[llvm-dev] Question about physical registers in ISel

Thomas Lively via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 19 10:29:33 PST 2019

To get into more detail, I'm trying to update WebAssembly's `call`
instruction. `call` is currently constrained to return one or zero
arguments, so in TableGen we have a separate call Instruction for each
possible return type. But I need to update calls to return arbitrarily many
values of any combination of WebAssembly types, so even if we imposed some
reasonable artificial limit like 8 return values, this approach would
require generating billions of Instructions in TableGen. So that won't work.

The other approach is to have just one call instruction that has variadic
outputs as well as inputs. This is the approach I am pursuing now, and I
have it working through the lowering to MachineSDNodes, but the approach is
breaking down in the InstrEmitter because it is trying to use physical

WebAssembly is a stack machine, so all instructions including calls push
their results onto the value stack as opposed to placing them in registers.
The value stack is not in memory and not addressable, so it is not the same
as the normal stack of call frames, which we also have. Instead, the value
stack takes the place of registers. The way we model the value stack is by
using virtual registers exclusively in the MachineInst layer, then
rearranging (i.e. "stackifying") the instructions and removing all
registers entirely right before lowering to MC. We skip register allocation
entirely, since WebAssembly does not have registers to allocate. We use
virtual registers instead of physical registers because the value stack can
contain an unbounded number of values.

In order to fit this modeling, I need a new virtual register to be
allocated for each def of the generalized call instruction. The
stackification passes in the WebAssembly backend will correctly interpret
this as the call's return values being pushed onto the value stack.

The other option I am considering would be to lower calls to contain a
chain of glued pseudo nodes that each explicitly defs a single register,
the collapsing that chain of nodes back into a single MachineInstruction
using a custom inserter hook. That seems similar to your suggestion of
using a pseudo, but I do still need to get just a single call
MachineInstruction after instruction selection. Using an ephemeral chain of
pseudo nodes seems like a hack, since it is working around a modeling
mismatch between the InstrEmitter and the WebAssembly backend.

To avoid that workaround, I would prefer to add a bit named something like
"implicitDefsAreVirtual" that I could just set for all WebAssembly
instructions. WebAssembly is the only upstream stack machine target, but
other downstream stack machine targets such as TON Labs' TVM have adopted
our approach of using virtual registers and would also benefit from having
such a bit.

On Tue, Nov 19, 2019 at 9:04 AM Quentin Colombet <qcolombet at apple.com>

> Hi Thomas,
> If you know a specific instruction pushes a value on the stack and want to
> represent that as a virtual register (as opposed to a frame index for
> instance), you could use an explicit def with a pseudo that you later lower
> to your actual instruction with the stack access.
> I am not sold on the bit approach for now. It seems to me that we are
> working around a proper modeling. If you want to pursue with that approach,
> I would need more information on the semantic of that bit, how it is going
> to be used, what problem it solves and so on, to give a better opinion.
> Cheers,
> Q
> Le 18 nov. 2019 à 19:35, Thomas Lively <tlively at google.com> a écrit :
> Hi Quentin,
> Thanks, that explanation makes sense. I can see that in a normal register
> machine, implicitly defs must be physical registers. In a stack machine
> like WebAssembly, though, implicit defs are known to be pushed onto the
> value stack just like any other defs. Slots on the value stack are
> represented by virtual registers until stackification, so for WebAssembly
> we do need the implicit defs to be stored in virtual registers. I guess the
> best thing to do for now would be to add a bit to the MCInstrDesc carrying
> this information.
> Thomas
> On Mon, Nov 18, 2019 at 6:53 PM Quentin Colombet <qcolombet at apple.com>
> wrote:
>> Hi Thomas,
>> The reason is simply because an implicit def of a virtual register
>> doesn’t make sense in theory.
>> The rationale is let say that an instruction implicitly defines a virtual
>> register. The implicit aspect of it means that this information cannot be
>> “seen”. Thus, when we finally emit the instruction, there is nothing in the
>> encoding or anywhere that is going to tell where the value has been
>> physically defined.
>> The only way something is implicitly defined is when everybody knows
>> where the value is going to end up and that’s only achievable in a physical
>> register.
>> Cheers,
>> -Quentin
>> On Nov 18, 2019, at 6:21 PM, Thomas Lively via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> Hi all,
>> I need to figure out why InstrEmitter::EmitMachineNode assumes that when
>> the number of outputs of a MachineSDNode is greater than the number of defs
>> in the corresponding MCInstrDesc, the outputs in the difference will be
>> placed into physical registers as opposed to virtual registers.
>> The specific line in question is:
>>     bool HasPhysRegOuts = NumResults > NumDefs &&
>> II.getImplicitDefs()!=nullptr;
>> Where NumResults is the number of outputs in the MachineSDNode and
>> NumDefs comes from the MCInstrDesc and ultimately the TableGen definition
>> of the instruction. I do not know why this assumption is made or what code
>> depends on it, but it is over 12 years old:
>> https://github.com/llvm/llvm-project/commit/c5549fc3a055710a3958a5b3164db3fa167c720c#diff-9dfdb934cb2b01ba001da4bbf3c5cb3cR632
>> .
>> The context for this question is that I am trying to implement an
>> instruction for the WebAssembly backend that returns a variable number of
>> operands. I am following the example of ARM's load multiple instructions,
>> but this assertion in the instruction emitter is causing problems because
>> WebAssembly, unlike ARM, does not use physical registers at all. Also,
>> almost all WebAssembly instructions have an implicit def of the register
>> that represents the argument stack, so II.getImplicitDefs() is necessarily
>> non-null.
>> I am open to ideas about the best way forward. I am currently thinking
>> that if this assumption is important for most targets, I might need to add
>> a new bit to MCInstrDesc to disable this assumption.
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191119/56a0bce7/attachment.html>

More information about the llvm-dev mailing list