GHC's internal confusion about Ints and Words
moritz.angermann at gmail.com
Tue Oct 20 09:48:22 UTC 2020
Alright, let me expand a bit. I've been looking at aarch64 NCG for ghc.
The Linux side of things is looking really good,
so I've moved onto the macOS side (I'm afraid I don't have any Windows
aarch64 hardware, nor much windows knowledge
to even attempt a Windows version yet).
When calling C functions, the usual approach is to pass the first few
arguments in registers, and then arguments that exceed
the argument passing slots on the stack. The Arm AArch64 Procedure Call
Standard (aapcs) for C does this by assigning 8byte
slots to each overflow argument on the stack. A company I won't name, has
decided to implement a slightly different variation of
the Procedure Call Standard, often referred to as darwinpcs. This deviates
from the aapcs for vargs, as well as for handling of
spilled arguments on the stack.
The aapcs allows us to generate calls to C functions without knowing the
actual prototype of the function, as all arguments are
simply spilled into 8byte slots on the stack. The darwinpcs however
requires us to know the size of the arguments, so we can
properly pack them onto the stack. Ints have 4 bytes, so we need to pack
them into 4byte slots.
In the process library we have this rather fun foreign import:
foreign import ccall unsafe "runInteractiveProcess"
:: Ptr CString
-> Ptr CString
-> Ptr FD
-> Ptr FD
-> Ptr FD
-> Ptr CGid
-> Ptr CUid
-> CInt -- reset child's SIGINT & SIGQUIT
-> CInt -- flags
-> Ptr CString
-> IO PHANDLE
with the corresponding C declaration:
extern ProcHandle runInteractiveProcess( char *const args,
This function thus takes 14 arguments. We pass only the first 8 arguments
in registers, and the others on the stack.
Argument 12 and 13 are of type int. On linux using the aapcs, we can pass
those in 8byte slots on the stack. That is
both of them are effectively 64bits wide when passed. However for
darwinpcs, it is expected that these adhere to their
size and are packed as such. Therefore Argument 12 and 13 need to be passed
as 4byte slots each on the stack.
This yields a moderate 8byte saving on the stack for the same function call
on darwinpcs compared to aapcs.
Now onto GHC. When we generate function calls for foreign C functions, we
deal with something like:
:: ForeignTarget -- function to call
-> [CmmFormal] -- where to put the result
-> [CmmActual] -- arguments (of mixed type)
-> BlockId -- The block we are in
-> NatM (InstrBlock, Maybe BlockId)
based on Cmm Nodes of the form CmmUnsafeForeignCall target result_regs args
The CmmActual in the care of runInteractiveProcess hold the arguments for
the function, however contrary to the function
declaration, it contains I64 slots for Argument 12 and 13. Thus computing
the space needed for them based on their Cmm
Representations yields 8bytes, when they should really be 32bit and consume
only 4 byte.
To illustrate this a bit better: here is what we see in the pretty printed
(_s6w3::I64) = call "ccall" arg hints: [PtrHint, PtrHint, PtrHint, signed,
signed, signed, PtrHint, PtrHint, PtrHint, PtrHint, PtrHint, signed,
signed, PtrHint] result hints: [signed] _runInteractiveProcess(I64[Sp +
96], I64[Sp + 88], I64[Sp + 104], I64[Sp + 112], I64[Sp + 120], I64[Sp +
56], I64[Sp + 64], I64[Sp + 72], I64[Sp + 24], 0, 0, I64[Sp + 8], I64[Sp +
40] | I64[Sp + 48] | I64[Sp + 80] | 3, I64[R1 + 7]);
I've added size information to the ForeignHints (NoHint, AddrHint,
SignedHint) we have, and computed both, which yields:
[(CmmReg (CmmLocal (LocalReg s6Gi (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6Gk (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6Gm (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6Go (CmmType BitsCat W64))),SignedHint W32)
,(CmmReg (CmmLocal (LocalReg s6Gq (CmmType BitsCat W64))),SignedHint W32)
,(CmmReg (CmmLocal (LocalReg s6Gs (CmmType BitsCat W64))),SignedHint W32)
,(CmmReg (CmmLocal (LocalReg s6Gu (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6Gw (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6Gy (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6Cp (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6FU (CmmType BitsCat W64))),AddrHint)
,(CmmReg (CmmLocal (LocalReg s6GA (CmmType BitsCat W64))),SignedHint W32)
,(CmmReg (CmmLocal (LocalReg s6GR (CmmType BitsCat W64))),SignedHint W32)
,(CmmReg (CmmLocal (LocalReg s6GM (CmmType BitsCat W64))),AddrHint)]
Thus, while we *do* know the right size from STG (which is what the Hints
are computed from), we loose this information when lowering
into Cmm, where we represent them with W64. This is what I was alluding to
in the previous email. In primRepCmmType, and mkIntCLit, we set their type
to 64bit for Ints; which on this platform does not hold.
Now I've gone ahead and effectively assume Cmm is lying to me when
generating Foreign Function Calls, and rely on the (new) sized
hints to produce the appropriate argument packing on the stack. However I
believe the correct way would be for GHC not to conflate Ints
and Words in Cmm; implicitly assuming they are the same width. Sadly it's
not as simple as having primRepCmmType and mkIntCLit produce 32bit types. I
fear GHC internally assumes "Int" means 64bit Integer, and then just
happens to make the Int ~ CInt assumption.
On Tue, Oct 20, 2020 at 3:33 PM Simon Peyton Jones <simonpj at microsoft.com>
> I’m afraid I don’t understand any of this. Not your fault, but I just
> don’t have enough context to know what you mean.
> Is there a current bug? If so, can you demonstrate it? If not, what is
> the problem you want to solve? Examples are always helpful.
> Maybe it’s worth opening a ticket too?
> *From:* ghc-devs <ghc-devs-bounces at haskell.org> *On Behalf Of *Moritz
> *Sent:* 20 October 2020 02:51
> *To:* ghc-devs <ghc-devs at haskell.org>
> *Subject:* GHC's internal confusion about Ints and Words
> Hi there!
> So there is a procedure calling convention that for reasons I did not
> fully understand, but seem to be historically grown, uses packed arguments
> for those that are spilled onto the stack. On top of that, CInt is 32bit,
> Word is 64bits. This provides the following spectacle:
> While we know in STG that the CInt is 32bits wide, when lowered into Cmm,
> it's represented as I64 in the arguments to the C function. Thus packing
> based on the format of the Cmm type would yield 8 bytes. And now, all
> further packed arguments have the wrong offset (by four).
> Specifically in GHC.Cmm.Utils we find:
> primRepCmmType :: Platform -> PrimRep -> CmmType
> primRepCmmType platform IntRep = bWord platform
> mkIntCLit :: Platform -> Int -> CmmLit
> mkIntCLit platform i = CmmInt (toInteger i) (wordWidth platform)
> The naive idea to just fix this and make them return cIntWidth instead,
> seemingly produces the correct Cmm expressions at a local level, but
> produces a broken compiler.
> A second approach could be to extend the Hints into providing sizes, and
> using those during the foreign call generation to pack spilled arguments.
> This however appears to be more of a patching up of some fundamental
> underlying issue, instead of rectifying it properly.
> Maybe I'll have to go down the Hint path, it does however break current Eq
> assumptions, as they are sized now, and what was equal before, is only
> equal now if they represent the same size.
> From a cursory glance at the issues with naively fixing the width for Int,
> it seems that GHC internally assumes sizeof(Int) = sizeof(Word). Maybe
> there is a whole level of HsInt vs CInt discrimination missing?
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the ghc-devs