StgRun and reserved C stack space

[Simon Peyton Jones]

Perhaps Simon Marlow, or others, may be able to help?

It would be good to capture the answers somewhere, perhaps in the GHC Commentary?
https://gitlab.haskell.org/ghc/ghc/wikis/commentary

Simon

| -----Original Message-----
| From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Stefan Schulze
| Frielinghaus
| Sent: 22 June 2019 10:23
| To: ghc-devs at haskell.org
| Subject: StgRun and reserved C stack space
| 
| Hi all,
| 
| I'm trying to understand how StgRun/StgReturn are supposed to be
| implemented.
| My current understanding is that StgRun is called from C land in order to
| save all callee-saved registers, allocate some stack space, and then
| finally jump to an STG function.  StgReturn is the counterpart of StgRun
| and is called from STG land in order to restore all callee-saved
| registers, free the previously allocated stack space, and finally return
| to C land.
| 
| Assuming that my intuition is correct I still have some concerns regarding
| the stack space which is allocated by StgRun.  StgRun allocates
| RESERVED_C_STACK_BYTES many bytes on the C stack.  However, I cannot see
| how an STG function is supposed to access this reserved area since no
| pointer is derived or whatsoever.  Does an STG function make use of the C
| stack pointer and assume that the reserved area is at the very bottom of
| the stack?
| 
| Lets make an example.  Assume the C ABI demands that every function
| allocates a Register Save Area (RSA for short) at the bottom of a stack
| frame which may be used by a subsequent C function.  That means a caller
| allocates some stack space for its callee.  This area must be below the
| local variable and parameter area which means it is at the very bottom of
| a stack frame.  A C stack frame then looks as follows:
| 
|       .                     .
|       .                     .
|       | previous frame      |
|       +---------------------+
|       | local variables     |
|       +---------------------+
|       | parameter area      |
|       +---------------------+
|       | register save area  |
|       +---------------------+  <---+ SP
| 
| Thus StgRun must also allocate a register save area which is used by
| subsequent C functions.  An example stack while calling StgRun could look
| as follows:
| 
|       .                     .
|       .                     .
|       |                     |
|       +---------------------+
| RSA1  | register save area  |
|       | used by StgRun      |
|       +---------------------+ xxx
|       |                     |   xx
|       |                     |    xx
|       |                     |     xx
|       |                     |     xx
|       |                     |      xxxxxx RESERVED_C_STACK_BYTES
|       |                     |     xx
|       |                     |     xx
|       |                     |    xx
|       |                     |   xx
|       +---------------------+ xxx
| RSA2  | register save area  |
|       | for further C calls |
|       +---------------------+  <---+ SP
| 
| RSA1 is allocated by the caller of StgRun and is used by StgRun in order
| to save all callee-saved registers.  StgRun itself then allocates
| RESERVED_C_STACK_BYTES many bytes and RSA2.  RSA2 will be used by
| subsequent C functions which are called from STG land.
| 
| Thus coming back to my initial question how is an STG function supposed to
| know where the reserved area starts?  One way would be to use SP +
| len(RSA2) but this would mean an STG function is aware of the register in
| which the C stack pointer is hold and the length of a register save area.
| I didn't find anything which suggests this while skimming over the code.
| 
| Many thanks and kind regards,
| Stefan
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