==================
                 System V ABI AMD64
                 ==================


This document describes concisely the subset of the amd64
ABI as it is implemented in QBE.  The subset can handle
correctly arbitrary standard C-like structs containing
float and integer types.  Structs that have unaligned
members are also supported through opaque types, see
the IL description document for more information about
them.


- ABI Subset Implemented
------------------------

Data classes of interest as defined by the ABI:
 * INTEGER
 * SSE
 * MEMORY


~ Classification

 1. The size of each argument gets rounded up to eightbytes.
    (It keeps the stack always 8 bytes aligned.)
 2. _Bool, char, short, int, long, long long and pointers
    are in the INTEGER class.  In the context of QBE, it
    means that 'l' and 'w' are in the INTEGER class.
 3. float and double are in the SSE class.  In the context
    of QBE, it means that 's' and 'd' are in the SSE class.
 4. If the size of an object is larger than two eightbytes
    or if contains unaligned fields, it has class MEMORY.
    In the context of QBE, those are big aggregate types
    and opaque types.
 5. Otherwise, recursively classify fields and determine
    the class of the two eightbytes using the classes of
    their components.  If any is INTEGER the result is
    INTEGER, otherwise the result is SSE.

~ Passing

  * Classify arguments in order.
  * INTEGER arguments use in order `%rdi` `%rsi` `%rdx`
    `%rcx` `%r8` `%r9`.
  * SSE arguments use in order `%xmm0` - `%xmm7`.
  * MEMORY gets passed on the stack.  They are "pushed"
    in the right-to-left order, so from the callee's
    point of view, the left-most argument appears first
    on the stack.
  * When we run out of registers for an aggregate, revert
    the assignment for the first eightbytes and pass it
    on the stack.
  * When all registers are taken, write arguments on the
    stack from right to left.
  * When calling a variadic function, %al stores the number
    of vector registers used to pass arguments (it must be
    an upper bound and does not have to be exact).
  * Registers `%rbx`, `%r12` - `%r15` are callee-save.

~ Returning

  * Classify the return type.
  * Use `%rax` and `%rdx` in order for INTEGER return
    values.
  * Use `%xmm0` and `%xmm1` in order for SSE return values.
  * If the return value's class is MEMORY, the first
    argument of the function `%rdi` was a pointer to an
    area big enough to fit the return value.  The function
    writes the return value there and returns the address
    (that was in `%rdi`) in `%rax`.


- Alignment on the Stack
------------------------

The ABI is unclear on the alignment requirement of the
stack.  What must be ensured is that, right before
executing a 'call' instruction, the stack pointer `%rsp`
is aligned on 16 bytes.  On entry of the called
function, the stack pointer is 8 modulo 16.  Since most
functions will have a prelude pushing `%rbp`, the frame
pointer, upon entry of the body code of the function is
also aligned on 16 bytes (== 0 mod 16).

Here is a diagram of the stack layout after a call from
g() to f().

                   |             |
                   | g() locals  |
                   +-------------+
              ^    |             | \
              |    | stack arg 2 |  '
              |    |xxxxxxxxxxxxx|  | f()'s MEMORY
     growing  |    +-------------+  |  arguments
    addresses |    | stack arg 1 |  ,
              |    |xxxxxxxxxxxxx| /
              |    +-------------+ -> 0 mod 16
              |    |   ret addr  |
                   +-------------+
                   |  saved %rbp |
                   +-------------+ -> f()'s %rbp
                   | f() locals  |    0 mod 16
                   |   ...       |
                                   -> %rsp

Legend:
 * `xxxxx`    Optional padding.


- Remarks
---------

  * A struct can be returned in registers in one of three
    ways.  Either `%rax`, `%rdx` are used, or `%xmm0`,
    `%xmm1`, or finally `%rax`, `%xmm0`.  The last case
    happens when a struct is returned with one half
    classified as INTEGER and the other as SSE. This
    is a consequence of the <@Returning> section above.

  * The size of the arguments area of the stack needs to
    be computed first, then arguments are packed starting
    from the bottom of the argument area, respecting
    alignment constraints.  The ABI mentions "pushing"
    arguments in right-to-left order, but I think it's a
    mistaken view because of the alignment constraints.

    Example: If three 8 bytes MEMORY arguments are passed
    to the callee and the caller's stack pointer is 16 bytes
    algined, the layout will be like this.

                   +-------------+
                   |xxxxxxxxxxxxx|    padding
                   | stack arg 3 |
                   | stack arg 2 |
                   | stack arg 1 |
                   +-------------+ -> 0 mod 16

    The padding must not be at the end of the stack area.
    A "pushing" logic would put it at the end.