Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3, Manual

Page: 934 / 1898

Volume 3: Instruction Reference 3:35
chk
chk — Speculation Check
Format: (
qp
) chk.s
r
2
,
target
25
pseudo-op
(
qp
) chk.s.i
r
2
,
target
25
control_form, i_unit_form, gr_form I20
(
qp
) chk.s.m
r
2
,
target
25
control_form, m_unit_form, gr_form M20
(
qp
) chk.s
f
2
,
target
25
control_form, fr_form M21
(
qp
) chk.a.
aclr r
1
,
target
25
data_form, gr_form M22
(
qp
) chk.a.
aclr f
1
,
target
25
data_form, fr_form M23
Description:
The result of a control- or data-speculative calculation is checked for success or failure.
If the check fails, a branch to
target
25
is taken.
In the control_form, success is determined by a NaT indication for the source register.
If the NaT bit corresponding to GR
r
2
is 1 (in the gr_form), or FR f
2
contains a NaTVal (in
the fr_form), the check fails.
In the data_form, success is determined by the ALAT. The ALAT is queried using the
general register specifier
r
1
(in the gr_form), or the floating-point register specifier f
1
(in the fr_form). If no ALAT entry matches, the check fails. An implementation may
optionally cause the check to fail independent of whether an ALAT entry matches. A
chk.a
with general register specifier r0 or floating-point register specifiers f0 or f1
always fails.
The target
25
operand, in assembly, specifies a label to branch to. This is encoded in the
instruction as a signed immediate displacement (
imm
21
) between the target bundle and
the bundle containing this instruction (
imm
21
=
target
25
- IP >> 4).
The branching behavior of this instruction can be optionally unimplemented. If the
instruction would have branched, and the branching behavior is not implemented, then
a Speculative Operation fault is taken and the value specified by
imm
21
is zero-extended
and placed in the Interruption Immediate control register (IIM). The fault handler
emulates the branch by sign-extending the IIM value, adding it to IIP and returning.
The control_form of this instruction for checking general registers can be encoded on
either an I-unit or an M-unit. The pseudo-op can be used if the unit type to execute on
is unimportant.
For the data_form, if an ALAT entry matches, the matching ALAT entry can be optionally
invalidated, based on the value of the
aclr
completer (See Table 2-14 ).
Note that if the
clr
value of the
aclr
completer is used and the check succeeds, the
matching ALAT entry is invalidated. However, if the check fails (which may happen even
if there is a matching ALAT entry), any matching ALAT entry may optionally be
invalidated, but this is not required. Recovery code for data speculation, therefore,
cannot rely on the absence of a matching ALAT entry.
Table 2-14. ALAT Clear Completer
aclr Completer Effect on ALAT
clr Invalidate matching ALAT entry
nc Don’t invalidate