Vector Comparison patch

[Richard Guenther]

On Tue, Aug 23, 2011 at 1:11 PM, Artem Shinkarov
<artyom.shinkaroff@gmail.com> wrote:
> On Tue, Aug 23, 2011 at 11:56 AM, Richard Guenther
> <richard.guenther@gmail.com> wrote:
>> On Tue, Aug 23, 2011 at 12:45 PM, Artem Shinkarov
>> <artyom.shinkaroff@gmail.com> wrote:
>>> I'm confused.
>>> There is a set of problems which are tightly connected and you address
>>> only one one of them.
>>>
>>> I need to do something with C_MAYBE_CONST_EXPR node to allow the
>>> gimplification of the expression. In order to achieve that I am
>>> wrapping expression which can contain C_MAYBE_EXPR_NODE into
>>> SAVE_EXPR. This works fine, but, the vector condition is lifted out.
>>> So the question is how to get rid of C_MAYBE_CONST_EXPR nodes, making
>>> sure that the expression is still inside VEC_COND_EXPR?
>>
>> I can't answer this, but no C_MAYBE_CONST_EXPR nodes may survive
>> until gimplification.  I thought c_fully_fold is exactly used (instead
>> of c_save_expr) because it _doesn't_ wrap things in C_MAYBE_CONST_EXPR
>> nodes.  Instead you delay that (well, commented out in your patch).
>
> Ok. So for the time being save_expr is the only way that we know to
> avoid C_MAYBE_CONST_EXPR nodes.
>
>>> All the rest is fine -- a > b is transformed to VEC_COND_EXPR of the
>>> integer type, and when we are using it we can add != 0 to the mask, no
>>> problem. The problem is to make sure that the vector expression is not
>>> lifted out from the VEC_COND_EXPR and that C_MAYBE_CONST_EXPRs are
>>> also no there at the same time.
>>
>> Well, for example for floating-point comparisons and -fnon-call-exceptions
>> you _will_ get comparisons lifted out of the VEC_COND_EXPR.  But
>> that shouldn't be an issue because C semantics are ensured for
>> the mask ? v0 : v1 source form by changing it to mask != 0 ? v0 : v1 and
>> the VEC_COND_EXPR semantic for a non-comparison mask operand
>> is (v0 & mask) | (v1 & ~mask).  Which means that we have to be able to
>> expand mask = v0 < v1 anyway, but we'll simply expand it if it were
>> VEC_COND_EXPR <v0<v1, {-1,}, {0,}>.
>
> Richard, I think you almost get it, but there is a tiny thing you have missed.
> Look, let's assume, that by some reason when we gimplified a > b, the
> comparison was lifted out. So we have the following situation:
>
> D.1 = a > b;
> comp = vcond<D.1, v0, v1>
> ...
>
> Ok?
> Now, I fully agree that we want to treat lifted a > b as VCOND. Now,
> what I am doing in the veclower is when I meet vector comparison a >
> b, I wrap it in the VCOND, otherwise it would not be recognized by
> optabs. literally I am doing:
>
> rhs = gimplify_build3 (gsi, VEC_COND_EXPR, a, b, {-1}, {0}>
>
> And here is a devil hidden. By some reason, when this expression is
> gimplified, a > b is lifted again and is left outside the
> VEC_COND_EXPR, and that is the problem I am trying to fight with. Have
> any ideas what could be done here?

Well, don't do it.  Check if the target can expand

 D.1 = a > b;

via feeding it vcond <a < b, {-1,...}, {0,...} > and if not, expand it piecewise
in veclower.  If it can handle it - leave it alone!

In expand_expr_real_2 add to the EQ_EXPR (etc.) case the case
of a vector-typed comparison and use the vcond optab for it, again
via vcond <a < b, {-1,...}, {0,...} >.  If you look at the EQ_EXPR case
it dispatches to do_store_flag - that's the best place to handle
vector-typed compares.

Richard.