Skips implementing it for the x86 JIT because that's a bit of a
nightmare to think about.
The ARM64 implementation requires sequential registers which means if
the incoming sources aren't sequential then we need to move the sources
in to the two vector temporaries. This is fine since we have zero-cycle
vector renames and the alternative is slower.
Arm64 store with writeback when source register is the same register as
the address is undefined behaviour.
Depending on hardware details this can do a whole bunch of things.
This situation happens when the x86 code does `push rsp` which is quite
common for applications to do. We would then convert this to a `str x8, [x8, #-8]!`
Which results in undefined behaviour.
Now that redundant loads are optimized this showed up as an issue. Adds
a unit test to ensure we don't hit this again.
When the destination overlaps one of the sources we must be careful to
follow a movprfx rule.
```
The destination register must not refer to architectural register state
referenced by any other source operand register of this instruction.
```
We ended up in a situation in the vpmulh{u,}w AVX tests where zm was
overlapping the destination which violated that rule. This also
generated invalid code for this instruction.
```
[INFO] movprfx z6, z4
[INFO] umulh z6.h, p6/m, z6.h, z6.h
```
As seen, we were overwriting one of the sources because the destination
overlapped it. Now instead check if each individual overlap so invalid
code isn't generated.
InstCountCI results aren't affected since this only happens in
situations with multiple instructions.
It turns out that pure SSA isn't a great choice for the sort of emulation we do.
On one hand, it discards information from the guest binary's register allocation
that would let us skip stuff. On the other hand, it doesn't have nearly as many
benefits in this setting as in a traditional compiler... We really *don't* want
to do global RA or really any global optimization. We assume the guest optimizer
did its job for x86, we just need to clean up the mess left from going x86 ->
arm. So we just need enough SSA to peephole optimize.
My concrete IR proposals are that:
* SSA values must be killed in the same block that they are defined.
* Explicit LoadGPR/StoreGPR instructions can be used for global persistence.
* LoadGPR/StoreGPR are eliminated in favour of SSA within a block.
This has a lot of nice properties for our setting:
* Except for some internal REP instruction emulation (etc), we already have
registers for everything that escapes block boundaries, so this form is very
easy to go into -- straightforward local value numbering, not a full into
SSA pass.
* Spilling is entirely local (if it happens at all), since everything is in
registers at block boundaries. This is excellent, because Belady's algorithm
lets us spill nearly optimally in linear-time for individual blocks. (And
the global version of Belady's algorithm is massively more complicated...)
A nice fit for a JIT.
Relatedly, it turns out allowing spilling is probably a decent decision,
since the same spiller code can be used to rematerialize constants in a
straightforward way. This is an issue with the current RA.
* Register assignment is entirely local. For the same reason, we can assign
registers "optimally" in linear time & memory (e.g. with linear scan). And
the impl is massively simpler than a full blown SSA-based tree scan RA. For
example, we don't have to worry about parallel copies or coalescing phis or
anything. Massively nicer algorithm to deal with.
* SSA value names can be block local which makes the validation implicit :~)
It also has remarkably few drawbacks, because we didn't want to do CFG global
optimization anyway given our time budget and the diminishng returns. The few
global optimizations we might want (flag escape analysis?) don't necessarily
benefit from pure SSA anyway.
Anyway, we explicitly don't want phi nodes in any of this. They're currently
unused. Let's just remove them so nobody gets the bright idea of changing that.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
This class is very expensive to initialize so if you happen to have the
disassembler configuration enabled you were eating a very bad
initialization cost for no reason.
Only initialize the data member if any disassembler runtime option is
enabled, this completely removes the overhead.
AddNZCV is a new op to return the NZCV for an addition directly, which lets us
skip software flag calculation in some cases. In the future it would be nice to
fuse this into the Add itself as a second destination to avoid repeating the
addition, but that's a very involved change and right now I'm building FEX on an
old Chromebook because my M1 kernel is FUBAR.
Similarly, SubNZCV returns flags for Sub. This has the extra twist of needing to
invert the carry bit due to the inverted definition between arm64 and x86_64.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
A bunch of the AES operations take a zero register upfront and we
currently materialize it for each instruction.
Considering that most AES operations are used back to back, we can
eliminate these materializations by caching it between instructions.
Additionally removes a move in the optimal case when destination matches
the state register, which is exactly what the SSE operation ends up
doing.
AESKeyGenAssist has an edge case that if the destination RA overlaps the
zero register then we still need to eat a move, hopefully doesn't happen
too frequently in practice. This is also the lesser used instruction so
it isn't a big deal. RA constraints could solve that still.
We can load the swizzle table from our constant pool now. This removes
the only usage of VTMP3 from our Arm64 JIT.
I would say the this is now optimal for the version without RCON set.
With RCON we could technically make some of the move of the constant
more optimal.
This now improves the instruction implementation from 17 instructions
down to 5 or 6 depending on if the host supports SVE.
I would say this is now optimal.
The range check and clamping is necessary in the cases of passing x86
shift amounts directly through VUSHL/VSSHR.
Some AVX operations are still using these with range clamping. A future
investigation task should be the check if they can be switched over to
the wide variants that we implemented for the SSE instructions.
When consuming our own controlled data, we don't want the range clamping
to be enabled.
The number of times the implicit size calculation in GPR operations has
bit us is immeasurable and was a mistake from the start of the project.
The vector based operations never had this problem since they were
explicitly sized for a long time now.
This converts the base IR operations to be explicitly sized, but adds
implicit sized helpers for the moment while we work on removing implicit
usage from the OpcodeDispatcher.
Should be NFC at this moment but it is a big enough change that I want
it in before the "real" work starts.
This takes the two independent VSXT{U}N{2,} operations and merges them
in to a single IR operations.
In some cases this can result in a more optimal implementation since
there is no need for moves inbetween.
Ryan Houdek