Re: [RFC] Add support for vectors in comparisons (like the C++ frontend does)

[Paul Iannetta <piannetta@kalrayinc.com>]

On Wed, Oct 12, 2022 at 01:18:19AM +0200, Paul Iannetta wrote:
> On Mon, Oct 10, 2022 at 11:07:06PM +0000, Joseph Myers wrote:
> > On Mon, 10 Oct 2022, Paul Iannetta via Gcc-patches wrote:
> > 
> > > I have a patch to bring this feature to the C front-end as well, and
> > > would like to hear your opinion on it, especially since it may affect
> > > the feature-set of the objc front-end as well.
> > 
> > > Currently, this is only a tentative patch and I did not add any tests
> > > to the testsuite.
> > 
> > I think tests (possibly existing C++ tests moved to c-c++-common?) are 
> > necessary to judge such a feature; it could better be judged based on 
> > tests without implementation than based on implementation without tests.
> 
> Currently, this feature has the following tests in g++.dg/ext/
>   - vector9.C
>   - vector19.C
>   - vector21.C
>   - vector22.C
>   - vector23.C
>   - vector27.C
>   - vector28.C
> provided by Marc Glisse when he implemented the feature for C++.
> 
> They are all handled by my mirror implementation (after removing
> C++-only features), save for a case in vector19.C ( v ? '1' : '2',
> where v is a vector of unsigned char, but '1' and '2' are considered
> as int, which results in a type mismatch.)
> 
> I'll move those tests to c-c++-common tomorrow, but will duplicate
> vector19.C and vector23.C which rely on C++-only features.
> 
> During my tests, I've been using variations around this:
> 
> typedef int v2si __attribute__((__vector_size__ (2 * sizeof(int))));
> 
> v2si f (v2si a, v2si b, v2si c)
> {
>   v2si d = a + !b;
>   v2si e = a || b;
>   return c ? (a + !b) && (c - e && a) : (!!b ^ c && e);
> }
> 
> It is already possible to express much of the same thing without the
> syntactic sugar but is is barely legible
> 
> typedef int v2si __attribute__((__vector_size__ (2 * sizeof(int))));
> 
> v2si f (v2si a, v2si b, v2si c)
> {
>   v2si d = a + (b == 0);
>   v2si e = (a != 0) | (b != 0);
>   return ((c != 0) & (((a + (b == 0)) != 0) & (((c - e) != 0) & (a != 0))))
>        | ((c == 0) & (((((b == 0) == 0) ^ c) != 0) & (e != 0)));
> }
> 
> Paul

I still need to check what is done by clang on the objc side, but in
order to not conflict with what was done before, a warning is
triggered by c_obj_common_truthvalue_conversion and
build_unary_operator warns if '!' is used with a vector.  Both warnings
are only triggered in pedantic mode as suggested by Iain Sandoe.

The support of the binary ops and unary ops works as the C++ front-end
does, there is however the case of the ternary conditional operator,
where the C standard mandates the promotion of the operands if they
have rank less than (unsigned) int, whereas C++ does not.

In any case, as per the documentation of VEC_COND_EXPR,
"vec0 = vector-condition ? vec1 : vec2" is equivalent to
``` (from tree.def)
  for (int i = 0 ; i < n ; ++i)
    vec0[i] = vector-condtion[i] ? vec1[i] : vec2[i];
```
But this is currently not the case, even in C++ where
``` (Ex1)
typedef signed char vec2 __attribute__((vector_size(16)));
typedef float vec2f __attribute__((vector_size( 2 * sizeof (float))));

void j (vec2 *x, vec2 *z, vec2f *y, vec2f *t)
{
  *x = (*y < *t) ? '1' : '0'; // error: inferred scalar type ‘char’ is
                              // not an integer or floating-point type
                              // of the same size as ‘float’.

  for (int i = 0 ; i < 2 ; ++i)                  // fine
    (*x)[i] = (*y)[i] < (*t)[i] ? '1' : '0';     //

  *z = (*x < *z) ? '1' : '0'; // fine
}
```

The documentation explicitly says:
> the ternary operator ?: is available. a?b:c, where b and c are
> vectors of the same type and a is an integer vector with the same
> number of elements of the same size as b and c, computes all three
> arguments and creates a vector {a[0]?b[0]:c[0], a[1]?b[1]:c[1], …}
Here, "*y < *t" is a boolean vector (and bool is an integral type
([basic.fundamental] 11), so this should be accepted.

An other point is that if we look at
```
  for (int i = 0 ; i < n ; ++i)
    vec0[i] = vector-condtion[i] ? vec1[i] : vec2[i];
```
implicit conversions may happen, which is completely over-looked
currently.  That is, the type of (1): "v = v0 ? v1 : v2" is the lowest
common type of v, v1 and v2; and the type of (2): "v0 ? v1 : v2" is the
lowest common type of v1 and v2.  (2) can appear as a parameter, but
even in that case, I think that (2) should be constrained by the type
of the parameter and we are back to case (1).

My points are that:
  - the current implementation has a bug: " *x = (*y < *t) ? '1' :
    '0';" from (Ex1) should be fine.
  - the current implementation does not explicetly follow the
    documented behavior of VEC_COND_EXPR.

What do you think?

Paul