generalized lvalues -- patch outline

[Ziemowit Laski <zlaski at apple dot com>]

On 19 Nov 2004, at 17.50, Ziemowit Laski wrote:

> To put it another way, I'm only concerned with cases where the 
> compiler currently complains
> about assigning to a non-lvalue, and the non-lvalue in question is a 
> cast of an lvalue.

Indeed, I now appear to have a mainline mod for C and C++ which allows 
assignment to lvalue casts for pointer types.  What follows is a 
high-level synopsis of what I did; if there is interest, I can whip up 
a full-fledged patch, complete with docs.  Please let me know.

Thanks,

--Zem

------

The lvalue cast assignment/increment/decrement can be enabled with the 
-flvalue-cast-assign flag (which is off by default).

The active ingredient of the C patch is:

Index: gcc/c-typeck.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-typeck.c,v
retrieving revision 1.400
diff -u -3 -p -r1.400 c-typeck.c
--- gcc/c-typeck.c      20 Nov 2004 20:31:43 -0000      1.400
+++ gcc/c-typeck.c      22 Nov 2004 20:32:00 -0000
@@ -2724,6 +2724,16 @@ lvalue_or_else (tree ref, enum lvalue_us
 {
   int win = lvalue_p (ref);

+  /* If -flvalue-cast-assignment is specified, we shall allow 
assignments
+     (including increment/decrement) to casts of lvalues, as long as
+     both the lvalue and the cast are pointers.  */
+  if (!win && flag_lvalue_cast_assign
+      && TREE_CODE (ref) == NOP_EXPR
+      && (use == lv_assign || use == lv_increment || use == 
lv_decrement)
+      && TREE_CODE (TREE_TYPE (ref)) == POINTER_TYPE
+      && TREE_CODE (TREE_TYPE (TREE_OPERAND (ref, 0))) == POINTER_TYPE)
+    win = lvalue_p (TREE_OPERAND (ref, 0));
+
   if (!win)
     {
       switch (use)

and will allow code as follows:

Index: gcc/testsuite/gcc.dg/lvalue-cast-1.c
===================================================================
RCS file: gcc/testsuite/gcc.dg/lvalue-cast-1.c
diff -N gcc/testsuite/gcc.dg/lvalue-cast-1.c
--- /dev/null   1 Jan 1970 00:00:00 -0000
+++ gcc/testsuite/gcc.dg/lvalue-cast-1.c        22 Nov 2004 20:32:50 
-0000
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-flvalue-cast-assign" } */
+
+int foo(void) {
+
+  char *p;
+  long l;
+  short s;
+
+  (long *)p = &l; /* ok */
+  ((long *)p)++;  /* ok */
+  (short)l = 2;   /* { dg-error "non-lvalue" } */
+  (long)s = 3;    /* { dg-error "non-lvalue" } */
+
+  return 0;
+}

Note that we're still erroring out on the non-pointer types, though 
that too can be changed.  FWIW, Microsoft's C compiler also allows the 
'(short)l = 2' (though not the '(long)s = 3').

The C++ side of things is very similar:

Index: gcc/cp/tree.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/cp/tree.c,v
retrieving revision 1.419
diff -u -3 -p -r1.419 tree.c
--- gcc/cp/tree.c       12 Nov 2004 21:47:09 -0000      1.419
+++ gcc/cp/tree.c       22 Nov 2004 20:34:50 -0000
@@ -223,6 +223,17 @@ lvalue_or_else (tree ref, const char* st
 {
   if (!lvalue_p (ref))
     {
+      /* If -flvalue-cast-assignment is specified, we shall allow 
assignments
+        (including increment/decrement) to casts of lvalues, as long as
+        both the lvalue and the cast are pointers.  */
+      if (flag_lvalue_cast_assign
+         && TREE_CODE (ref) == NOP_EXPR
+         && string && string[0] == 'a'
+         && TREE_CODE (TREE_TYPE (ref)) == POINTER_TYPE
+         && TREE_CODE (TREE_TYPE (TREE_OPERAND (ref, 0))) == 
POINTER_TYPE
+         && lvalue_p (TREE_OPERAND (ref, 0)))
+       return 1;
+
       error ("non-lvalue in %s", string);
       return 0;
     }

although we clearly could use an 'enum lvalue_use' here as well.  At 
any rate, this patch causes the following to compile and run 
successfully:

Index: gcc/testsuite/g++.dg/ext/lvalue-cast-1.cpp
===================================================================
RCS file: gcc/testsuite/g++.dg/ext/lvalue-cast-1.cpp
diff -N gcc/testsuite/g++.dg/ext/lvalue-cast-1.cpp
--- /dev/null   1 Jan 1970 00:00:00 -0000
+++ gcc/testsuite/g++.dg/ext/lvalue-cast-1.cpp  22 Nov 2004 20:36:43 
-0000
@@ -0,0 +1,29 @@
+/* { dg-do run } */
+/* { dg-options "-flvalue-cast-assign" } */
+
+#include <stdlib.h>
+#define CHECK_IF(expr) if (!(expr)) abort ()
+
+static int global;
+
+void f(int &) { global = 35; }
+void f(const int &) { global = 78; }
+
+long long_arr[2];
+
+int main(void) {
+
+  char *p;
+
+  (long *)p = long_arr;
+  ((long *)p)++;
+  *(long *)p = -1;
+  *p = -2;
+  CHECK_IF(p[-1] == 0 && p[0] == -2 && p[1] == -1);
+
+  long x = 0;
+  f((int)x);
+  CHECK_IF(global == 78);
+
+  return 0;
+}

The rest of the patch (not shown) deals with defining/handling of the 
-flvalue-cast-assign flag.