Re: [PATCH GCC]Improve loop bound info by simplifying conversions in iv base

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

On Tue, Jul 28, 2015 at 11:38 AM, Bin Cheng <bin.cheng@arm.com> wrote:
> Hi,
> For now, SCEV may compute iv base in the form of "(signed T)((unsigned
> T)base + step))".  This complicates other optimizations/analysis depending
> on SCEV because it's hard to dive into type conversions.  For many cases,
> such type conversions can be simplified with additional range information
> implied by loop initial conditions.  This patch does such simplification.
> With simplified iv base, loop niter analysis can compute more accurate bound
> information since sensible value range can be derived for "base+step".  For
> example, accurate loop bound&may_be_zero information is computed for cases
> added by this patch.
> The code is actually borrowed from loop_exits_before_overflow.  Moreover,
> with simplified iv base, the second case handled in that function now
> becomes the first case.  I didn't remove that part of code because it may(?)
> still be visited in scev analysis itself and simple_iv isn't an interface
> for that.
>
> Is it OK?

It looks quite special given it only handles a very specific pattern.  Did you
do any larger collecting of statistics on how many times this triggers,
esp. how many times simplify_using_initial_conditions succeeds and
how many times not?  This function is somewhat expensive.

+      || !operand_equal_p (iv->step,
+                          fold_convert (type,
+                                        TREE_OPERAND (e, 1)), 0))

operand_equal_p can handle sign-differences in integer constants,
no need to fold_convert here.  Also if you know that you are comparing
integer constants please use tree_int_cst_equal_p.

+      extreme = lower_bound_in_type (type, type);

that's a strange function to call here (with two same types).  Looks like
just wide_int_to_tree (type, wi::max/min_value (type)).

+  extreme = fold_build2 (MINUS_EXPR, type, extreme, iv->step);

so as iv->step is an INTEGER_CST please do this whole thing using
wide_ints and only build trees here:

+  e = fold_build2 (code, boolean_type_node, base, extreme);

Thanks,
Richard.

> Thanks,
> bin
>
> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>
>         * tree-ssa-loop-niter.c (tree_simplify_using_condition): Export
>         the interface.
>         * tree-ssa-loop-niter.h (tree_simplify_using_condition): Declare.
>         * tree-scalar-evolution.c (simple_iv): Simplify type conversions
>         in iv base using loop initial conditions.
>
> gcc/testsuite/ChangeLog
> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>
>         * gcc.dg/tree-ssa/loop-bound-2.c: New test.
>         * gcc.dg/tree-ssa/loop-bound-4.c: New test.
>         * gcc.dg/tree-ssa/loop-bound-6.c: New test.