[9fans] c compiler bug

[erik quanstrom <quanstro@quanstro.net>]

it appears that ?c do not properly do structure
assignment if the structure is packed.

i've included a patch just for 8c and a sample
program that generates two "packed assignment"
warnings with the modified compiler.

feedback appreciated unless it's "don't do that."

if this looks entirely reasonable, i'll submit a patch
for all the compilers.

- erik

---

; diffy -c /sys/src/cmd/8c/cgen.c
/n/dump/2010/0914/sys/src/cmd/8c/cgen.c:1824,1829 - /sys/src/cmd/8c/cgen.c:1824,1836
  	gins(ACLD, Z, Z);
  	gins(AREP, Z, Z);
  	gins(AMOVSL, Z, Z);
+ 	if(x = w & SZ_LONG-1){
+ 		warn(n, "packed assignment");
+ 		gins(AMOVL, nodconst(x), &nod3);
+ 	//	gins(ACLD, Z, Z);
+ 		gins(AREP, Z, Z);
+ 		gins(AMOVSB, Z, Z);
+ 	}
  	if(c & 4) {
  		gins(APOPL, Z, &nod3);
  		reg[D_CX]--;

---

#include <u.h>
#include <libc.h>

#pragma pack on
typedef struct S S;
struct	S
{
	uchar	a;
	ushort	b;
	uint	c;
};
#pragma pack off

S
func(void)
{
	S s;

	s.a = 1;
	s.b = 2;
	s.c = 3;
	return s;
}

void
main(void)
{
	S s, t;

	memset(&s, 0xff, sizeof s);
	print("sizeof s = %d\n", sizeof s);
	s = func();
	memset(&t, 0xff, sizeof t);
	t = s;
	print("a=%d b=%d c=%d\n", t.a, t.b, t.c);
	exits(nil);
}