Is it possible to enhance the vector STG registers(Xmm, Ymm, Zmm) with more information?

[Abhiroop Sarkar]

Hello Simon,

Thanks for your response. I had written a patch[1] for this and the
approach I took was quite similar to what you pointed out.

data GlobalReg = ...
                          |  XmmReg
                              (Maybe (Length, Width))
                              (Maybe GlobalVecRegTy)

data GlobalVecRegTy = Integer | Float

-- Width and Length are already defined
data Width = W8 | W16 | W32 .....

type Length = Int


I wrapped the types inside a `Maybe` because when initializing a GlobalReg
(in the `activeStgRegs`[2] function), I was not sure what value to
initialize the register with, so I used a `Nothing` when initializing.

I see now in the case of `VanillaReg` it is initialized with the `VGcPtr`
constructor: VanillaReg 1 VGcPtr etc

I think I should modify my patch as well to remove the Maybe and initialize
with some default Length, Width and GlobalRegTy.  Thanks for the help.

Abhiroop


[1] https://phabricator.haskell.org/D4922
[2]
https://github.com/ghc/ghc/blob/master/includes/CodeGen.Platform.hs#L450-L623
<https://github.com/ghc/ghc/blob/master/includes/CodeGen.Platform.hs#L450>

On Fri, Jul 13, 2018 at 10:58 AM Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> Abhiroop
>
>
>
> Did anyone reply?
>
>
>
> My instinct is this. You want to use the same register (say Xmm reg 3) in
> different ways.  We already have this for ‘VanillaReg’:
>
> data GlobalReg
>
>   = VanillaReg      -- pointers, unboxed ints and chars
>
>         Int         -- its number
>
>         VGcPtr
>
>    | …
>
>
>
> data VGcPtr = VGcPtr | VNonGcPtr
>
>
>
> We use VanillaReg for both pointers and non-pointers, so (VanillaReg 3
> VGcPtr) is register 3 used as a pointer, and (VanillaReg 3 VNonGcPtr) is
> register 3 used as a non-pointer.  And notice that globalRegType looks at
> this field to decide what type to return.
>
>
>
> I think you can do exactly the same: add a field to Xmm that explains how
> you are gong to divide it up.  Would that work?
>
>
>
> Simon
>
>
>
> *From:* ghc-devs <ghc-devs-bounces at haskell.org> *On Behalf Of *Abhiroop
> Sarkar
> *Sent:* 27 June 2018 22:32
> *To:* ghc-devs at haskell.org
> *Subject:* Is it possible to enhance the vector STG registers(Xmm, Ymm,
> Zmm) with more information?
>
>
>
> Hello all,
>
>
>
> I am currently working on adding support for SIMD operations to the native
> code generator. One of the roadblocks I faced recently was the definition
> of the `globalRegType` function in "compiler/cmm/CmmExpr.hs". The
> `globalRegType` function maps the STG registers to the respective `CmmType`
> datatype.
>
>
>
> For Xmm, Ymm, Zmm registers the function defines globalRegType like this:
> https://github.com/ghc/ghc/blob/master/compiler/cmm/CmmExpr.hs#L585-L587
> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Fblob%2Fmaster%2Fcompiler%2Fcmm%2FCmmExpr.hs%23L585-L587&data=02%7C01%7Csimonpj%40microsoft.com%7C982283cef5194f34c1e508d5dc758c48%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C636657319780067709&sdata=3SB86SoRXM2q%2BPE0tPcbuTsTZ6w7suxPX95qM%2BNLPA8%3D&reserved=0>
>
>
>
> Consider the case for an Xmm register, the above definition limits an Xmm
> register to hold only vectors of size 4. However we can store 2 64-bit
> Doubles or 16 Int8s or 8 Int16s and so on
>
>
>
> The function `globalRegType` is internally called by the function
> `cmmRegType` (
> https://github.com/ghc/ghc/blob/838b69032566ce6ab3918d70e8d5e098d0bcee02/compiler/cmm/CmmExpr.hs#L275
> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc%2Fghc%2Fblob%2F838b69032566ce6ab3918d70e8d5e098d0bcee02%2Fcompiler%2Fcmm%2FCmmExpr.hs%23L275&data=02%7C01%7Csimonpj%40microsoft.com%7C982283cef5194f34c1e508d5dc758c48%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C636657319780077717&sdata=V7McH5OXYD%2Bbfl2jjUHmquDxmr1BlZF8q1L%2Bq%2Bscq%2BY%3D&reserved=0>)
> which is itself used in a number of places in the x86 code generator.
>
>
>
> In fact depending on the result of the `cmmRegType` function is another
> important function `cmmTypeFormat` defined in Format.hs whose result is
> used to print the actual assembly instruction.
>
>
>
> I have extended all the other Format types to include VectorFormats,
> however this definition of the `globalRegType` seems incorrect to me.
> Looking at the signature of the function itself:
>
>
>
> `globalRegType :: DynFlags -> GlobalReg -> CmmType`
>
> its actually difficult to predict the CmmType by just looking at the
> GlobalReg in case of Xmm, Ymm, Zmm. So thats why my original question how
> do I go about solving this. Should I modify the GlobalReg type to contain
> more information like Width and Length(for Xmm, Ymm, Zmm)  or do I somehow
> pass the length and width information to the globalRegType function?
>
>
>
> Thanks
>
> Abhiroop Sakar
>


-- 
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