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

Quentin Colombet via llvm-dev
2020-05-05T16:59:53Z

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.
>
> 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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