[M3devel] a trouble with passing records by value..

[Jay K]

Given something like this:

jbook2:p247 jay$ more 1.c
#include <assert.h>

typedef struct { long code; long value; } T1;

void ActionLookup(T1 t, long code, long value);

void ActionLookup(T1 t, long code, long value)
{
  assert(t.code == code);
  assert(t.value == value);
}

int main()
{
  T1 t1 = {2,2};
  ActionLookup(t1, 2, 2);
  return 0;
}
j


on some platforms, such as AMD64_DARWIN, T1 is passed in registers. Good.


However, one of the unfortunate aspects of our Modula-3 system is that when you reference e.g. t1.value,
the backend isn't told you are accessing the "value" "field" of "t1", and it figures out where that is,
but rather 64bits at offset 64bits into t1. Therefore t1 must have an address. Therefore it can't be in registers.
Darn.


If m3cg were higher level this could be better.


There should be a viable compromise where the parameter is passed in registers, and only "homed"
to some stack location if its address is used -- e.g. to pass unused parameters in registers.
But given the inefficiency of field accesses, I'm not sure it is worth trying?


Maybe we should have M3CG include field references?


There is this basic problem that the interface between m3front and m3cc isn't really at the
right level for m3cc. But it is probably for m3front. m3front wants a lower level code generator.


 - Jay