[PATCH] handling of non-lvalue compound objects

Handling of non-lvalue compound objects:

We introduce a new primitive - EXPR_SLICE.  Meaning is "that many bits
from that offset in that non-lvalue struct or union".  It is used when
we try to get a member out of a non-lvalue struct or union (subsequent .<field>
just narrow the slice).  And as far as scalar, struct and union fields count,
that's it.  The only subtle point is handling of array fields.  And there
I'm doing what C99 requires - they *do* decay to real, honest pointers,
causing a copy of object to memory if needed.  We get an anonymous object
that lives until the next sequence point; optimizer is certainly free to
get rid of it completely if it can make do with the value we'd copied there.

Note that you _are_ allowed to say
        foo().a[1] = 0;
It doesn't make sense, since the value you've assigned will be immediately
lost (and any optimizer will turn that into f()), but it is legitimate and 
it avoids a *lot* of PITA in describing semantics.

It covers only array decay - any other member of non-lvalue struct or union
is *not* an lvalue and in
	struct foo {int x; int y[2];};
	struct foo a(void);
        ...
	a().x = 0;              /* not allowed, non-lvalue */
	a().y[0] = 1;           /* allowed, but pointless */
you will get an error from the first assignment, but not from the second
one.

Signed-off-by: Al Viro <viro@parcelfarce.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/expression.h b/expression.h
index d03cdc35..4c10db8e 100644
--- a/expression.h
+++ b/expression.h
@@ -40,6 +40,7 @@ enum expression_type {
 	EXPR_INDEX,		// index in initializer
 	EXPR_POS,		// position in initializer
 	EXPR_FVALUE,
+	EXPR_SLICE,
 };
 
 struct expression {
@@ -78,6 +79,11 @@ struct expression {
 			struct expression *deref;
 			struct ident *member;
 		};
+		// EXPR_SLICE
+		struct /* slice */ {
+			struct expression *base;
+			unsigned r_bitpos, r_nrbits;
+		};
 		// EXPR_CAST and EXPR_SIZEOF
 		struct /* cast_arg */ {
 			struct symbol *cast_type;