Re: [PATCH GCC]Improve bound information in loop niter analysis

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Jul 28, 2015 at 11:36 AM, Bin Cheng <bin.cheng@arm.com> wrote:
> Hi,
> Loop niter computes inaccurate bound information for different loops.  This
> patch is to improve it by using loop initial condition in
> determine_value_range.  Generally, loop niter is computed by subtracting
> start var from end var in loop exit condition.  Moreover, loop bound is
> computed using value range information of both start and end variables.
> Basic idea of this patch is to check if loop initial condition implies more
> range information for both start/end variables.  If yes, we refine range
> information and use that to compute loop bound.
> With this improvement, more accurate loop bound information is computed for
> test cases added by this patch.

+      c0 = fold_convert (type, c0);
+      c1 = fold_convert (type, c1);
+
+      if (operand_equal_p (var, c0, 0))

I believe if c0 is not already of type type operand-equal_p will never succeed.

(side-note: we should get rid of the GMP use, that's expensive and now we
have wide-int available which should do the trick as well)

+         /* Case of comparing with the bounds of the type.  */
+         if (TYPE_MIN_VALUE (type)
+             && operand_equal_p (c1, TYPE_MIN_VALUE (type), 0))
+           cmp = GT_EXPR;
+         if (TYPE_MAX_VALUE (type)
+             && operand_equal_p (c1, TYPE_MAX_VALUE (type), 0))
+           cmp = LT_EXPR;

don't use TYPE_MIN/MAX_VALUE.  Instead use the types precision
and all wide_int operations (see match.pd wi::max_value use).

+  else if (!operand_equal_p (var, varc0, 0))
+    goto end_2;

ick - goto.  We need sth like a auto_mpz class with a destructor.

struct auto_mpz
{
  auto_mpz () { mpz_init (m_val); }
  ~auto_mpz () { mpz_clear (m_val); }
  mpz& operator() { return m_val; }
  mpz m_val;
};

> Is it OK?

I see the code follows existing practice in niter analysis even though
my overall plan was to transition its copying of value-range related
optimizations to use VRP infrastructure.

I'm still ok with improving the existing code on the basis that I won't
get to that for GCC 6.

So - ok with the TYPE_MIN/MAX_VALUE change suggested above.

Refactoring with auto_mpz welcome.

Thanks,
RIchard.

> Thanks,
> bin
>
> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>
>         * tree-ssa-loop-niter.c (refine_value_range_using_guard): New.
>         (determine_value_range): Call refine_value_range_using_guard for
>         each loop initial condition to improve value range.
>
> gcc/testsuite/ChangeLog
> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>
>         * gcc.dg/tree-ssa/loop-bound-1.c: New test.
>         * gcc.dg/tree-ssa/loop-bound-3.c: New test.
>         * gcc.dg/tree-ssa/loop-bound-5.c: New test.