From mboxrd@z Thu Jan  1 00:00:00 1970
From: Zack Weinberg <zack@rabi.columbia.edu>
To: egcs@egcs.cygnus.com
Subject: making aliases into the middle of a structure
Date: Fri, 30 Apr 1999 23:15:00 -0000
Message-ID: <199904221813.OAA07239@blastula.phys.columbia.edu>
X-SW-Source: 1999-04n/msg00769.html
Message-ID: <19990430231500.MglaktnoAISrnYZxp_L6eXrHB6wrMptpNOOz2bu1PsM@z>

Consider this bit of code.

struct s
{
  int a, b;
};

struct s x = { 0, 0 };

extern __typeof(x.b) y __attribute__ ((alias("x.b")));

The goal is to have x.b and y refer to the same memory location.  I am
aware that this is somewhat perverse, but there are legitimate reasons
(binary compatibility) for it.

If gcc were to translate this into

.data
.globl x
        .align 4
        .type    x,@object
        .size    x,8
x:
        .zero   8
.globl y
        .set    y, x+4

then the assembler would generate a correct object module, and
references to `x.b' and `y' would indeed operate on the same memory
location.

Unfortunately this is not what gcc produces; instead I get

.data
.globl x
        .align 4
        .type    x,@object
        .size    x,8
x:
        .zero   8
.globl y
        .set    y,x.b


The assembler generates an undefined reference to the symbol `x.b' and
forgets about `y'.

I have experimented with workarounds involving offsetof();
unfortunately, the best this can do is

	.set y,x + ((size_t) &((s *)0)->b)

which is not an expression that the assembler can evaluate.  I do not
want to write a raw assembly module, because that will break if the
padding inside the structure, or the sizes of the component types,
change.

Any suggestions?

zw