Hi all,

I'm having trouble with calling struct returning functions with alien on a
64bit linux vm using the hidden first argument mechanism [1]. The function
gets called, but the result is not copied into the struct pointed by the
hidden argument.

I tried to debug what happens with gdb, but everything looks fine, except
not getting the result back. The address is copied into %rdi (regs[0]) in
dax64business.h:92 [2], which is exactly what the spec says [3].

Here is a minimal example calling a C function returning a struct with two
doubles:

at34 := Alien lookup: 'at34' inLibrary: 'libalientest.so'.
at34 primFFICallResult: nil with: (r := Alien newC: 16) pointer.
vec := (r doubleAt: 1) @ (r doubleAt: 9). r free. vec " 0.0 at 0.0 "

returns 0.0 at 0.0 instead of 3.0 at 4.0

the C code goes like this:

typedef struct atvect{double x, y;} atvect;

atvect atv(double x, double y) {
  atvect v = {x, y};
  return v;
}

atvect at34() {
  return atv(3.0, 4.0);
}

The vm is sqcogspur64linuxht 5.0-201910291408. The image is a recent trunk
image (update: #19142), image format 68021 (64 bit). Alien is
Alien-Core-TorstenBergmann.101.mcz.

What am I missing?

Thanks, Balázs

[1] "The rules for structure results vary slightly by platform. Most
functions returning structures expect a “hidden” first parameter holding
the result struct’s address. Because the FFI provides no abstraction one
must pass this parameter explicitly. ... On linux all struct results are
re-turned through the hidden first argument mechanism ... ".

https://wiki.squeak.org/squeak/uploads/6100/Alien%20FFI.2.pdf

[2]
https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/a8a1dc1e33267e0fa2dab22959e41d0a072420d9/platforms/Cross/plugins/IA32ABI/dax64business.h#L92

[3] "2. If the type has class MEMORY, then the caller provides space for
the return value and passes the address of this storage in %rdi as if it
were the first
argument to the function. In effect, this address becomes a “hidden” first
ar-
gument. This storage must not overlap any data visible to the callee through
other names than this argument.
On return %rax will contain the address that has been passed in by the
caller in %rdi."

https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf p.24.
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