Re: IMA vs tree-ssa

[Geoff Keating <geoffk at geoffk dot org>]

Richard Henderson <rth@redhat.com> writes:

> On Thu, Feb 26, 2004 at 02:20:09PM -0800, Dale Johannesen wrote:
> > There are several places where tree-ssa does type comparison by
> > comparing TYPE_MAIN_VARIANT() for equality.  This doesn't work with
> > IMA, and leads to several different ICEs in various SPECmarks (you
> > can probably figure out which one the example came from :)
> 
> This is a bug in IMA.  When unifying the types, you should either
> replace all of them or make all copies be variants.  Clearly the
> later is much easier than the former.

As I commented, will this really work?

Consider:

a.c:

struct bar {
  int x;
};

extern void foo (struct bar *bp);

// call foo()

b.c:

struct not_bar {
  int x;
};

void foo (struct not_bar *bp) {
  ...
}


This is valid C89.

Also, consider:

a.c:

union x_u {
  int i[2];
  double d;
};

union x_u x_u_i = { 0x1234, 0x5678 };

b.c:

extern union {
  double d;
  int i[2];
} x_u_i;

// use x_u_i

which is also valid in C89.  The rules got tightened significantly in
C99, but even then you can have one anonymous structure and another
which has a tag.

In addition, structure compatilibity between modules is not
transitive, even in C99.  For instance,

struct { int x; }

is compatible with both

struct x_1 { int x; }
and
struct x_2 { int x; }

but x_1 and x_2 are not compatible with each other.


The relevant paragraphs in the standards are 6.1.2.6 para 1 in C89,
and 6.2.7 para 1 in C99; note that the paragraphs are quite different,
this is one of the more significant changes between C89 and C99.


If it does work, I agree this is the best solution.

-- 
- Geoffrey Keating <geoffk@geoffk.org>