What is the GlobalVecRegTy field doing?
Don’t you need an Int for the register number, like all the rest?
Generally, sounds good though
S
From: Abhiroop Sarkar <asiamgenius@gmail.com>
Sent: 13 July 2018 14:07
To: Simon Peyton Jones <simonpj@microsoft.com>
Cc: ghc-devs@haskell.org
Subject: Re: Is it possible to enhance the vector STG registers(Xmm, Ymm, Zmm) with more information?
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
On Fri, Jul 13, 2018 at 10:58 AM Simon Peyton Jones <simonpj@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@haskell.org> On Behalf Of Abhiroop Sarkar
Sent: 27 June 2018 22:32
To: ghc-devs@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
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) 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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