
sptk Presaged branch should be predicted Static Taken
dptk Presaged branch should be predicted Dynamically
Table 2-13. Importance Hint
ih Completer Branch Predict Importance Hint
none Less important
imp More important