The size of a union is the size of the largest
element aligned with the largest alignment.
For example, the size of the following union is
16, not 13 (as returned before this patch).
union {
char c[13];
int i;
};
The spec says that numbers can be arbitrarily
big, and only the last 64 bits will be taken
into consideration. Calling sscanf does not
implement this, so I wrote an ad-hoc function.
Compiling languages with closures often requires passing
an extra environment parameter to the called function.
One solution is to use a convention, and reserve, say,
the first argument for that purpose. However, that
makes binding to C a little less smooth.
Alternatively, QBE now provides a way to remain fully
ABI compatible with C by having a "hidden" environment
argument (marked with the keyword 'env'). Calling a
function expecting an environment from C will make the
contents of the environment undefined, but the normal
arguments will be passed without alteration. Conversely,
calling a C function like it is a closure by passing
it an environemnt will work smoothly.
This happens to be needed for C. The standard mandates that a return
value is used if the caller uses it. Surprisingly, if the return
"value" is not used, the callee can use "return;".
A better solution is to add an "undef" value and return it, "undef"
would also have other use cases for compiling C.
I thought it would be harder (and maybe it is).
My fear was that a call must be always followed by
a parallel move from machine registers (this is an
assumption in both spill and rega). This however
remains true, because the ABI code generates a
dummy "copy RAX" by accident!
