v8.1M's "low-overhead-loop" extension has three instructions
for looping:
* DLS (start of a do-loop)
* WLS (start of a while-loop)
* LE (end of a loop)
The loop-start instructions are both simple operations to start a
loop whose iteration count (if any) is in LR. The loop-end
instruction handles "decrement iteration count and jump back to loop
start"; it also caches the information about the branch back to the
start of the loop to improve performance of the branch on subsequent
iterations.
As with the branch-future instructions, the architecture permits an
implementation to discard the LO_BRANCH_INFO cache at any time, and
QEMU takes the IMPDEF option to never set it in the first place
(equivalent to discarding it immediately), because for us a "real"
implementation would be unnecessary complexity.
(This implementation only provides the simple looping constructs; the
vector extension MVE (Helium) adds some extra variants to handle
looping across vectors. We'll add those later when we implement
MVE.)
Backports commit b7226369721896ab9ef71544e4fe95b40710e05a
v8.1M implements a new 'branch future' feature, which is a
set of instructions that request the CPU to perform a branch
"in the future", when it reaches a particular execution address.
In hardware, the expected implementation is that the information
about the branch location and destination is cached and then
acted upon when execution reaches the specified address.
However the architecture permits an implementation to discard
this cached information at any point, and so guest code must
always include a normal branch insn at the branch point as
a fallback. In particular, an implementation is specifically
permitted to treat all BF insns as NOPs (which is equivalent
to discarding the cached information immediately).
For QEMU, implementing this caching of branch information
would be complicated and would not improve the speed of
execution at all, so we make the IMPDEF choice to implement
all BF insns as NOPs.
Backports commit 05903f036edba8e3ed940cc215b8e27fb49265b9
The t32 decode has a group which represents a set of insns
which overlap with B_cond_thumb because they have [25:23]=111
(which is an invalid condition code field for the branch insn).
This group is currently defined using the {} overlap-OK syntax,
but it is almost entirely non-overlapping patterns. Switch
it over to use a non-overlapping group.
For this to be valid syntactically, CPS must move into the same
overlapping-group as the hint insns (CPS vs hints was the
only actual use of the overlap facility for the group).
The non-overlapping subgroup for CLREX/DSB/DMB/ISB/SB is no longer
necessary and so we can remove it (promoting those insns to
be members of the parent group).
Backports 45f11876ae86128bdee27e0b089045de43cc88e4
Convert the T32 coprocessor instructions to decodetree.
As with the A32 conversion, this corrects an underdecoding
where we did not check that MRRC/MCRR [24:21] were 0b0010
and so treated some kinds of LDC/STC and MRRC/MCRR rather
than UNDEFing them.
Backports commit 4c498dcfd84281f20bd55072630027d1b3c115fd
The miscellaneous control instructions are mutually exclusive
within the t32 decode sub-group.
Backports commit d6084fba47bb9aef79775c1102d4b647eb58c365 from qemu
This includes a minor bug fix to LDM (user), which requires
bit 21 to be 0, which means no writeback.
Backports commit c5c426d4c680f908a1e262091a17b088b5709200 from qemu
In op_bfx, note that tcg_gen_{,s}extract_i32 already checks
for width == 32, so we don't need to special case that here.
Backports commit 86d21e4b509a2835ed79f234f476a4c5191d435b from qemu
Pass the T5 encoding of SUBS PC, LR, #IMM through the normal SUBS path
to make it clear exactly what's happening -- we hit ALUExceptionReturn
along that path.
Backports commit ef11bc3c461e2c650e8bef552146a4b08f81884e from qemu
Document our choice about the T32 CONSTRAINED UNPREDICTABLE behaviour.
This matches the undocumented choice made by the legacy decoder.
Backports commit 4c97f5b2f0fa9b37f9ff497f15411d809e6fd098 from qemu
The m-profile and a-profile decodings overlap. Only return false
for the case of wrong profile; handle UNDEFINED for permission failure
directly. This ensures that we don't accidentally pass an insn that
applies to the wrong profile.
Backports commit d0b26644502103ca97093ef67749812dc1df7eea from qemu
Convert the modified immediate form of the data processing insns.
For A32, we can finally remove any code that was intertwined with
the register and register-shifted-register forms.
Backports commit 581c6ebd17c8f56ad52772216e6c6d8cc8997e8b from qemu
Convert the register shifted by register form of the data
processing insns. For A32, we cannot yet remove any code
because the legacy decoder intertwines the immediate form.
Backports commit 5be2c12337f4cbdbda4efe6ab485350f730faaad from qemu
Convert the register shifted by immediate form of the data
processing insns. For A32, we cannot yet remove any code
because the legacy decoder intertwines the reg-shifted-reg
and immediate forms.
Backports commit 25ae32c558182c07fc6ad01b936e9151cbf00c44 from qemu
Add the infrastructure that will become the new decoder.
No instructions adjusted so far.
Backports commit 51409b9e8cfe997b1ac3365df7400e0c6e844437 from qemu
