[llvm-dev] RFC: [GlobalISel] propagating int/float type information

Amara Emerson via llvm-dev
2020-05-05T18:14:50Z

> On May 5, 2020, at 9:59 AM, Quentin Colombet <qcolombet at apple.com>
wrote:
>
> I don’t think bfloat should be handled this way. What Amara is suggesting
is an optimization, i.e., if we drop the information we are still correct.
> With bfloat, if we do an operation on float16 instead of bfloat16 this is a
correctness problem.
>
> So that means that either we need to have new opcodes for bfloat or we need
to carry around the floating point type in MIR. I think it would be more
manageable to have the floating point type long term.
> That said, it also depends on what we decide to do at the IR level. For
instance, if bfloat support in the IR is limited to intrinsics, we wouldn’t need
to go down that road.
>
>> On May 5, 2020, at 4:19 AM, Ties Stuij via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>>
>> I don't know too much about GlobalIsel, but I'm working on
adding a new bfloat IR/MVT type (16-bit float type) to LLVM, and on one of the
patches Amara raised the issue what we would to to disambiguate between a half
and a bfloat for GlobalIsel.
I don’t think it was me who said that but it’s a good point.

I see no good reason not to re-use the existing IR instructions for computation
on BFloat, given that we do so for other uncommon FP formats as well. Which I
think in turn leaves us with little choice but to mirror the IR here (adding
separate opcodes for this case just seems wrong).
>>
>> Just wanted to highlight that BFloat might be another use-case for
this.
>>
>> Cheers,
>> /Ties
>>
>> ________________________________________
>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Amara Emerson via llvm-dev <llvm-dev at lists.llvm.org>
>> Sent: 01 May 2020 18:12
>> To: LLVM Developers' List
>> Cc: Matt Arsenault
>> Subject: [llvm-dev] RFC: [GlobalISel] propagating int/float type
information
>>
>> Hi,
>>
>> GlobalISel currently drops all type information relating to the
integer/FP distinction during the IR translation pass, as the LLT types only
represent whether a value is a scalar/vector/pointer and it’s size/shape. To
compensate, later passes use the FP operations on those values to guess what
kind of value is being stored within that virtual register.
>>
>> This means that i32/float loads get translated into the same thing, and
only when that value is used, say by an fadd, then will we know that it was an
FP value. The regbankselect pass on AArch64 currently tries to walk uses/defs in
order to guess what kind fo regbank to assign to vregs. This however doesn’t
work all the time, and most commonly, it doesn’t work when a load of an FP value
is used in a loop. In that case, the FP users are obscured by PHIs which make it
difficult (although not strictly impossible) to guess what regbank to assign.
This has drastic consequences for performance on FP workloads.
>>
>> But this isn’t the first time we’ve had this kind of issue, and it
probably won’t be the last [1].  propose that we have some form of type hint
propagation done at the IRTranslator stage in order to make this whole situation
easier (and faster in compile-time).
>>
>> Option 1) We use some form of metadata on the MIR instructions like
G_LOADs to signify that the vreg defined likely has an FP IR type. IIUC the
current Metadata MachineOperand type is only intended for debug info. This
approach is probably the cheapest in compile time/complexity and is the least
invasive, but we’d need to find somewhere in MachineInstr to store this extra
information.
>>
>> Option 2) Store the type hints in an analysis. In its simplest form at
translation time we could keep a set of all the vregs that we know have FP types
and then try to maintain that as new vregs are created to replace those
throughout the pipeline. Keeping it updated might turn out to be expensive
during passes like the legalizer.
>>
>> Any thoughts?
>>
>> Cheers,
>> Amara
>>
>> [1] Currently we have a workaround for the specific case in
https://reviews.llvm.org/D79207, but as Matt correctly points out, this isn’t
viable in the long term because using the IR value type from the
MachineMemOperand won’t work when opaque pointers finally land.
>>
>>
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