vectorizer: Fix up vectorization using WIDEN_MINUS_EXPR [PR102124]

Hi!

The following testcase is miscompiled on aarch64-linux at -O3 since the
introduction of WIDEN_MINUS_EXPR.
The problem is if the inner type (half_type) is unsigned and the result
type in which the subtraction is performed (type) has precision more than
twice as larger as the inner type's precision.
For other widening operations like WIDEN_{PLUS,MULT}_EXPR, if half_type
is unsigned, the addition/multiplication result in itype is also unsigned
and needs to be zero-extended to type.
But subtraction is special, even when half_type is unsigned, the subtraction
behaves as signed (also regardless of whether the result type is signed or
unsigned), 0xfeU - 0xffU is -1 or 0xffffffffU, not 0x0000ffff.

I think it is better not to use mixed signedness of types in
WIDEN_MINUS_EXPR (have unsigned vector of operands and signed result
vector), so this patch instead adds another cast to make sure we always
sign-extend the result from itype to type if type is wider than itype.

Bootstrapped/regtested on aarch64-linux, x86_64-linux and i686-linux, ok
for trunk/11.3?

2021-08-31  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102124
	* tree-vect-patterns.c (vect_recog_widen_op_pattern): For ORIG_CODE
	MINUS_EXPR, if itype is unsigned with smaller precision than type,
	add an extra cast to signed variant of itype to ensure sign-extension.

	* gcc.dg/torture/pr102124.c: New test.


	Jakub

Jakub Jelinek <jakub@redhat.com> writes:
> Hi!
>
> The following testcase is miscompiled on aarch64-linux at -O3 since the
> introduction of WIDEN_MINUS_EXPR.
> The problem is if the inner type (half_type) is unsigned and the result
> type in which the subtraction is performed (type) has precision more than
> twice as larger as the inner type's precision.
> For other widening operations like WIDEN_{PLUS,MULT}_EXPR, if half_type
> is unsigned, the addition/multiplication result in itype is also unsigned
> and needs to be zero-extended to type.
> But subtraction is special, even when half_type is unsigned, the subtraction
> behaves as signed (also regardless of whether the result type is signed or
> unsigned), 0xfeU - 0xffU is -1 or 0xffffffffU, not 0x0000ffff.
>
> I think it is better not to use mixed signedness of types in
> WIDEN_MINUS_EXPR (have unsigned vector of operands and signed result
> vector), so this patch instead adds another cast to make sure we always
> sign-extend the result from itype to type if type is wider than itype.
>
> Bootstrapped/regtested on aarch64-linux, x86_64-linux and i686-linux, ok
> for trunk/11.3?
>
> 2021-08-31  Jakub Jelinek  <jakub@redhat.com>
>
> 	PR tree-optimization/102124
> 	* tree-vect-patterns.c (vect_recog_widen_op_pattern): For ORIG_CODE
> 	MINUS_EXPR, if itype is unsigned with smaller precision than type,
> 	add an extra cast to signed variant of itype to ensure sign-extension.
>
> 	* gcc.dg/torture/pr102124.c: New test.

LGTM.  I was wondering whether it would be better to add a new
non-default parameter to select this behaviour, so that any new
callers have to think about it too.  It would also be more consistent
with the shift_p parameter.

Either way's fine though, so the patch is OK as-is.

Thanks,
Richard

> --- gcc/tree-vect-patterns.c.jj	2021-08-17 21:05:07.000000000 +0200
> +++ gcc/tree-vect-patterns.c	2021-08-30 11:54:03.651474845 +0200
> @@ -1268,11 +1268,31 @@ vect_recog_widen_op_pattern (vec_info *v
>    /* Check target support  */
>    tree vectype = get_vectype_for_scalar_type (vinfo, half_type);
>    tree vecitype = get_vectype_for_scalar_type (vinfo, itype);
> +  tree ctype = itype;
> +  tree vecctype = vecitype;
> +  if (orig_code == MINUS_EXPR
> +      && TYPE_UNSIGNED (itype)
> +      && TYPE_PRECISION (type) > TYPE_PRECISION (itype))
> +    {
> +      /* Subtraction is special, even if half_type is unsigned and no matter
> +	 whether type is signed or unsigned, if type is wider than itype,
> +	 we need to sign-extend from the widening operation result to the
> +	 result type.
> +	 Consider half_type unsigned char, operand 1 0xfe, operand 2 0xff,
> +	 itype unsigned short and type either int or unsigned int.
> +	 Widened (unsigned short) 0xfe - (unsigned short) 0xff is
> +	 (unsigned short) 0xffff, but for type int we want the result -1
> +	 and for type unsigned int 0xffffffff rather than 0xffff.  */
> +      ctype = build_nonstandard_integer_type (TYPE_PRECISION (itype), 0);
> +      vecctype = get_vectype_for_scalar_type (vinfo, ctype);
> +    }
> +
>    enum tree_code dummy_code;
>    int dummy_int;
>    auto_vec<tree> dummy_vec;
>    if (!vectype
>        || !vecitype
> +      || !vecctype
>        || !supportable_widening_operation (vinfo, wide_code, last_stmt_info,
>  					  vecitype, vectype,
>  					  &dummy_code, &dummy_code,
> @@ -1291,8 +1311,12 @@ vect_recog_widen_op_pattern (vec_info *v
>    gimple *pattern_stmt = gimple_build_assign (var, wide_code,
>  					      oprnd[0], oprnd[1]);
>  
> +  if (vecctype != vecitype)
> +    pattern_stmt = vect_convert_output (vinfo, last_stmt_info, ctype,
> +					pattern_stmt, vecitype);
> +
>    return vect_convert_output (vinfo, last_stmt_info,
> -			      type, pattern_stmt, vecitype);
> +			      type, pattern_stmt, vecctype);
>  }
>  
>  /* Try to detect multiplication on widened inputs, converting MULT_EXPR
> --- gcc/testsuite/gcc.dg/torture/pr102124.c.jj	2021-08-30 12:08:05.838649133 +0200
> +++ gcc/testsuite/gcc.dg/torture/pr102124.c	2021-08-30 12:07:52.669834031 +0200
> @@ -0,0 +1,27 @@
> +/* PR tree-optimization/102124 */
> +
> +int
> +foo (const unsigned char *a, const unsigned char *b, unsigned long len)
> +{
> +  int ab, ba; 
> +  unsigned long i;
> +  for (i = 0, ab = 0, ba = 0; i < len; i++)
> +    {
> +      ab |= a[i] - b[i];
> +      ba |= b[i] - a[i];
> +    }   
> +  return (ab | ba) >= 0;
> +}
> +
> +int
> +main ()
> +{
> +  unsigned char a[32] = { 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a' };
> +  unsigned char b[32] = { 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a' };
> +  unsigned char c[32] = { 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b' };
> +  if (!foo (a, b, 16))
> +    __builtin_abort ();
> +  if (foo (a, c, 16))
> +    __builtin_abort ();
> +  return 0;
> +}
>
> 	Jakub