Problem with std_expand_builtin_va_start on targets where sizetype and ptr_type_node don't have the same mode

[Zack Weinberg]

I'm trying to help DJ with converting m32c to define_constraint, and
I'm running into an unrelated problem.  Specifically, the unmodified
compiler targeting m32c-elf, in -mcpu=m32cm mode, crashes on any use
of __builtin_va_start, even this:

int foo(int x, ...)
{
 __builtin_va_list t;
 __builtin_va_start (t, x);
}

test.c:4: internal compiler error: in expand_expr_real_1, at expr.c:7109

The problem appears to be with std_expand_builtin_va_start, which does

 tree t;
 t = make_tree (sizetype, nextarg);
 t = fold_convert (ptr_type_node, t);

 t = build2 (MODIFY_EXPR, TREE_TYPE (valist), valist, t);
 TREE_SIDE_EFFECTS (t) = 1;

 expand_expr (t, const0_rtx, VOIDmode, EXPAND_NORMAL);

When -mcpu=m32cm, sizetype is HImode (16 bits) but ptr_type_node is
PSImode (24 bits).  The VAR_DECL constructed by the first three lines
of std_expand_builtin_va_start has a DECL_RTL in PSImode, but a
TREE_TYPE in HImode.  This is not acceptable to expand_expr_real_1.

Naively speaking, I do not see why the function could not be written

 tree t = build2 (MODIFY_EXPR, TREE_TYPE (valist), valist),
                  make_tree (ptr_type_node, nextarg);
 expand_expr (t, const0_rtx, VOIDmode, EXPAND_NORMAL);

or even

 rtx va_r = expand_normal (valist);
 emit_move_insn (va_r, nextarg);

but this is a part of the compiler I am not at all familiar with...

zw