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 <[email protected]> On Behalf Of Abhiroop Sarkar
Sent: 27 June 2018 22:32
To: [email protected]
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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