3914 – Struct as argument that fits in register has member accessed wrong

Depending on the build options and where in a function the struct parameter is used, referencing one member will actually reference another one.
Given the following program:
import std.stdio;
struct SS
{
	char a;
	char b;
	char c;
	char d;
//	char e;
}
void A( SS ss1 )
{
C:	//writeln( ss1.a, ss1.b, ss1.c, ss1.d );
	char temp;
	temp = ss1.a;
	ss1.a = ss1.d;
	ss1.d = temp;
	temp = ss1.b;
	ss1.b = ss1.c;
	ss1.c = temp;
B:	//writeln( ss1.a, ss1.b, ss1.c, ss1.d );
	temp = ss1.a;
	ss1.a = ss1.d;
	ss1.d = temp;
	temp = ss1.b;
	ss1.b = ss1.c;
	ss1.c = temp;
A:	writeln( ss1.a, ss1.b, ss1.c, ss1.d );
}
void main()
{
	SS ss3;
	ss3.a = 'A';
	ss3.b = 'L';
	ss3.c = 'D';
	ss3.d = 'O';
	A( ss3 );
}
If lines A, B, and C are enabled in the following pattern along with the given build options, you get these results:
	(none)	-g	-rel	-rel-O	-O	-g -O
A	ALDO	ALDO	ALDO	ALDA	ALDA	ALDA
						
B, A	ODLA	ODLA	ODLA	ADLA	ADLA	ADLA
	ALDO	ALDO	ALDO	ALDA	ALDA	ALDA
						
C, B, A	ALDA	ALDO	ALDA	ALDA	ALDA	ALDA
	ODLA	ODLA	ODLA	ODLA	ODLA	ODLA
	ALDO	ALDO	ALDO	ALDO	ALDO	ALDO
In particular, notice that field 'd' starts off wrong.
If I uncomment field 'e', or change one of them to an int, the problem goes away.
At C, "ALDO" should be printed.
At B, "ODLA" should be printed.
At A, "ALDO" should be printed.

Here's a slightly reduced test case which doesn't require any compiler flags. 
Not a regression, fails even on DMD0.165.
It's a problem with variadic function parameters and fastpar arguments.
------------
struct SS {
 char a, b, c, d;
}
void show(char[] args...) {
 assert(args[0]=='A');
 assert(args[1]=='L');
 assert(args[2]=='D');
 assert(args[3]=='O');
}
void A( SS ss ) {
 show( ss.a, ss.b, ss.c, ss.d );
}
void main(){
 SS ss3;
 ss3.a = 'A';
 ss3.b = 'L';
 ss3.c = 'D';
 ss3.d = 'O';
 A( ss3 );
}

PATCH: The struct ss is passed in EAX. The backend wants ss.b. It sees that ss.b is already in EAX, so it doesn't reload it into AX. But AX actually contains ss.a.
Solution: Disable the optimisation in cod1.loaddata() if it's a subsequent member of the struct.
Index: cod1.c
===================================================================
--- cod1.c	(revision 413)
+++ cod1.c	(working copy)
@@ -3453,6 +3453,7 @@
 // See if we can use register that parameter was passed in
 if (regcon.params && e->EV.sp.Vsym->Sclass == SCfastpar &&
 	regcon.params & mask[e->EV.sp.Vsym->Spreg] &&
+	!(e->Eoper == OPvar && e->EV.sp.Voffset > 0) && // Must be at the base of that variable
 	sz <= REGSIZE)			// make sure no 'paint' to a larger size happened
 {
 	reg = e->EV.sp.Vsym->Spreg;
=========================
Reduced test case:
=========================
struct Snake {
 short a, b;
}
void venom(short dd)
{
 assert(dd == 'B');
}
void serpent( Snake ss ) {
 venom(ss.b);
}
void main(){
 Snake s;
 s.a = 'A';
 s.b = 'B';
 serpent( s );
}

changeset 416

Fixed DMD1.058 and DMD2.042.